A male medium ground finch (left) next to a female small ground finch (right). It's unclear what the nature of their relationship is—but if the male is small for his kind, love could blossom. Photo Credit: Nick Athanas, flickr

When Looking for Love, Darwin’s Finches Choose Mates Just Like Their Parents

“Wow, you look just like my father” can be a surefire way to bring a date to a screeching halt. Unless, that is, you have hollow bones and live on a remote Pacific island.Today, in the journal PNAS, researchers <a href="http://www.pnas.org/cgi/doi/10.1073/pnas.1813662115">report</a> that the famous finches once studied by Darwin on the Galápagos Islands tend to choose mates that bear an uncanny resemblance to their parents. This strategy helps ensure that birds boost their chances of bunking up with partners of their own species—and may even help solve an age-old evolutionary puzzle.On the small sliver of Earth that Darwin’s finches call home, <a href="https://www.princeton.edu/news/2017/11/27/study-darwins-finches-reveals-new-species-can-develop-little-two-generations">18</a> <a href="http://science.sciencemag.org/content/early/2017/11/20/science.aao4593">species</a> of these birds speckle the skies. Each lineage has its own little quirks. For instance, the shapes and sizes of different finch beaks have been whittled around specific foods: Some more suited to crack open little seeds, while others are particularly adept at pecking at cacti.
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But it was only in the last million years or so that these diverse lineages truly start to crystallize. Even today, their distinguishing features are few and far between—which means breeding season can be a chaotic cross between speed dating and a family reunion. For years, biologists have wondered how so many finch lineages diverged in such close proximity, in such a short amount of time. Without geographical barriers to keep species separate, how did birds avoid interbreeding?Luckily for finches, they’re born with the most reliable role models nature can provide: their parents. By simply paying attention to the subtle features and habits of their primary caretakers, little finches can begin to acquire a sense of their species identity—and which subtle features distinguish one lineage from another. This time-sensitive process of learning is called imprinting.Some imprinted traits are vocal. When a male finch is single and ready to mingle, he can’t be a slouch with song: His winsome, species-specific warble is a big part of what will ultimately woo a female. But young birds aren’t born belting arias. Each male must earn his vocal heritage by studying dutifully under his father’s tutelage during his youth. Because these serenades are sung or recognized on a lineage-by-lineage basis, they help maintain finch fidelity—but imprinting is far from foolproof. If impressionable young males are exposed to the vocal stylings of another species during this period, they may spend the rest of their lives chasing the wrong tail.
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<figcaption>A female cactus finch. When it comes time for her to select a mate, she may trend towards choosing with with a beak shaped like her father's. Photo Credit: Sergey Pisarevskiy, flickr
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Clearly, deciding on a date through one trait alone can be quite the gamble. If a finch is to carry on with its own kind, there’s a good chance it’s imprinting on other cues to tell who’s who in the avian community, reasoned evolutionary biologists <a href="https://eeb.princeton.edu/people/peter-grant">Peter</a> and <a href="https://eeb.princeton.edu/people/b-rosemary-grant">Rosemary</a> Grant of Princeton University. And if the Grants have a theory about Darwin’s finches, it’s probably right.The pair has studied the Galápagos Islands’ winged residents <a href="https://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html">since 1973</a>, observing evolution in real time as lineages of finches ebbed and flowed through the years. From 1976 to 2012, the Grants tracked thousands of finches on the tiny volcanic outcrop of Daphne Major, meticulously recording their beak and body dimensions and noting each bird’s parentage and mating habits (according to their report, the duo attempted to find every nest and identify the parents in every year of breeding from 1976 to 1998).
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“There is so much power in long-term studies like these,” says <a href="http://www.massey.ac.nz/massey/expertise/profile.cfm?stref=136040">James Dale</a>, an evolutionary biologist who studies bird behavior at Massey University in New Zealand. The Grants, he adds, have “accumulated a vast database that allows them to test so many new theories.”The breadth of the Grants’ body of work is truly staggering. By 2012, the pair had amassed nearly 40 years of anatomical and behavioral data on two species of finch: the <a href="https://neotropical.birds.cornell.edu/Species-Account/nb/species/megfin1/overview">medium ground finch</a> (Geospiza fortis), a puff of a bird with a stubby beak suited for snacking on seeds; and the <a href="https://neotropical.birds.cornell.edu/Species-Account/nb/species/cocfin3">cactus finch</a> (Geospiza scandens), which sports a svelte, spiky beak apt for chowing down on—you guessed it—prickly pear cactus.The Grants quickly discovered each species of finch had a strong tendency to stick to its own kind—and, as they expected, the cues for their choices weren’t limited to song. For instance, in the medium ground finch, body size matched up consistently between mates and parents—especially the father. In the cactus finch, on the other hand, females also went for daddy doppelgängers, but instead prioritized the shape of their fathers’ beaks. Regardless of anatomy, these preferences meant that when most flirty finches were hot to trot, they were recalling the faces and forms they saw most often in their youth.“It was very rewarding to discover that [features] of the mates of both sons and daughters can be predicted from the same characteristics of the parents,” the Grants wrote in an email.Incredibly, because the differences between individual birds are so minor, many finches were selecting mates among anatomical variants of “just millimeters,” says <a href="http://www.utsc.utoronto.ca/~jweir/">Jason Weir</a>, a biologist who studies the evolution of birds at the University of Toronto, but was not involved in the work. Such a subtle pattern, he says, was only detectable through the sheer numbers of birds the Grants tagged and tracked over the years. “They basically know every individual and have every pedigree,” Weir explains.
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<figcaption>A male cactus finch. His pointed beak may appeal to a female looking to couple up—if it resembles her father's, that is. Credit: David Cook, flickr
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Every population, however, is graced by a few renegade romantics. Some medium ground finches ultimately strayed outside their own lineage, blurring the lines between species. But even mismatched lovebirds found common ground. Rather than sticking to their own, some medium ground finches sought the company of <a href="https://neotropical.birds.cornell.edu/Species-Account/nb/species/smgfin1">small ground finches</a> (Geospiza fuliginosa), which are closely related, but, as their name indicates, a bit more diminutive in size. When the Grants examined these oddballs, they realized the medium ground finches most likely to seek mini-mates were often pint-sized themselves.Some large finches took on a similar habit. Over the course of his 14-year life, one lothario from the <a href="https://neotropical.birds.cornell.edu/Species-Account/nb/species/larcaf2/overview">large cactus finch</a> (Geospiza conirostris) population swept no less than six female medium ground finches off their feet—several of whom were particularly big-bodied for their breed. Even in extra-species couplings, like mated with like.Imprinting on parents to find a mate “is a learning process that helps finches to navigate the adult world,” the Grants explain.
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And the findings of this study speak to much more than individual finches’ flights of fancy. By picking partners that parallel their parents, finches increase the likelihood that their mates—and future offspring—will be of their same ilk. Reusing the same imprinting template over and over means that the differences between lineages can get enhanced until they split away entirely. With this mirroring strategy, finches can breed diversity under even the most unexpected of circumstances.Turns out, birds of a feather really do flock together—especially in the Galapagos.

Bringing home fellas that look just like dad might be a red flag to some. But for Galápagos finches, a pairing like this might just do papa proud.

Adult African clawed frogs (Xenopus laevis) usually develop spike-like structures at sites of limb amputation. But researchers can now equip these frogs with bioreactors that help them partially regenerate their hind legs. Photo Credit: TimVickers, Wikimedia Commons

A Progesterone-Pumping Device Helps Frogs Regenerate Lost Limbs

Somewhere along our evolutionary trajectory, we humans lost our regenerative powers. Sure, we can manage <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2701258/">a hunk of liver here</a>, <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5454966/">the tip of a finger there</a>—but in contrast to the <a href="https://www.pbs.org/wgbh/nova/article/saving-axolotls/">salamanders</a>, flatworms, and <a href="https://www.popularmechanics.com/science/animals/a27571/starfish-disease-science/">sea stars</a> of the world, the spontaneous remaking of entire organs and limbs eludes humankind.For biologist <a href="http://ase.tufts.edu/biology/labs/levin/">Michael Levin</a> of Tufts University, however, “lost” isn’t the same as “nonexistent.”Only a few million years separate humans from our closest regenerating relatives. If our animal ancestors built bone and tissue from scratch throughout adulthood, these superpowers may still lie buried within our own tissues, waiting to be reawakened. All it would take, says Levin, is the right signal.Now, Levin and his colleagues may have taken the next steps in pinpointing that very trigger by reviving these clandestine capabilities in African clawed frogs. Their work, <a href="https://www.cell.com/cell-reports/fulltext/S2211-1247(18)31573-0">published today</a> in the journal Cell Reports, kick-started the growth of partial but high-functioning hind legs in amphibian amputees after 24 hours of hormonal treatment. Though the work is still preliminary, such findings may someday inform researchers on the path to finding—and flipping—a similar switch in humans.
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Cut the tail off a tadpole, and <a href="https://www.nationalgeographic.com/science/phenomena/2013/01/13/tadpole-heal-thyself-vilified-chemicals-spur-regeneration/">another will take its place</a>. But even in the amphibian world, there are sobering consequences to growing up: If a mature frog loses a leg, the best it can hope for is a frail cartilaginous spike to take its place. This ability is technically regenerative—certainly far more than what any human could accomplish—but the resulting appendage does little for a creature that relies heavily on its hind limbs to navigate its aquatic environment.As the frog ages, its regenerative skills seem to go into hibernation—making it “a good stepping stone between animals that [fully] regenerate, like salamanders, and animals that don’t, like mammals,” Levin explains.To rouse regeneration from its slumber, the researchers first needed to identify the right molecular alarm. The ideal candidate, they reasoned, would be a powerful chemical cue that is both naturally produced by animal bodies and capable of directing a vast array of cellular functions—a winning combination that, for the researchers, pointed to progesterone.
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Every month, as a natural part of the female menstrual cycle, progesterone regenerates the entire endometrium, or lining of the uterus. “We thought, ‘It’s impossible that progesterone is only acting at this level,’” explains lead author <a href="http://sites.tufts.edu/celiaherrerarincon/">Celia Herrera-Rincon</a>, a neuroscientist at Tufts University.Progesterone wields far more power than simply preparing the uterus for pregnancy. Receptors for this hormone exist all over the bodies of both male and female mammals, and in the past several decades, researchers have shown it to be a potent catalyst for wound healing, promoting <a href="https://docplayer.net/74761056-Role-of-progesterone-in-peripheral-nerve-repair.html">nerve repair</a>, <a href="https://www.cell.com/developmental-cell/fulltext/S1534-5807(01)00064-8?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS1534580701000648%3Fshowall%3Dtrue">bone remodeling</a>, and <a href="https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1524-475X.2008.00440.x">blood vessel formation</a>. If applied in the right place at the right time, progesterone could very well coax an injured limb onto a regenerative cascade.“Progesterone is very useful and safe,” explains <a href="https://www.popcouncil.org/research/expert/regine-sitruk-ware">Régine Sitruk-Ware</a>, a reproductive endocrinologist at the Population Council’s Center for Biomedical Research who did not participate in the new research. “It’s helpful for the uterus, but in many other layers of the body—in vessels, in bone, in the brain, and in the [nervous system] in general—progesterone has a positive, regenerative aspect.”
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To harness the power of progesterone, the researchers designed a silicon apparatus filled with a hydrating gel and stitched it onto <a href="https://www.sciencedaily.com/releases/2016/10/161019155533.htm">African clawed frogs</a> (Xenopus laevis) whose right hind legs had been amputated three hours prior. Some of the devices also contained a dose of progesterone, which was steadily fed to the wound site for 24 hours. When the day was up, the team carefully detached the device—and waited.Within a matter of weeks, the progesterone-treated frogs had a serious leg up on their peers. In the absence of the hormone, frog amputees produced the typical featureless rods, regardless of whether or not they’d been fitted with the apparatus. But the frogs hit with progesterone began to sprout bigger, more structured appendages. And nearly ten months out from the initial spark of progesterone, partial, paddle-like limbs had pushed their way out of the wound site.When the researchers inspected the burgeoning limbs more closely, they found that, unlike the simple, cartilaginous spikes of typical amputees, they seemed to have the form and patterning of true legs. The limbs were thick and full of new blood vessels, nerves, and bone, and were forming what looked like tentative knees: the beginnings of actual joints. These frogs’ legs weren’t just regenerating more than their untreated counterparts—they were regenerating better. It’s a phenomenon that regenerative biologist <a href="http://firm.uni-frankfurt.de/john-barker">John Barker</a> of the Frankfurt Initiative for Regenerative Medicine praises as “smart proliferation”: the ability to form order out of the chaos of growing, dividing cells.
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<figcaption>Amputation of adult frogs' hind limbs usually results in the formation of a cartilaginous "spike" (left). But attaching a device that feeds progesterone to the amputation site for just 24 hours stimulates the growth of a paddle-like structure (right). The dashed line indicates the plane of amputation. Photo Credit: Celia Herrera-Rincon, Tufts University
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“It was amazing that we were creating structures with only 24 hours of treatment,” Herrera-Rincon says. “I remember showing Mike [Levin] and being fascinated.”When the frogs got the chance to stretch their legs, the untreated, spike-toting regenerates proved to be feeble swimmers and spent more time loitering in place. Meanwhile, the animals that had worn the progesterone-dosing device glided merrily by, almost indistinguishable from their unamputated kin.Though the legs could function, the regeneration was far from complete. While the limb that appeared had clear features, the growth stopped abruptly before the frog’s hind digits would otherwise appear. And, importantly, not all the treated frogs responded to the same extent, indicating that the approach is unlikely to be a one-size-fits-all treatment. But for a first pass, Levin says, the amount of regeneration achieved was “really striking.”“If the person grew a limb like the frogs did, no one would say it was perfect,” says <a href="https://www.whitedlab.com/">Jessica Whited</a>, a regenerative biologist at Harvard University’s Brigham Regenerative Medicine Center who frequently collaborates with Levin’s lab, but was not involved in the newest finding. “But it’s important to demonstrate something that would be better than what grows naturally.”
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Levin and his team are currently tinkering with their cocktail of apparatus-administered drugs, as well as with the duration for which the device is worn after amputation. With just a few additions and adjustments, he says, the preliminary results are already looking good.“To rebuild any structure, you need to give the cells some signal about what they’re rebuilding,” Levin explains. “And progesterone [alone] gives an incomplete message.”The rest of this message, Whited adds, will involve a lot of fine-tuning. When limbs grow (and regrow) naturally, they rely on a suite of complex anatomical signals that both drive the production of new tissue and bone and tell it when and where to stop. But the domino effect likely still applies: The key will be finding the right recipe of chemicals to jumpstart the process, then sit back and let nature take its course.
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<figcaption>Deer antlers are mammalian appendages capable of repeated rounds of regeneration. Male deer (and female reindeer) shed and sprout their antlers anew each year. Credit: USFWS Mountain-Prairie, flickr
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“This isn’t ready for human trials… but it’s also not science fiction,” Levin says of the device. “I don’t think humans are barred from regeneration in any fundamental way.”We’re not exactly a hop, skip, and a jump away from regenerating limbs in people—even partial ones. Though frogs and humans share quite a few genes and basic anatomical structures, Barker explains, we’re still fundamentally very different creatures. But, even if the packaging varies from species to species, the basic toolkit of regeneration might translate across even fairly far-flung species. There are even <a href="/wgbh/nova/video/extreme-animal-weapons/">mammals that naturally regenerate</a>: Deer regularly <a href="https://www.sciencedirect.com/science/article/pii/S135727251400226X">regrow their antlers</a> well into adulthood—a process that, not-so-coincidentally, is also driven by sex hormones.Even as technology advances, it’s unclear if humans will ever regenerate as slickly as salamanders. But maybe these able-bodied amphibians are inching us just a bit closer: One small leap for a frog may someday mean one giant step for humankind.

Twenty-four hours of the hormone progesterone can kickstart over nine months of limb regeneration in amphibian amputees.

The Cabbage tree emperor moth (Bunaea alcinoe) decorates its wings with tiny, sound-absorbing scales. By blocking echolocation, these scales lower the chances of the moth being pinpointed and eaten by a bat. Photo Credit: Bart Wursten, flickr

Moths Muffle Bat Sonar with Sound-Absorbing Wings

It’s not easy being deaf in the dark—especially when your greatest enemy is a master of sound.Such is the twilight plight of the humble cabbage tree emperor moth (Bunaea alcinoe): It’s all these umber underdogs can do to avoid becoming a hungry <a href="http://www.pbs.org/wnet/nature/group/mammals/bat/">bat’s</a> dinner. Bat sonar is so sensitive that these predators attain an impressive intimacy with their surroundings—one they manipulate to easily locate and devour the minute, wriggling bodies of their prey. But even moths that can’t hear their stalkers’ telltale screech aren’t as powerless as you might expect.Today, in the journal PNAS, researchers <a href="http://www.pnas.org/cgi/doi/10.1073/pnas.1810025115">report</a> that the wings of cabbage tree emperor moths are flecked with tiny scales capable of absorbing the sounds of echolocating bats, dampening reverberations that might give away their location. The evolution of this stealth-mode strategy may have helped moths evade detection by their nocturnal predators.
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Often dismissed as drab <a href="https://www.pbs.org/wgbh/nova/nature/journey-butterflies.html">butterfly</a> alternates or light-loving nuisances, moths might just be one of the most underappreciated members of the insect world. Like their butterfly relatives, moths are essential pollinators and sources of food, supporting diverse ecosystems worldwide. Even in the face of danger, the modest moth is mighty: In recent years, researchers have uncovered a dizzying array of anti-bat defenses in these little insects. Because bats rely heavily on sonar to navigate the world around them, much of the moth’s modus operandi is auditory. Some moths will produce their own ultrasonic clicks to <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4443353/">jam bats’ sonar</a> or <a href="https://link.springer.com/article/10.1007%2Fs00359-014-0924-0">vocally advertise their toxicity</a>, while others have adopted a strategy of constant vigilance by <a href="https://academic.oup.com/bioscience/article/51/7/570/268256">evolving the insect equivalent of ears</a>, which are sensitive to the audio of approaching bats.However, one group of oddballs exhibits none of the aforementioned tricks of the trade—and it was these miscellaneous moths that piqued the interest of <a href="http://www.bristol.ac.uk/biology/people/marc-w-holderied/">Marc Holderied</a>, a sensory ecologist at the University of Bristol in the United Kingdom.<a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0027190">Previous research</a> had hinted that these mysterious moths, while effectively “deaf” to bats, were literally flying under the radar: They absorbed the sounds of sonar using the <a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0027190">exquisite architecture</a> of tiny, leaf-shaped scales that decorate their <a href="https://www.jstor.org/stable/1539496?seq=1#page_scan_tab_contents">wings</a>. These scales might act like the auditory equivalent of an invisibility cloak, allowing moths to pass through bat territory neither seen nor heard. But no one had yet figured out exactly how these little scales were accomplishing such an acoustic feat.
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<figcaption>The Cape serotine (Neoromicia capensis) is a sub-Saharan species of bat that feasts on insects like moths, using echolocation to home in on its prey's location. Photo Credit: Bernard Dupont, flickr
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To puzzle out the mechanics of the muffling, Holderied and his colleagues, led by acoustician <a href="http://www.bristol.ac.uk/biology/people/zhiyuan-shen/overview.html">Zhiyuan Shen</a>, plucked individual scales from cabbage tree emperor moths and constructed 3D models of their structure to see how they would behave in the presence of sound waves.When bat sonar hits a typical object, some of the sound waves will ricochet off the surface and boomerang batward. This outcome is ideal for the bat, which can harvest the information contained in the echo and home in on its prey. All objects, however, have what’s called a resonant frequency, or a natural frequency at which they vibrate. (This is the phenomenon behind the common trope of opera singers shattering wine glasses—the sound hits the glass at its resonant frequency, causing the poor vessel to vibrate itself to pieces.) If incoming sonar meets its target at its resonant frequency, the energy of the sound wave can be converted into motion. This curbs the sound waves’ ability to reverberate and muddies the bat’s estimation of the vibrating object’s whereabouts.For this strategy to be viable for a moth in a bat’s crosshairs, the resonant frequency of its scales would need to overlap with the frequency of bat sonar, typically between 20 to 150 kilohertz. When the researchers analyzed their model scale, they found one resonant frequency. And then another. And then another. To Holderied’s amazement, an individual insect scale was capable of vibrating at not one, but three specific frequencies—28.4, 65.2, and 153.1 kilohertz—depending on how exactly the scale trembled in space.
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“They were perfectly spaced to cover the bat echolocation range,” Holderied explains. “At that moment, we know this wasn’t a coincidence: They’re there for a reason.”As a point of comparison, the researchers repeated their analysis with a modified scale resembling that of a butterfly, a close relative of the moth. Like moths, butterflies adorn their wings with scales—but because most butterflies are most active during the day, as a group, they probably have much less in the way of anti-bat defenses, Shen explains.Like the B. alcinoe model scale, the butterfly-like scale exhibited three resonant frequencies—but this time, they fell less cleanly within the relevant range of bat sonar, clocking in at 88.4, 150.9, and 406.0 kilohertz. It seemed that, under pressure from their bat predators, moths had embarked on this specific evolutionary trajectory alone.
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<figcaption>A single scale from the wing of a cabbage tree emperor moth (Bunaea alcinoe). Typical scales are less than 0.25 millimeters wide. Photo Credit: Marc W. Holderied and Zhiyuan Shen, University of Bristol
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But bats are not to be outdone. They wield a <a href="https://www.nature.com/articles/nature11664">high degree of control</a> <a href="http://www.pnas.org/content/114/41/10978">over the frequency</a> of their own sonar, meaning there’s a chance they could pinpoint gaps in the moth’s acoustic armor. However, not all moth scales are created equal, and every wing is dotted with units of different shapes and sizes, Holderied explains. With such diversity, B. alcinoe wings may stifle an even broader range of frequencies. It’s not yet clear exactly how much these moths compromise bat echolocation in a more natural context—but in the arms race between predator and prey, any amount of sound absorption could tip the scales in favor of Team Moth.Accordingly, one of the clearest next steps is to return to the field and see if the findings hold true for moths on the run from actual bats, says <a href="https://eeb.boisestate.edu/home/current-students/">Juliette Rubin</a>, a sensory ecologist at Boise State University who also studies anti-bat strategies in moths, but did not contribute to the newest finding. For instance, it will be important to test if sound-absorbing moths actually fall prey to bats less often than their not-so-covert cousins—or if shaving scales off B. alcinoe makes them more vulnerable to the hunt. After all, subtle sculpting of scales doesn’t guarantee that B. alcinoe can actually flummox an experienced, living predator. But if these wings are truly as functional as they seem, moths may just fly in the face of bats’ acoustic aptitude.“If the scales are really functioning to absorb sonar, this shows you the incredible adaptations that moths have made to bats,” says <a href="https://www.floridamuseum.ufl.edu/mcguire/kawahara/akito-kawahara/">Akito Kawahara</a>, a sensory ecologist at the Florida Museum of Natural History who did not participate in the research. “It really highlights the importance of bats in the nocturnal landscape and how much impact they have on insects.”
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The researchers are also currently investigating whether these scaly structures are present on other moths—especially those known to engage in other anti-bat antics. Doubling up on defenses could pay some serious dividends, but these adaptations often come with a cost. For instance, Holderied explains, soundproofing scales may have made moth wings heavier, potentially impeding flight.On the other hand, tinkering with the microstructure of a tiny scale is nothing compared to some of the evolutionary triumphs of some other moths—like growing a set of ears, explains biologist <a href="https://sites.google.com/site/cwrufoxlab/people">Jessica Fox</a>, who studies insect flight at Case Western Reserve University but was not involved in the new research.This is not to undersell the exquisite nature of these little scales. While humans continue to painstakingly construct more lightweight, effective resonant absorbers to soundproof buildings and conceal military operations, moths have executed a feat of engineering that’s eluded us for years. Evolution, it seems, operates on even the smallest of scales in the war waged between moth and bat.“Natural evolution and human engineering are converging on the same point,” Shen adds. “But nature is better than engineering.”

To shield themselves from bat echolocation, moths don an acoustic cloak of invisibility—using the sound-absorbing scales on their wings.

This unfortunate bull came afoul of a few stems of jumping cholla (Cylindropuntia fulgida). The spines of this cactus are decorated with backward-facing barbs that facilitate entry into flesh, but make extraction excruciating. Photo Credit: Lon&Queta, flickr

A Single Spine From This Cactus Can Lift a Half-Pound Slab of Pork

In the arid deserts of the Southwestern United States, there exists a humble-looking cactus by the name of <a href="https://plants.usda.gov/core/profile?symbol=cyfu10">Cylindropuntia fulgida</a>. With its stout stature and knobby tentacles, C. fulgida is a sight to behold—but best admired from afar.At the slightest provocation, the spiny stems of this cactus will detach and lodge themselves in whatever’s nearby, be it fur, clothes, or flesh—earning it the nickname “jumping cholla.” It’s actually a bit of an exaggeration: This plant is not going to physically leap out and attack you. But its trigger-happy strategy is not one to toy with unless you’re eager for a world of hurt.The prickliest part of the problem are the tiny, backward-facing barbs that decorate the tips of jumping cholla spines. These barbs not only enable the spines to impale flesh, but also make extraction excruciating. According to <a href="http://dx.doi.org/10.1098/rspb.2018.2280">a new study</a>, once anchored, the barbs cling so tightly that a single inch-long spine can lift a half-pound slab of pork.
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“This work really brought home for us how tenacious these spines are,” says study author <a href="https://croftss.wordpress.com/">Stephanie Crofts</a>, a biomechanist at the University of Illinois at Urbana-Champaign. “It blew us away.”Thousands of species of cacti exist, spanning all sorts of shapes, sizes, and statures; even their spines (which are actually <a href="https://www.britannica.com/plant/cactus">a type of leaf</a>) are no stranger to diversity. Some spines serve as a blatant deterrent to hungry herbivores, but others, like those from cacti in the Opuntioideae group, which include both the prickly pear and jumping cholla, are thought to play a different role. They help the plant latch onto hapless passersby, giving them a shot at dispersing into new territory like the sharpest of stowaways. To up their staying power, these spines bear ornaments unlike anything you’d ever want to adorn your Christmas tree: microscopic barbs that enable the needly structures to stick and stay stuck. But until now, no one has quantified the difference these embellishments really make.That’s where Crofts and her advisor, <a href="https://www.philipslanderson.com/">Philip Anderson</a>, an evolutionary biomechanist at the University of Illinois at Urbana-Champaign, come in. Crofts’ work focuses on the power of <a href="http://jeb.biologists.org/content/221/22/jeb187294">puncture</a>—work that entails a lot of jabbing, piercing, and perforating. With their naturally prickly arsenal of weaponry, cacti stood out to Crofts as a natural subject for her research.
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<figcaption>Despite its name, the jumping cholla won’t actually leap out and attack you. But brush up against one, a segment of this cactus will happily bury itself in your clothes, hair, or flesh. Good luck getting it out. Photo Credit: L. Brian Stauffer, University of Illinois at Urbana-Champaign
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In Crofts’ and Anderson’s new work, published today in the journal Proceedings of the Royal Society B, they collected five spines each from six species of cacti. Three of them—Pereskia grandifolia, or rose cactus; Echinopsis terscheckii, or Argentine saguaro; and Echinocactus grusonii, or golden barrel cactus—bore unbarbed spines, while the others—Opuntia fragilis, or brittle prickly pear; Opuntia polyacantha, or plains prickly pear; and, of course, Cylindropuntia fulgida, or jumping cholla—had barbed spines.Cactus cuttings encased in newspaper clippings, were shipped to the University of Illinois by the Huntington Botanical Gardens in California. Crofts bravely unwrapped the contents by hand and clipped the spines from each specimen. “It was an exciting few days when we had loose cacti rolling around,” Crofts recalls. “I definitely ended up having to use the tweezers on my Swiss Army knife.” (Crofts graciously thanks the makers of the multitool in the acknowledgments of her publication.)To test the puncturing prowess of each set of spines, Crofts next selected her victims: skinless chicken breasts; raw, skin-on pork shoulders; and a silicon-based polymer. Chicken breast is soft, relatively unprotected muscle—something that might easily mimic the inside of a hungry animal’s mouth, Crofts explains. Pig tissue, on the other hand, would have a similar texture and toughness of human flesh (if anything, pig’s a bit thicker). Lastly, the polymer has an amorphous, elastic texture, and is devoid of the collagen and muscle fibers present in animal flesh. 
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When Crofts got to stabbing, she found that barbed spines were easier to embed into both animal tissue and the polymer. It seems a bit counterintuitive at first, Crofts explains, but the barbs are actually a huge asset to the spines’ otherwise needly structure: Each concentrates pressure onto a small area, reducing the amount of force necessary to grind through its unfortunate target—much like slicing with a serrated knife.Extricating the spines, however, was quite a different story. After barbs deploy within tissue, withdrawal goes against the grain, making the spines a nightmare to extract. And the more fibrous the tissue, the more there is for the barbs to snag on. Because of this, removal of barbed spines from the jello-y polymer made for smooth sailing, but the chicken and pig weren’t quite so lucky.The pork shoulder in particular proved to be astoundingly stubborn. In one trial, Crofts found that a lone, inch-long spine snipped from a jumping cholla was enough to hoist a half-pound cut of pig into the air. Only a couple millimeters of the spine had penetrated the tissue—just enough to clear the layer of skin—but there the pork dangled, as if completely weightless.
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<figcaption>A one-inch spine from a jumping cholla was able to hoist a half-pound slab of pork shoulder into the air by its skin. The researchers had to tug the meat off the spine to free it. Photo Credit: L. Brian Stauffer, University of Illinois at Urbana-Champaign
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“It was this big old chunk of meat just hanging there, a suspended chunk of pig,” Crofts says. “We physically had to pull the pig off the spine.”And after jumping cholla spines like these were wrested from their porcine prisons, Crofts discovered the ordeal had stripped a handful of barbs from their tips. They didn’t penetrate so well after that.Of course, cacti are far from the only plants prone to puncture—or even the only organisms. Throughout their work, the researchers were struck by the resemblance of barbed cactus spines to an unexpected kindred spirit: <a href="http://www.pnas.org/content/109/52/21289">North American porcupine quills</a>. Six years ago, a <a href="http://www.pnas.org/content/109/52/21289">study</a> led by biomedical engineer <a href="http://www.karplab.net/">Jeffrey Karp</a> of Harvard Medical School conducted a similar series of experiments on these quills; when Crofts compared the two sets of results side by side, she was astounded by what she refers to as a “weird cross-kingdom convergence.” Though porcupine quills are much bigger and primarily serve as a defense mechanism, both structures penetrate flesh with ease and stay firmly put—a dangerous double whammy of entry and retention.
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“This is really exciting,” says <a href="http://www.marthamunoz.com/">Martha Muñoz</a>, an evolutionary biologist at Virginia Polytechnic Institute and State University who didn’t participate in the new finding. “Here, we see similarities in the puncture properties in organisms as different as cacti and porcupines. It’s hard to get more distant than that.”However, Crofts and Anderson’s work didn’t directly compare the two structures—something that would probably illuminate the similarities and differences between them more clearly, points out <a href="https://juliemessier.org/">Julie Messier</a>, a plant ecologist at the University of Waterloo who was not involved in the new research. Still, Karp says it’s “wonderful” that this new work has hinted that the benefits of barbs transcend the branches of the tree of life.In fact, shortly after Karp’s group published their initial findings on porcupines, he and his son Joshua also took note of the spiky resemblance between porcupine and cactus—so much so that the younger Karp, then 10 years old, pitted the two head-to-head in his next science fair project, entitled, “The Secret of the Barb: Porcupine Quills vs. Cactus Needles.” In one experiment, Joshua drove quills and spines into raw chicken to see how much flesh the barbs retained upon withdrawal; in another, he pitted the structures tip-to-tip and made them duel to the death.
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<figcaption>North American porcupine quills (top) and jumping cholla cactus spines (bottom) are covered in overlapping, backwards-facing barbs that serve two purposes: aiding penetration into flesh and making extraction extremely difficult. While porcupine quills are much bigger and serve primarily as a mode of defense, there are surprising similarities in structure between the two. White bars represent 100 micrometers in each image. Photo Credit: Jeffrey Karp, Harvard Medical School (top); Stephanie Crofts, University of Illinois at Urbana-Champaign (bottom)
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The results were mixed: The porcupine quill took a bigger chunk out of the chicken—but a cactus spine conclusively skewered a porcupine quill. For now, the jury’s still out on the victor of this jabby joust.Still, one thing’s for sure, according to Crofts: “I do not ever want to brush up against a jumping cholla,” she says. “They do what they do very well. But I don’t want to be what they’re puncturing.”

The spines of the jumping cholla cactus slide in with ease, but they’re grueling to extract. You can thank their barbs.

A new study shows that, in contrast to a longstanding rule in human biology, mitochondrial DNA can be inherited from fathers as well as mothers. Photo Credit: seal1837, Pixabay

Not your mom’s genes: Mitochondrial DNA can come from Dad

The first time <a href="https://www.cincinnatichildrens.org/bio/h/taosheng-huang">Taosheng Huang</a> saw the test results, he was sure there’d been a mistake. Even after a technician repeated the diagnostic, Huang didn’t believe it. “That’s impossible,” he said.Huang, a pediatrician and geneticist at <a href="https://www.cincinnatichildrens.org/">Cincinnati Children’s Hospital Medical Center</a>, asked the patient to come back and provide fresh samples of blood, which Huang then split among several research labs to reduce any chance of error. It was a finding that Huang knew would break a central tenet of human genetics—but time and time again, the outcome was the same.Huang’s patient, a four-year-old boy, was carrying two different sets of <a href="https://www.pbs.org/wgbh/nova/article/hybrid-dolphin-gives-scientists-rare-window-into-evolution/">mitochondrial DNA</a>: one from his mother, as expected—and another, from his father.This was only the beginning. Using modern DNA sequencing technology, Huang and his colleagues have conclusively verified paternally-inherited mitochondrial DNA in 17 individuals spanning three unrelated families. Their <a href="http://www.pnas.org/cgi/doi/10.1073/pnas.1810946115">work</a> appears today in the journal PNAS.“This is a really groundbreaking discovery,” says <a href="http://med.stanford.edu/xinnanwanglab.html">Xinnan Wang</a>, a biologist at Stanford University who studies mitochondria but was not involved in the new findings. “It could open up an entirely new field... and change how we look for the cause of [certain] diseases.”
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Broadly speaking, we’re each a genetic mix of Mom and Dad. In the nucleus, which contains our chromosomes, this holds true. But the nucleus isn’t the only part of the cell that contains DNA. Cells contain power centers called mitochondria that also carry their own sets of DNA—and in nearly all known animals, mitochondrial DNA is <a href="https://www.sciencemag.org/news/2004/05/mitochondrial-dna-mixes-it">inherited exclusively</a> from the mother. This lopsided acquisition is so ingrained that researchers often analyze mitochondrial DNA to trace maternal lineages back in time.Scientists still aren’t completely sure why this process has evolved to be strictly matrilineal, but a few theories have come to the forefront. For one, the mitochondria of sperm are believed to experience <a href="http://www.nrcresearchpress.com/doi/10.1139/gen-2015-0090?url_ver=Z39.88-2003&amp;rfr_id=ori:rid:crossref.org&amp;rfr_dat=cr_pub%3dwww.ncbi.nlm.nih.gov#.W_iwCZNKiLJ">higher rates of mutation</a> than those in eggs, making their input somewhat riskier. Additionally, having only one type of mitochondria makes it easier for the genomes in the nucleus and mitochondria to <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6037425/">coordinate</a>, as both generate the raw materials necessary for proper cellular function, explains <a href="https://bio.umontreal.ca/repertoire-departement/vue/breton-sophie-1/">Sophie Breton</a>, a mitochondrial geneticist at the University of Montreal who did not participate in the study. The addition of another mitochondrial stakeholder, so the theory goes, could muddle this intimate two-way dialogue.
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<figcaption>Cow cells with nuclei stained in blue and mitochondria stained in yellow. Photo Credit: Torsten Wittmann, University of California, San Francisco
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Whatever is driving maternal mitochondrial inheritance, it’s clear our cells have taken quite a few precautions to keep it that way. A formidable bevy of molecular machines stands at the ready to ensure that no unwanted paternal contribution slips through in the process of fertilization. Even though a sperm’s mitochondria help power its odyssey to the egg, its mitochondria aren’t meant to last. Some mitochondria are thought to be lost as sperm develop; the few that make it through the fertilization process are tagged with a chemical marker that allows the egg to <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5282644/">easily recognize and annihilate</a> any rogue paternal interlopers.<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5282644/">A handful of animals</a>, including flies and mice, are known to occasionally <a href="http://www.nrcresearchpress.com/doi/10.1139/gen-2015-0090?url_ver=Z39.88-2003&amp;rfr_id=ori:rid:crossref.org&amp;rfr_dat=cr_pub%3dwww.ncbi.nlm.nih.gov#.W_iwCZNKiLJ">“leak” paternal mitochondrial DNA</a> into offspring alongside maternal input, but humans have historically been excluded from this club. Only a few reports of dad-derived mitochondrial DNA exist, and most have been chalked up to contamination or laboratory sample mix-ups. <a href="https://www.nejm.org/doi/10.1056/NEJMoa020350?url_ver=Z39.88-2003&amp;rfr_id=ori%3Arid%3Acrossref.org&amp;rfr_dat=cr_pub%3Dwww.ncbi.nlm.nih.gov">One case</a>, described in 2002, received considerable attention when it was independently confirmed by another research group. But in the 16 years since, the search for another instance of this genetic anomaly has been unfruitful.
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So even after Huang was convinced that his remarkable four-year-old patient had inherited mitochondrial DNA from both parents, he knew the work was far from done. When the researchers next traced this odd genetic signature back through the boy’s family tree, they found that, across three generations, 10 individuals in the patient’s family appeared to harbor mixed mitochondrial DNA. Now that the researchers knew where to look—and what to look for—as they expanded their search. Not long after, seven additional individuals from two other unrelated families were confirmed to harbor the same condition.Amazingly, all three families seemed to show similar patterns of inheritance of this unusual trait. Not all members were afflicted, meaning that some individuals passed on their genes in the typical fashion, conceiving children that inherited only their mothers’ mitochondrial DNA. But it was clear that the mitochondrial genomes of several men in these families were breaking the biological rules of fatherhood. Rather than being discarded, these paternal packages of genetic information somehow kept pace with their maternal counterparts during fertilization, leaving some of their kids with mixed mitochondria.Daughters with these mixed genomes then went on bequeath their heterogeneous heirlooms onto their own children in carbon copy. This meant their kids inherited mixtures indirectly—even though the children possessed none of their own father’s mitochondrial DNA. Such was the case with Huang’s original four-year-old patient.
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<figcaption>Sheep are one of the few animals in which mitochondrial DNA can occasionally be inherited from both the mother and the father. Photo Credit: herbert2512, Pixabay
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Sons who acquired mixed mitochondrial genomes presented a slightly more complex picture. The ability to pass on paternal DNA seemed to be a dominant trait, or one that requires a gene from only one parent to show itself in offspring. This meant that some of the men who had inherited hybrid mitochondrial genomes were able to transfer them into their children, who displayed even more diversity as they mixed Dad’s genes with Mom’s. But not all men with mixtures retained this ability, leading to occasional dead-ends.Exactly how paternal mitochondrial DNA infiltrates the embryo still isn’t entirely clear. It’s definitely a genetic trait—but not one, ironically, found in the mitochondrial genome itself. In fact, apart from the unusual mode of inheritance, there don’t seem to be any defects present in any of these patients’ mitochondria. Instead, whatever allows dads to deposit their mitochondrial DNA in their offspring is likely a mutation encoded in the nucleus—where things are supposed to come from both mom and dad.<a href="https://icahn.mssm.edu/profiles/florence-marlow">Florence Marlow</a>, a developmental biologist at the Icahn School of Medicine at Mount Sinai who was not involved in the research, theorizes that the anomaly could be at the step where the mitochondria of sperm are normally tagged for destruction. If this labeling step never occurs, she explains, the fertilized egg has no way of identifying and eliminating these paternal interlopers—meaning Dad’s mitochondria get a clear pass to proceed alongside Mom’s.
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Since putting together their initial findings, Huang and his team have already identified several more candidate families with mixed mitochondrial DNA. According to his preliminary estimates, this phenomenon may be present in as many as 1 in every 5,000 people. While Wang stresses that more research by other groups is needed to confirm these numbers, the work from Huang’s team indicates that paternal transmission of mitochondria may be far more widespread than researchers once thought.So how has this been overlooked for so long? “Most people have taken for granted that mitochondrial inheritance is strictly maternal,” Breton explains. “But sequencing techniques are so much more powerful than they were even just a few years ago.”
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<figcaption>Cells taken from the connective tissue of a mouse. The nuclei of the cell are stained in blue and the mitochondria are stained in green. Photo Credit: D. Burnette, J. Lippincott-Schwartz/NICHD
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The possibility of paternal inheritance could also expand our options for assisted reproductive technology in the distant future. Huang’s group previously played a part in the successful conception and delivery of a <a href="https://www.rbmojournal.com/article/S1472-6483(17)30041-X/fulltext">“three-parent” baby</a> in 2016. The child’s mother suffered from a mitochondrial disease, so Huang and his colleagues transferred her nuclear DNA into a donor egg, which had been stripped of its own chromosomes but could supply healthy mitochondria. This hybrid egg, containing the genetic information from two mothers, was then fertilized with the father’s sperm.In theory, Breton says, if paternal mitochondria are viable, they could be added to—or perhaps even replace—defective maternal mitochondria, obviating the need for a third parent.But that’s a big if, especially considering that the mitochondrial DNA of sperm is more prone to mutation. We’re far from understanding the full repercussions of inheriting bi-parental mitochondria under natural circumstances, much less through artificial medical procedures, many of which are likely to fuel <a href="https://www.npr.org/sections/health-shots/2018/06/06/615909572/inside-the-ukrainian-clinic-making-3-parent-babies-for-women-who-are-infertile">continued debates on ethics</a>.For now, maternal inheritance of mitochondrial DNA is still the norm. But simply expanding our view of genetic inheritance opens up countless doors. “These results will change the way we describe mitochondrial inheritance,” Breton says. “I’m 100 percent sure we’ll find more cases like these in the future.”

A new study provides compelling evidence that children can inherit mitochondrial DNA from both their parents.

Humpbacks and many other whales use baleen—the overlapping structures in their mouths where teeth might otherwise be—to filter tiny prey like krill and plankton out of water. But because baleen has the texture of hair or fingernails, it doesn't preserve well in the fossil record. New research now indicates that some whales may have fed by suction in the evolutionary interim between teeth and baleen. Photo Credit: Ari S. Friedlaender/University of California, Santa Cruz under NOAA permit

This 33-Million-Year-Old Whale Sucked Up Food Like A Giant Vacuum

It takes a lot to feed a <a href="http://www.pbs.org/big-blue-live/animals/blue-whale/">blue whale</a>. One of these super-sized swimmers can consume as much as 8,000 pounds of food—about the weight of a Hummer H2—on a daily basis. Which means blue whales need to be exquisitely equipped for some serious eating.The secret to this behemoth’s success? Baleen: a hyper-efficient filter-feeding system found exclusively in whales, composed of a comb-like series of overlapping plates with the texture of fingernails. After taking a massive gulp of ocean water, <a href="http://www.pbs.org/wnet/nature/ocean-giants-whale-fact-sheet/7567/">baleen whales</a>—including blue whales and <a href="https://www.pbs.org/video/life-reef-humpback-whales/">humpbacks</a>—expel unwanted liquid through this sieve, leaving behind a nutrient-dense mouthful of tiny, freshly-strained prey like krill and plankton that are easily swallowed whole.But scientists continue to grapple with the evolutionary origins of nature’s biggest oral colander. Baleen whales’ ancestors had teeth—and many of the details behind the transition to baleen, which preserves poorly in the fossil record, remain mysterious.Now, with the re-examination of a 33-million-year-old fossil, researchers are filling in a piece of this long-standing puzzle. According to <a href="https://www.cell.com/current-biology/fulltext/S0960-9822(18)31414-3">a new study</a> published today in the journal Current Biology, some whales jettisoned their teeth entirely before the acquisition of baleen, and in the interim, adopted a third method of feeding: suctioning up their food like giant vacuum cleaners.
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“If this is right, this fossil fills in a missing spot on the [whale family] tree,” says <a href="https://www.uprovidence.edu/bio/jessica-martin/">Jessica Martin</a>, a paleontologist at the University of Providence who was not involved in the research. “This might change the way we look at the big picture of this transition.”The fossil in question, newly anointed Maiabalaena nesbittae, was first unearthed in Oregon in the 1970s, but spent decades in the collections of the Smithsonian’s National Museum of Natural History still shrouded in rock. “This fossil’s been languishing for years,” says study author <a href="https://paleobiology.si.edu/staff/individuals/pyenson.html">Nicholas Pyenson</a>, the Museum’s curator of fossil marine mammals. “It’s amazing to finally be able to weigh in on it.”At about 33 million years old, M. nesbittae likely swam the seas of the early Oligocene—a period of rapid and vast geologic change. It was around this time that the southern tip of what’s now South America broke free of Antarctica, rejiggering ocean currents and cooling marine temperatures worldwide. Though it’s still unclear exactly how this transition affected ocean biodiversity, if there was a paleontological moment for a lineage of whales to ditch its dentition, it very well could have been this.
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<figcaption>The upper jaw and skull of Maiabalaena nesbittae, an extinct whale species that lived 33 million years ago. M. nesbittae was toothless, and may have been a suction feeder that gave rise to modern baleen whales. Photo Credit: Smithsonian Institution
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Which makes M. nesbittae a potential watershed whale. Only one precious specimen of the species has been uncovered to date, and the fossil is incomplete. Luckily, most of what’s left involves the skull—which is key to studying how this species managed its massive, yet often microscopic, meals. To interrogate this fossil’s feeding forte, a team of scientists led by Carlos Peredo, a palaeobiologist at George Mason University, decided to reopen the 40-year-old case on M. nesbittae.One thing was quickly obvious to Peredo. “I looked for teeth, and it had no teeth,” he explains with a laugh. “That was the easy one.” Teeth, unlike baleen, are hardened and encased in enamel, and thus far more likely to preserve as fossils. Even if its chompers had somehow been lost to time, the roof of M. nesbittae’s mouth also appeared to be structurally inhospitable to sockets that might otherwise house true dentition. This made it unlikely that this whale was munching on large-bodied prey, à la its toothier ancestors.
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Next, the team used CT scanning to more closely analyze the skull’s internal architecture without compromising the fossil. As there was little point in searching for ephemeral baleen, Peredo and his colleagues instead searched for attachment sites from which the baleen might have been suspended like vertical window blinds. They discovered the upper jaw was thin and narrow, without enough surface area to accommodate the gums from which baleen plates might grow. To the researchers, this was a clear strike against filter feeding.The researchers believe M. nesbittae utilized another option entirely on the road to baleen: simply slurping prey out of the surrounding waters through the sheer force of suction. Although the fossil’s facial muscles were long gone, the bones of its throat indicated that the whale was capable of retracting its tongue, creating negative pressure that would have drawn in water and small creatures from the surrounding environment. The toothless whale would have had no means to chew its prey, simply swallowing them whole. But the switch to suction was likely a move that would have averted a significant cost: Teeth are energetically expensive to produce. And it was on this blank canvas, the authors propose, that baleen eventually took shape a few million years later.
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<figcaption>An artist's imagining of a Maiabalaena nesbittae calf nursing from its mother. Artist Credit: Alex Boersma
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As modern humans can attest, teeth come in handy. They allow creatures to both latch onto their prey and grind them down to swallowable portions, significantly increasing the size and breadth of an animal’s viable menu items. Suction feeding, on the other hand, doesn’t pulverize prey before it goes down the gullet, and favors smaller, softer-bodied meals. But suction feeding’s lack of oral ornamentation means it’s a relatively cheap strategy. Even toothy mammals like humans are no stranger to its efficiency: Suckling is the only way babies can eat before their teeth come in.Some scientists, however, are a little more hesitant to embrace the idea that M. nesbittae was a suction feeder. “I’m not convinced it didn’t have teeth,” says <a href="https://www.rachelracicot.org/">Rachel Racicot</a>, a paleobiologist who studies marine mammals at the Keck Science Center. Critical pieces of the fossil’s jaw are fairly worn or incomplete, she points out, muddying the conclusions that can be drawn about the presence or absence of dentition. Until more fossils from this time period reveal themselves, suction feeding will be difficult to confirm, Racicot adds. Alternatively, evidence of <a href="https://www.frontiersin.org/articles/10.3389/fmars.2017.00067/full">a more stepwise transition</a> from teeth to baleen may exist—which could mean there were once intermediate whale species that harbored both in their mouths. The two theories on this interim aren’t necessarily mutually exclusive.
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Regardless of how these whales fed in the years after they lost their teeth, the ultimate advent of baleen was revolutionary. According to Peredo, “it’s like growing fingernails out of the roof of your mouth.” There’s nothing else quite like baleen in the animal kingdom, and we probably wouldn’t have the sea’s biggest creatures without it. By dispensing of the sea water “filler” in each gulp, filter-feeding ensures that baleen whales can engulf gargantuan amounts of food in one go: A blue whale can take up <a href="http://blogs.discovermagazine.com/notrocketscience/2010/12/09/blue-whales-can-eat-half-a-million-calories-in-a-single-mouthful/">half a million calories</a> in a single mouthful.“If you drink coffee, you’re taking advantage of filter feeding,” Pyenson explains. “It’s just separating fluid out from a substance.” Though in the case of whales, he adds, they’re hungry for the grinds, not the liquid coffee, so to speak. A good filter means that especially tiny forms of prey—which don’t require chewing—can comprise a viable meal.
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<figcaption>Modern baleen whales, like humpbacks, employ a filter-feeding strategy that enormously ups the nutritional value of each mouthful compared to suction feeding. This has enabled many whale species to grow to gargantuan sizes. Photo Credit: Ari S. Friedlaender/University of California, Santa Cruz under NOAA permit
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Chomping and suction feeding still have their perks, though. All three feeding strategies exist in modern whales, helping many species comfortably share the seas. And adopting one tactic doesn’t preclude a whale from utilizing another. While some toothed whales do chew on their prey, many like orcas or beluga whales simply use their teeth as graspers before swallowing food whole. Others like the beaked whale or narwhal have only rudimentary tooth structures, and must rely on suction to more or less inhale their prey. Though gray whales are equipped with baleen, they happily suction feed as well.This sort of flexibility may become increasingly important as the planet continues to change under the influence of human intervention. “Maiabalaena is like its own little case study,” Peredo says. “The oceans changed dramatically, and this is what happened—whales changed what they ate, the physical structures of their mouths.”But whales aren’t infinitely adaptable; even baleen took millions of years to come into being. It remains unclear how sea life will fare in an age of <a href="https://www.pbs.org/wgbh/nova/article/climate-change-plants-high-temperature/">rising temperatures</a> and increasingly pervasive <a href="https://www.pbs.org/wgbh/nova/article/ships-noises-humpback-whale-song/">acoustic pollution</a>. For now, researchers are keeping an eye on the past. The search for baleen’s origins may yet reveal the climactic changes that triggered its rise—and how whales weathered the storm with their mysterious shuttered smiles.

After discarding their teeth, the ancestors of today’s filter-feeding whales may have fed by suction before evolving baleen.

The size and shape of rodent sperm varies drastically across the many species that inhabit South American forests. Photo Credit: Júlio Vilela, The Field Museum of Natural History

What Rodent Sperm Can Teach Us About Biodiversity

In the summer of 2008, waylaid on a trip to Buenos Aires, <a href="https://www.researchgate.net/profile/Noe_De_La_Sancha">Noé de la Sancha</a> found himself face-to-face for the first time with his future in-laws, struggling to explain the hundreds of pickled testicles slung over his shoulder.But de la Sancha, a biologist at the Field Museum of Natural History and Chicago State University, had good reason for toting around such precious cargo: The package was for a friend (really, it was). Awaiting him at the end of his journey was a team of researchers at the University of Buenos Aires, ready to extract hordes of formalin-preserved sperm of all shapes and sizes from the rodent reproductive tracts that de la Sancha had collected during his field studies.The team’s work, <a href="https://doi.org/10.1093/jmammal/gyy146">published today</a> in the Journal of Mammalogy, showcases the surprising extent of diversity in South American rodents. In some cases, the physical characteristics of sperm might even help researchers distinguish between the many closely-related species that share this rich landscape.
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From the meek house mouse to the bizarrely behemoth capybara, the breadth of rodents in our modern world is vast: About <a href="http://www.ucmp.berkeley.edu/mammal/rodentia/rodentia.html">1,500 species</a> scurry the globe. Some, like chipmunks or voles, are capable of carrying deadly diseases that can be passed to people; others, like <a href="https://www.iucnredlist.org/species/4651/22191157">the uber-fluffy chinchilla</a>, have been driven to the brink of extinction by human hunters. Despite this diversity, it’s sometimes impossible to tell these cuddly critters apart by looks alone—especially when related species scamper around in close proximity. So de la Sancha and his colleagues, led by biologist <a href="https://www.researchgate.net/scientific-contributions/58645939_Luis_F_Rossi">Luis Rossi</a> of the University of Buenos Aires, decided to go microscopic.“Sperm are some of the most biologically diverse cell types in nature,” explains <a href="http://biology.umd.edu/heidi-fisher.html">Heidi Fisher</a>, who studies the sexual evolution of rodents at the University of Maryland, College Park but did not contribute to the new research. Eyeballing sperm might just help researchers pick apart subtle differences between species.To test this possibility, de la Sancha, Rossi, and their colleagues sampled the sperm of 58 individual rodents spanning 18 different species native to South America. All the specimens had been chemically preserved during de la Sancha’s travels, enabling the researchers to extract the contents of the rodents’ epididymides—the long, highly convoluted tubes that cradle the testes.
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<figcaption>These taxidermied rodents showcase several of the species that romp the forests of South America. While many of these rodents are only distantly related, they look very similar, making species distinctions tough in the field. Photo Credit: Field Museum of Natural History
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When the researchers peered at their pickled prizes under the microscope, they were surprised to find just how much sperm differed in physique. Sperm are sculpted a bit like balloons: Each has a head, a slight neck, a midpiece, and a tail. The head contains the sperm’s genetic payload—what will ultimately combine with the DNA of the female egg—while the midpiece is chock full of energy-generating machinery to power the whiplike movements of the sperm’s tail. But beyond these general motifs, the similarities end.Two of the species surveyed—<a href="https://www.iucnredlist.org/species/3613/115065791">Calomys laucha</a>, or the small vesper mouse, and <a href="https://www.iucnredlist.org/species/750/22333428">Thaptomys nigrita</a>, or the blackish grass mouse—appeared to produce sperm with oval heads, not unlike the ones found on human variants. The other 16 species, however, harbored sperm with “hooked” heads, whose tops resembled the curved peaks of meringues; of these, some were rounded at the junction with the neck, while others were more square-jawed. Additionally, some species’ sperm swam with highly elongated midpieces and tails, while others were substantially more compact.
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Unexpectedly, more closely related species did not tend to have more similar looking sperm. Divergences in sperm size and shape could help researchers quickly and efficiently sort rodents in the future, de la Sancha says—especially in cases where rodent populations look alike and occupy overlapping habitats. Many researchers rely on DNA sequencing, which has gotten faster and cheaper in recent years, to sort species, but genetic information hasn’t been banked for all rodents. Even when the sequence of a genome is known, the boundaries between populations often blur. In these cases, starkly divergent sperm could offer some identifying clues.“There isn’t one magic bullet for distinguishing species,” de la Sancha says. “To describe a new species, you have to use as many types of evidence as you can.”In fact, there’s precedent for the utility of scrutinizing rodent sperm for exactly this purpose, says <a href="https://www.adelaide.edu.au/directory/bill.breed">William Breed</a>, a reproductive biologist at the University of Adelaide who has dedicated his career to the study of sperm in Australia’s rodents and marsupials. Breed’s previous work has identified at least <a href="https://drmc.library.adelaide.edu.au/dspace/handle/2440/5329">two</a> <a href="https://www.researchgate.net/publication/230191076_Spermatozoa_of_murid_rodents_from_Africa_morphological_diversity_and_evolutionary_trends">cases</a> in which very different forms of sperm appear in males supposedly of the same mouse or rat species, hinting at the possibility that some subpopulations have begun to drift apart genetically.
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/Figure_2.width-990_7UB2iR6.jpg"/>
<figcaption>The heads of sperm of vesper mice in the Calomys genus come in different shapes. The tops of some are hooked, while others are oval; at the base, some are polygonal, while others are entirely round. Credit: Rossi et al. 2018
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But the system is far from perfect. For one thing, sperm are finicky, and there can be quite a bit of variation in how they develop. Within a single species—or even a single individual—not all sperm are identical. Sperm are churned out en masse throughout a male’s lifetime, and suffer high rates of DNA mutation. These sensitive swimmers are also prone to shapeshifting in response to changing environments, which can introduce stress or toxins, Fisher explains. Little tweaks in sperm anatomy can even occur over time as an individual ages, or when he comes into heat during the mating season, Breed adds.With such caveats in mind, Breed says, researchers leading future studies will need to be conscious of changing conditions to get an accurate picture of these eccentric ejaculates. What’s more, several of the species surveyed in the current study were represented by only one individual—and though the researchers collected at least 50 sperm from each species, the build of one rodent’s sperm isn’t necessarily representative of his brethren.
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Regardless of its utility for species identification, this study highlights the incredible range of sperm shapes and sizes—even within a single group of rodents, says <a href="https://hoekstra.oeb.harvard.edu/">Hopi Hoekstra</a>, an evolutionary biologist at Harvard University who did not participate in the research. And in the future, this kind of diversity could speak volumes about these and other understudied species.Sperm <a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.531.1032&amp;rep=rep1&amp;type=pdf">vary immensely</a> <a href="https://www.sciencedirect.com/science/article/pii/B9780123725684000033">across the tree of life</a>. Fruit flies, for instance, can produce sperm cells that run <a href="https://www.washingtonpost.com/news/speaking-of-science/wp/2016/05/25/fruit-fly-sperm-is-monstrously-long-and-we-might-finally-know-why/?utm_term=.4e307bb8d9b2">about 20 times the length</a> of their own bodies. But rodents in particular—which, by species count, make up about 40 percent of mammals—seem to <a href="https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.1992.tb01193.x">really run the gamut</a> of this manly morphology.And the heterogeneity isn’t just for kicks. Sperm architecture, for instance, might be intricately linked to sexual behavior. Sperm with <a href="https://hoekstra.oeb.harvard.edu/files/hoekstra/files/fisheretal2016.pdf">longer midpieces</a>, which supply the power necessary to weather the labyrinthine journey to the egg, tend to swim faster, and are often found in rodent species with more promiscuous males.
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/DSC06686-JFV191_Akodon_montensis_photo_by__Jul.width-990_XIxpsEl.jpg"/>
<figcaption>This montane grass mouse (Akodon montensis) has sperm with a pyriform head shape: The tops look like the tips of meringues, and the base is rounded.
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Hooked heads—which are <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/j.1420-9101.2009.01867.x">especially common</a> in rodents—have also been the source of much study, though the impetus behind their clawed appearance remains mostly mysterious. Some scientists theorize that hooks could come in handy as sperm snake eggward, acting as anchors that allows these stalwart swimmers to <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/j.1420-9101.2009.01867.x">lash themselves</a> to the sides of the <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/jeb.12857">female reproductive tract</a>. Especially long and curvy hooks might also help sperm <a href="https://watermark.silverchair.com/biolreprod0025.pdf?token=AQECAHi208BE49Ooan9kkhW_Ercy7Dm3ZL_9Cf3qfKAc485ysgAAAmEwggJdBgkqhkiG9w0BBwagggJOMIICSgIBADCCAkMGCSqGSIb3DQEHATAeBglghkgBZQMEAS4wEQQMVuX8B5rk1RbpZfG8AgEQgIICFOocqyjcAA1S-Ui5GZfqK4mAUMiZF9_nHeTTuMXi6E8BvaMDtdd9KLrRc0Bsw2fAPdZ-7g4e8Y88frZX6WbvB8JZddQ3nQbWIXVhjav_9-U8c91Fl_RFRuYlpP3AN0DpjUNKZ4-Uyayy1CDXS2-_pmV7RBqm7-_wDFLN3vstVrPJBuR2YQa9nZFGHaUnboW0gH2K9YhKifbU0SojbFo7gnjSLWu_idYyCR8gs2VnnkgrHhnYwWcNLOZTrvOylHTVhxUoUu9o1_WGh1O1-1M_j4WkogR0Qq64nDxTTy7hEmIT16eCYgm7iAprCmXHTElSUWQZYYhwSRdqVIC5RMxSixNxsjDh7JeZrNd2Bc102cKuQueStFk0X-GUXLTJEZJchqI3Yt3CpHBFlxNzY_AnWPM7V-ZY8YlXiviF2f5xqQOw1_DXAPQGu2COh7BZ1uzyQ0GoBv9ahBa_P0xU1B07kWEYwD2oQZunIvIvVMW0Rpfupbm1XeV4cV0v6uaa9xsFBwlSaFmRvA47E_-BsYRrYCOXgGWEbRU8MH4SOSIAGkqctF65bcLCO8DrV_td0PaWAVY8ipeR9Frkc6D6Za7_UQQ7vDYa3xZw1lbzcHNdZC39BaRzQt2J-Opl1eNR5tBpw57dPMi9gQoADizbs1bTV6QhVVPlyCGKteMa4yZ9CKNQVs12McXGsmtzuOD4ZvmT_rq8jW8">speed along</a> by cutting through the current of the vaginal milieu like the bow of a boat.Currently, too little is known about the South American rodents in question to determine which of these theories will pan out. But de la Sancha hopes studies like his will renew interest in conservation, especially in an era of such rapid species loss and habitat destruction. The clock is ticking—and every new bit of information counts.“If these species go extinct before we can understand them,” Fisher says, “we won’t know what we’re losing. There’s inherent value in looking at diversity.”

When it comes to understanding the diversity of South American rodents, sperm might have a lot to say—if you’re willing to listen.

Katherine J. Wu

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