Field of Science


Actual Top Ten Animals of 2013

Not so much a popularity contest as an altitude contest.

10: Himalayan Jumping Spider

The fuzzy and springy Euophrys omnisuperstes lives on the world's highest mountains. It's been found 6,700 meters (a little over four miles) high on Mount Everest. Some birds can fly at higher altitudes, but the spider lives there full-time, apparently subsisting on unlucky insects carried up to it by the wind.

9, 8: Iranian Space Monkeys

Twice this year, Iran announced that it had successfully sent a monkey on a sub-orbital rocket flight. The trips allegedly happened in January and December, both reaching heights of 120 kilometers (75 miles) before safely returning their rhesus macaque-nauts to Earth. But the Associated Press pointed out that the launches couldn't be confirmed by outside sources, and that one post-launch photo showed an entirely different monkey than pre-launch.

7, 6, 5, 4, 3: Russian Rocket Zoo

On April 19 of this year, a Soyuz rocket took off for space carrying a capsule full of animals. Called Bion-M1, the capsule held mice, mongolian gerbils, geckos, fish, and snails. The mice were shared between Russian scientists and NASA, whose researchers would study the effects of low gravity on cell growth, blood flow, joint movement, and sperm motility, among other body functions. The mission orbited Earth for 30 days.

NASA writes, "All of the mice that Russian scientists had shared with their U.S. colleagues returned from space in good health." The Russian scientists must have kept the dead ones for themselves, then, because 29 out of the 45 mice didn't survive. All 8 gerbils perished too, along with the fish. There appeared to have been technical problems with the systems built to feed and keep the animals alive onboard.

2, 1: Imaginary Mars Rock Animals

"NASA Curiosity Rover spots iguana on Mars," declared a Fox News headline in November. It was actually a rock shaped vaguely like a lizard. But someone from a website called UFO Sightings Daily was calling it an "animal" and doing news interviews. Back in May, the same paranormal website had announced the discovery of a "Mars rat" in another one of the rover's photos. It was, of course, also a rock.

Will any Earth animals travel farther from the planet's surface in 2014? If they do, here's hoping they fare better than a Mongolian gerbil.

Honorable mention for effort: NASA launchpad frog.

Why It's Nearly Impossible to Castrate a Hippo

Chances are you've never wondered how difficult it is to remove the testes of a hippopotamus. Other people have been thinking hard about it, though, because in fact it's almost impossible.

Before sitting down to emasculate a common hippopotamus, Hippopotamus amphibius, it would be reasonable to ask why. They're a threatened species, so usually conservationists try to make more baby hippos—not fewer. But in zoos, hippos turn out to be prolific baby-makers. Females can live for 40 years and may birth 25 calves in that time. This would be great news in the wild, but zookeepers don't always have someplace to store a new two-ton animal.

Male hippos can also be aggressive toward each other, at least while they have all their man parts. For both of these reasons, zoos may want to have their male hippos fixed. But there are a few factors working against them, explains a new paper in the journal Theriogenology (that's reproductive science for vets) by an international group of authors.

The first challenge is that hippopotamuses hide their genitals. The testes are inside the body, instead of outside in a scrotum. (Other mammals in the internal-testes club, since you asked, include the armadillo, sloth, whale, and platypus.) This makes the hippo's testes totally invisible from the outside. Combined with a penis that the paper's authors describe as "discreet," it means it's hard to tell males from females at a distance.

Another problem is that testes aren't in the same place from one hippo to the next, and they may "retract" even farther during surgery. Hippopotamuses are also difficult to safely put to sleep. "In the past, hippopotamus anesthesia has been fraught with serious complications," the authors explain.

After moving past the anesthesia problem (they used an apparently safer blend of drugs, delivered via a dart to the hippo's ear), the researchers turned to the anatomical problems. Their answer was ultrasound. Once they had positioned the animal, they used ultrasound imaging to find the testes—then used it again after cutting into the hippo, if the testis they were looking for had scooted farther away from them.

Even after finding the sneaky organs, the procedure wasn't simple. The depth of the testes' hiding places varied by as much as 16 inches from one hippo to the next. Everything had to be done deep inside the animal's body, making it hard to see what was going on. "Grasping the testicle with forceps proved laborious" in most of the animals, the authors write. They also mention using a "two-handed technique" and "moderate traction." The whole hour-and-a-half procedure, based on a method for castrating horses, is described in detail for anyone who wants to try it themselves.

All ten hippos in the study were successfully castrated, though one died shortly afterward, following a complication from a unknown pre-existing condition. Over the next six months, the authors checked in with the zoos housing the hippos to see whether their behavior or interaction with other animals had changed. There were four cases where zoos wanted their hippos fixed to ease aggression between males; in all four, the problem seemed better. (One zoo, though, reported that castrated males were harassed more by females.) Overall, the authors think their technique will help zoos take better care of their hippos.

The final challenge to hippopotamus surgery—what should be a challenge, anyway—is that the animals spend most of their time in a pool of water packed with feces. The animals in the study lowered their stitched-up bellies into this infectious slurry as soon as they had a chance. Yet all of them healed from surgery without trouble. Hippos in general seem to be especially good healers, the authors write.

A possible explanation for the animals' healing superpower is the "red sweat" or "blood sweat" that oozes from their skin. It's not really sweat and it doesn't contain blood, though it is red. The pigments in this skin secretion have been found to absorb UV light, making the "sweat" a potential sunscreen. The pigments can also keep bacteria from growing. So a built-in antibiotic may be what keeps hippos from getting infections after they tussle and bite each other (or after meddling vets come and cut out their manhood). However the red sweat works, it shows that a hippo's secrets don't end with the location of his testicles.

Images: Charlesjsharp (via Wikimedia Commons); drawings by Eva Polsterer (from Walzer et al.).

Walzer C, Petit T, Stalder GL, Horowitz I, Saragusty J, & Hermes R (2013). Surgical castration of the male common hippopotamus (Hippopotamus amphibius). Theriogenology PMID: 24246424

Spider Acts Like Ruthless Carnivore, Is Really Flexitarian

Even deadly predators crave a salad sometimes. Certain orb-weaver spiders—apparent full-time carnivores who eat by trapping prey, covering it with digestive juices, and then slurping it down like an insect smoothie—have been secretly taking their meals with a plant-based side dish. Namely, pollen.

Orb weavers are a family of spiders common all across the world; they're the ones that weave the classic concentric-circle webs you see in picture books. Earlier studies have shown that those webs can collect a lot of pollen on their sticky strands, and that the spiders are willing to eat pollen in the lab. Benjamin Eggs, a graduate student at the University of Bern in Switzerland, and ecologist Dirk Sanders studied two kinds of orb weavers to see what truth there was to this rumor.

For the first part of the experiment, Eggs gathered 20 young Aculepeira ceropegia spiders from outdoors in the spring and brought them into the lab. He coaxed each spider to build its nest inside a cardboard box, where he supplied it with several fruit flies per week. Eggs also sprinkled half the spiderwebs with birch pollen, as they might be in the wild.

After a month, Eggs broke down the spiders' bodies and examined the carbon and nitrogen isotopes inside them. Isotopes, if it's been a while since your last chemistry class, are different forms of the same element. For example, most carbon atoms in the world have 6 protons plus 6 neutrons in their nuclei, making them carbon-12. But a small percentage of carbon atoms, called carbon-13, have an extra neutron. Animals incorporate the atoms they eat into their bodies. So by comparing the ratio of lighter to heavier isotopes in spiders' bodies to the signature ratios of their various foods, the researchers could see what the spiders were eating.

In the lab, orb weavers supplied with pollen had a different isotope ratio than their neighbors who only received fruit flies. This told Eggs that his spiders were, in fact, eating the pollen. But would they do it in the wild?

He returned outdoors and gathered young Araneus diadematus spiders, another orb-weaver species, from the trees. He used nets and bug vacuums to gather as many other insects from the area as possible—the spiders' potential prey. And he collected samples of pollen from neighboring trees.

Eggs analyzed the carbon and nitrogen isotopes in the outdoor spiders, along with all the insects and pollen that might be on their menu. Based on those ratios, he calculated that pollen made up 25 percent of the orb weavers' diet.

It's convenient for orb weavers to eat pollen, because they frequently dismantle and eat their webs in order to recycle the silk proteins. As long as they're at it, they may as well suck down the vegetables trapped there. Eggs points out, though, that the pollen grains he found in his study are too big for spiders to swallow accidentally. So his orb weavers must have deliberately eaten pollen grains by first covering them in digestive enzymes, then slurping them up.

Eating pollen might be most practical in the spring, when young spiders have just hatched and built webs, but insects are still hard to find. The spiders in this study were all juveniles. Yet Eggs, who carried out this research for his undergraduate thesis, thinks there's no reason adult spiders wouldn't eat pollen too. He says, "The anatomy of araneids does not change dramatically when they reach maturity." (He adds intriguingly, "Exception: genitals.")

It's even possible that other kinds of spiders nibble on pollen too. "There is evidence that other web-building spiders like the sheet weavers (Linyphiidae) also feed on pollen," Eggs says, "although they don’t eat their own web." Every diet needs a cheat day, after all.

Photo by Dirk Sanders.

Eggs B, & Sanders D (2013). Herbivory in spiders: the importance of pollen for orb-weavers. PloS one, 8 (11) PMID: 24312430

Higher Altitude Protects Teens from Concussions

The human brain is a vulnerable thing, perched in its peanut shell on top of our walking, stumbling bodies. Humans who enjoy collision-heavy pastimes—say, tackle sports—put their brains in particular danger. And when it comes to concussions, young people are at even more risk than adults. Yet kids who play at at higher altitudes seem to be safer than their peers. The reason, hidden somewhere in the brain's squishy dynamics, might help protect kids and adults who are smashing into each other everywhere.

You don't have to travel to Denver's Mile High stadium for your body to start responding to altitude. "Relatively small changes in altitude can have significant changes upon the physiology of the body," say Gregory Myer and David Smith, both in the sports medicine department at Cincinnati Children's Hospital Medical Center. (The coauthors responded to my email jointly.)

At just 600 feet above sea level, the authors point out, oxygen in the atmosphere has already dropped from 21 percent to 20 percent. Your body notices this slight change and adjusts. One measure it takes, upon noticing there's less oxygen available than usual, is to send a little more blood to your brain. "This leads to a slight filling up of the brain space," Myer and Smith say. Your brain ends up squeezed just a tad more tightly into your head.

Wherever you are, if you get suddenly knocked on the head, your brain will ricochet around inside your skull's fluids. In actual scientific terms, it "sloshes." The delicate brain squishes and twists, and hosts of neurons fire all at once. You may black out. Afterward, you might have memory loss, confusion, nausea, dizziness, and other symptoms that can last for days or months. The looser, stretchier blood vessels in the brains of people under age 20 may explain why they're at even greater risk.

Concussions might be prevented if the skull could keep the brain from sloshing by holding onto it a little tighter—as it does at higher altitudes. To find out whether this works, Myer, Smith and their colleagues used data from the National High School Sports-Related Injury Surveillance System. Run by the University of Colorado, Denver, this study collects data on injuries from high schools across the country.

The authors looked at nearly 6,000 concussions from about 500 schools. The concussed kids were athletes in all kinds of sports, at schools ranging from sea level to 6,900 feet. When the researchers divided student athletes into those living above and below the median altitude—which was 600 feet—they saw a significant difference in concussions. Across all sports, kids at higher altitudes had a 31 percent lower risk of concussion. Among football players only, the results were essentially the same: a 30 percent lower risk at higher altitude.

It's an intriguing difference. As sports organizations and the public learn more about chronic traumatic encephalopathy (CTE) and the long-term risks for athletes with head injuries, the quest to prevent concussions is growing more urgent. High schoolers, though, don't travel to play like professional athletes do. Could some of their lower risk have to do with changes in their bodies that happen over a lifetime of living at a certain altitude? "Visiting altitude will begin creating a tighter fit the minute you arrive," Myer and Smith say. However, adjustment happens over the long term too. "Everyone is likely different in how quickly they respond [to altitude] and how protection occurs for them," the authors say. "This is why we are working to evaluate technologies that can give this same protection whether you are in Denver or Miami." They'll be looking next at adults and professional athletes to try to find answers.

One hint comes from an earlier study David Smith performed on rats. While wearing a collar that slightly squeezed their jugular veins, the rats were hit hard on the head. The collar seemed to make rats less vulnerable to concussion, apparently because more blood was in their heads, squeezing their brains more tightly and preventing sloshing. This all sounds pretty unpleasant for the rats, but Myer and Smith insist that "the technologies we are studying are no more risky than yawning or even the act of lying down."

Animals like woodpeckers and head-ramming sheep manage to protect their brains from damage, the researchers point out. So why can't we? Of course, in our case the head ramming is in the name of fun. But there might be ways to safeguard our brains, like these animals do, from the inside out.

Image: Rocky Mountain High School in Colorado, by Paul L. Dineen (via Wikimedia Commons)

David W. Smith, Gregory D. Myer, Dustin W. Currie, R. Dawn Comstock, Joseph F. Clark, & Julian E. Bailes (2013). Altitude Modulates Concussion Incidence: Implications for Optimizing Brain Compliance to Prevent Brain Injury in Athletes. Orthopaedic Journal of Sports Medicine DOI: 10.1177/2325967113511588

Laughter Is OK Medicine, Unless It Kills You

Careful with the bedside banter, doctors. Before you put on your best Patch Adams impression, you might want to consider whether your attempts at humor will ease your patient's discomfort or give him a protruding hernia.

That's the conclusion of a review paper in the Christmas issue of BMJ that asks the jolly question of whether laughter can kill. The two authors, R. E. Ferner of the University of Birmingham and J. K. Aronson of Oxford University—no JK-ing, those are his real initials—take a tongue-in-cheek approach. They even give their research question an acronym: MIRTH (Methodical Investigation of Risibility, Therapeutic and Harmful).

Ferner and Aronson scoured medical literature for studies having to do with laughter. After "excluding papers on the Caribbean sponge Prosuberites laughlini and with authors called Laughing, Laughter, Laughton, or McLaughlin," they were left with three categories of study. One had to do with the benefits of laughter, one with its dangers, and the third with medical conditions that have laughter as a symptom.

Let's hear the bad news first. Laughter, according to various researchers, can lead to syncope (fainting), arrhythmia, and cardiac rupture. In asthmatics, laughing can trigger an attack. Laughing can even cause pneumothorax, a collapsed lung. People with cataplexy, a rare condition tied to narcolepsy, may suddenly lose all their muscle strength and collapse during a fit of laughter. An especially good laugh can make a person's hernia protrude, or dislocate someone's jaw.

Among the more pedestrian dangers, breathing in sharply when you start to laugh can make you choke. Laughing in someone's face can spread germs. And, of course, there's the danger of pee coming out when you laugh, which doctors call "giggle incontinence."

The authors also gathered a list of about three dozen medical conditions that have been reported—commonly or not—to cause laughter. These include epilepsy, brain tumors, multiple sclerosis, and kuru (a disease you are unlikely to contract unless you're a practicing cannibal).

Now for the good news. Laughter may increase your pain tolerance, reduce stiffness in the walls of your arteries, and even lower your risk of a heart attack. In patients with chronic obstructive pulmonary disease (COPD), laughter can improve lung function. Fifteen minutes of laughter reportedly burns 40 calories, which fitness-wise makes it similar to a very slow walk (or, according to Fitness magazine, barbecuing.)

Most strangely, one study used clowns to try to (indirectly) get women pregnant. Immediately after undergoing IVF, women were subjected to 12 to 15 minutes of entertainment by "a clown, dressed as a chef de cuisine." Among these women, 36 percent became pregnant, compared to just 20 percent in a control group.

Perhaps aspiring clowns themselves, the authors can't resist throwing in a few puns of their own: "Laughing fit to burst can cause cardiac rupture." "Perhaps surgical patients derive no advantage from being in stitches." "It remains to be seen whether...sick jokes make you ill, [or] dry wit causes dehydration." I'll give them the benefit of the doubt and assume that, knowing the potentially serious side effects of laughter, they chose to spare their audience the risk.

Image: Urban Combing (Ultrastar175g) (via Flickr)

R E Ferner, & J K Aronson (2013). Laughter and MIRTH (Methodical Investigation of Risibility, Therapeutic and Harmful): narrative synthesis. BMJ DOI: 10.1136/bmj.f7274

Newly Discovered Flower Makes Fake Pollen to Fool Bees

"I was certain it was something new when I saw it," says Chris Martine of the bush tomato species he discovered in the Australian outback. It's a scrappy, spiny shrub with crinkly purple flowers that thrives on fire. It also uses treachery to survive, disguising its female flowers with fake male parts and even fake pollen.

A botanist and biodiversity scientist at Bucknell University, Martine explains that the new plant "was on the radar of a few local botanists as being an oddball." Martine had been studying related species for a decade, so when his lab analyzed this plant's DNA, he recognized that it was something different. He went to Australia to document the species in person and dubbed it Solanum cowiei after botanist Ian Cowie at the Northern Territory Herbarium, who first introduced him to the plant.

The diverse Solanum genus includes plants ranging from potatoes and tomatoes to eggplant and nightshade. The Australian bush tomatoes that Martine studies grow little fruits that can be edible or quite poisonous, depending on the species.

Martine discovered that compared to its relatives, S. cowiei is especially well adapted to the fires that sometimes sweep through its habitat. The plants live in large groups of clones tied together by underground root systems. In an area that had recently been burned clear, Martine found S. cowiei plants springing up and blooming while other species lagged behind. This means that after a fire, these plants have a competitive edge over all their neighbors in getting to pollinators.

The new species's method of reaching those pollinators is a weird one. S. cowiei grows separate male and female flowers, and like about a dozen of its close relatives, it disguises the female flowers with fake male parts and pollen. Under an electron microscope, the false pollen grains look like brand-new tennis balls. Real pollen grains are closer to old ping-pong balls, with large dents or grooves on their surface—these are the weak spots where a narrow tube may later burst out of the wall of the pollen grain, carrying the plant's sperm to an egg.

Why bother with the ruse? Solanum flowers don't have any attractive fragrance to lure their pollinators, or nectar for insects or birds to drink while they're dusted with pollen. Instead, these plants rely on pollinators that want to eat the pollen itself. Certain foraging bees use pollen to feed their young, Martine says. "So if you want one of these bees to visit your flowers, you have to have to have the visual cue of the anthers," a flower's male parts. "And if you want them to come back to flowers like yours again, you have to give them some reward to take away."

Martine and his collaborators are now studying whether this fake pollen is any better or worse for young bees to eat than the real stuff. "Is there a difference in what they are getting?" he says. "Can they tell?"

The "oddball" bush tomato isn't the only plant Martine finds intriguing. He produces an online video series called "Plants Are Cool, Too!" ("Can an animal make its own food? No! Can an animal feed the whole world? No!" the theme song declares.) Martine started this series after working with kids who were interested in science and realizing that they knew a lot about animals, but not so much about plants. People browsing online are more likely to encounter a cute cat video, after all, than one about cattails. So he started putting together episodes that highlight some of the "coolest" plants, along with the botanists who study them.

The next full episode should be out in January, he says, and it includes an especially cool moment: a new species of mustard plant being discovered. "Our guest expert looked down during shooting and said, 'Hey, wait a minute,'" he says. "I don't know how often new species are discovered while a camera is running, but it can't be very common."

Photo by Kym Brennan.

Christopher T. Martine, David E. Symon, & Elizabeth C. Evans (2013). A new cryptically dioecious species of bush tomato (Solanum) from the Northern Territory, Australia. PhytoKeys DOI: 10.3897/phytokeys.30.6003

Leaping Land Fish Has Perfect Camouflage, Is Not a Hoax

You might never spot them if not for the jumping. On the coast of Guam, Pacific leaping blennies blend in perfectly with the rocks they live on, their limbless bodies maintaining a sleek profile. But the creatures give themselves away when they coil their tails to one side and shoot like a spring from rock to rock. These unsettling animals are fish that live on land. How they pull it off could give us hints about the evolution of our first earthbound ancestors.

Terry Ord, an evolutionary ecologist at the University of New South Wales, calls the Pacific leaping blenny "an extraordinary animal." It lives its adult life out of water, hopping between rocks and breathing through its skin as well as gills. It relies on splashes from waves to stay wet, but it rarely—or never—goes for a swim.

Even though the coast of Guam teems with leaping blennies, Ord says, "we know surprisingly little about this land fish." Ord and his graduate student Courtney Morgans investigated one mysterious feature: the fish's conveniently rock-like coloration. Without being so well camouflaged, could the blennies have ever made their first leap onto land?

Morgans and Ord traveled around the periphery of Guam and visited five different blenny populations. At each site, they took photographs of the fish and their background rocks. (The blennies aren't always stone-colored; during courtship, males darken to a charcoal hue while females fade nearly to white. Both sexes can flash a bright-red fin on their backs that they normally keep hidden. But the researchers kept a close eye on their subjects during the experiment to make sure they didn't change colors.)

Computer analysis of the photos showed that the blennies' normal skin color is a perfect match to the rocks they live on. Some birds use UV light, which the researchers didn't analyze, to find their prey. But for most of the hungry lizards and crabs patrolling Guam, the blennies should blend right in to the rocks.

To find out how well this camouflage really protects leaping blennies, Morgans and Ord set up 70 fake fish as bait. They molded plasticine blenny bodies with realistic coloration and anchored them to spots around the island with fishing line. Some fake blennies sat on the rocks, while others were on the sand, where they don't blend in as well.

After three days, Morgans and Ord returned to their fake fish. If the props were nicked, punctured, or had bites taken out of them, the scientists assumed predators had come by. They saw that predators attacked blennies on the sand much more often than those on the rocks.

So their coloration seems to be crucial to the leaping blennies' survival on shore. The scientists think that in this regard, the fish may have just been lucky. When they compared the Pacific leaping blenny to 12 closely related blenny species, they found that all the relatives have similar coloration. (Some of these relatives also spend time out of water, but the Pacific leaping blenny is the only one to live on land full-time.) If the ancestor to all these species had the same rocky skin color, then it was well prepared to wriggle out of the ocean and start a new life on land.

That doesn't mean the transition was easy. Blennies also had to evolve a way to breathe air through their skin, like frogs do. Their tail-jumping trick is helpful too, letting the legless fish propel themselves through their habitat. "Obviously moving about on land is critical," Ord says. (You can watch them leaping in the video below, from his lab's YouTube channel.)

Ord says this freak-show fish actually has a lot to tell us about evolution. For one thing, it demonstrates the kinds of adaptations an animal can make after it transitions to a new home. It also speaks to our own ancestry. "In the late Devonian, fish made the first transition onto land, and from that event evolved all of the land vertebrates we now have in the world," he says. The land fish represent a "snapshot of one of the most important evolutionary events in our history." Our ancestors may have looked equally ridiculous as they floundered on land—but like the leaping blenny, they were going places.

Image: Courtney Morgans, UNSW

Courtney L. Morgans, & Terry J. Ord (2013). Natural selection in novel environments: predation selects for background matching in the body colour of a land fish. Animal Behaviour DOI: 10.1016/j.anbehav.2013.09.027

Turtle Moms Choose Their Babies' Genders by Where They Build Their Nests

If turtles had realtors, their motto would also be "Location, location, location!"—but not because they care about a scenic vista. The spot a mother turtle chooses to dig her nest determines whether her young will be males or females. This might even be the most important factor in her decision.

A female painted turtle (Chrysemys picta) is not an over-involved parent. She digs a hole in the dirt, lays a batch of eggs there, and buries them. Then she returns to her freshwater life without giving the nest another thought. The eggs incubate and develop under the soil while the summer wears on. Hatchlings finally chip their way free in the late summer or early fall, but in cooler parts of North America they don't leave the nest right away; they stay hunkered down with their siblings to hibernate until the following spring.

Sometime in the middle of the summer, a significant event happens inside each buried egg: the developing turtle becomes male or female. Its sex hasn't been determined by its genes like ours is. Instead, as with many other reptiles, the temperature in the nest tilts the egg toward one sex or the other. Cooler nests produce males and warmer ones make females. If the nest stays within a narrow temperature range, hatchlings of both sexes will crawl out at the end of the season.

Timothy Mitchell, a Ph.D. student in ecology at Iowa State University, studies a population of painted turtles living in northwestern Illinois. These particular turtles have been under close watch by scientists for more than two decades, but it hasn't become clear whether turtle moms are active in determining their hatchlings' sex—do they choose nest sites that will best balance the sex ratio of their eggs? To find out, Mitchell set up a kind of nest-building competition between himself and the mother reptiles.

Mitchell scoped out 20 nests in his study site, a forested area near the Mississippi River. Right after the mothers left their nests behind, he went in and dug the eggs up. Then he tucked the eggs into artificial, Styrofoam-box nests. Half the eggs from each batch went into a box right next to where their mother had left them, buried at the same depth to create a controlled replica of the original nest. The other half went into a box at a site Mitchell selected at random.

(How do you randomly place a turtle nest? Mitchell used a random number generator to choose a distance away from the original nest, up to 30 meters. Then he flung a pencil in the air and walked in whatever direction in was pointing when it fell. If the resulting location was, say, in the Mississippi, he tried again.)

Just before they were due to hatch, the eggs were dug up and brought to a lab. Mitchell monitored their hatching and then returned the tiny turtles to their artificial nests for hibernation (along with a sprinkling of eggshells, as if the turtles had been there the whole time). He checked on the baby turtles once more in the spring.

Temperature sensors hidden in the nests revealed that sites chosen by turtle moms were a little warmer than Mitchell's randomly selected ones. This meant they were more open to the sun; nests that were shaded by vegetation were cooler.

Between the original nest sites and the random ones, there was no difference in the number of eggs that survived all the way through hatching and hibernation. But there was a major difference in sex ratio: while the turtle moms' nest sites produced roughly equal numbers of boy and girl turtles, the hatchlings from Mitchell's randomly placed nests were about 80 percent male.

"This strongly suggests this process of sex ratio selection is influencing where Mom chooses to nest," Mitchell says, "as opposed to selection to have eggs survive."

Wherever she builds her nest within this forest, a painted turtle mother can be assured that her young will survive equally well. But it's in her best interest to keep the sex ratio balanced. In the long term, turtles that tend to build male-heavy or female-heavy nests will lose out when the population swings in that direction, because young turtles of the opposite sex will then have better mating prospects.

A warming climate is a threat to any species whose sex ratios depend on the temperature. But if female turtles are savvy enough to leave their eggs in exactly the right sex-balancing spot, should we stop worrying about them? "That is still the big question in the field!" Mitchell says. He thinks moms' choice of nest sites will be a crucial part of how this species responds to climate change. But many other factors will matter too, like the fragmentation of the turtles' habitat and how the climate affects their predators. Turtle moms today know how to build a perfect nest for their offspring , but that balance may be as fragile as eggshells.

Image: Timothy Mitchell.

Timothy S. Mitchell, Jessica A. Maciel, & Fredric J. Janzen (2013). Does sex-ratio selection influence nest-site choice in a reptile with temperature-dependent sex determination? Proceedings of the Royal Society B DOI: 10.1098/rspb.2013.2460