False vampire bats attack a rodent in India in 2010. Photograph by Stephen Dalton, Minden Pictures/Corbis

If bats ever used a cell phone, they could forgo the version with caller ID: The mammals can identify each other by their voices, a new study says.

Bats aren’t the only mammals to use voice recognition—people do it, too. Even in the days before caller ID, a simple “Hi, it’s me,” from a close friend or loved one was usually enough to figure out who’s on the other end. Recognizing a person by voice, however, requires previous knowledge: We can’t identify a stranger on the phone by voice alone because we have never met them before.

People can, however, discriminate between a familiar voice and an unfamiliar one, even if they’ve never met the other person. We can also distinguish between two individuals by voice alone even if we’ve never met them before.

Hanna Kastein and colleagues at the University of Veterinary Medicine in Hannover, Germany, wanted to know whether bats could perform these same tasks.

“Bats are totally interesting mammals to study voice perception since they are dependent on their vocalizations for orientation and communication due to their nocturnal lifestyle. In addition, they are socially living animals that frequently communicate acoustically with other members of their species,” Kastein said. (Also see “‘Whispering’ Bat Evolved to Trick Prey.”)

Besides their social lifestyles, bats and people share a number of physical characteristics. Both produce sounds using a combination of the larynx, vocal cords, and nasal cavities. These structures work together with an animal’s physical makeup to produce an individual’s unique voice.

“In stressful situations, voices become higher pitched, or ‘squeaky,’ in bats as in humans. Also, each individual bat has a slightly different morphology, and thus its voice sounds different from any other individual, just as voices in humans differ individually,” Kastein said.

You Had Me at Hello

Kastein and colleagues wanted to know whether bats could use vocal calls to identify individuals with which they shared a roost, and whether they could use these same calls to distinguish between two different individuals.

The researchers worked with the greater false vampire bat (Megaderma lyra) because the species has a rich array of calls that it uses in several contexts. (See “Vampire Bats Have Vein Sensors.”)

The team observed two groups of bats kept in separate artificial roosts for two months. They hypothesized that bats that had the most body contact while roosting would form the closest relationships. Kastein and colleagues then recorded various vocal calls from both groups of bats.

When Kastein played the recording of a vocal call over a loudspeaker, bats in both roosts universally turned their heads toward the speaker regardless of whether the call was from a bat with which they had close body contact, a bat from the same roost, or a bat from the other roost. (Interactive: Hear tropical bat calls.)

Given that the artificial roosts had much lower rates of vocal calls, due to the lack of stimuli, the researchers thought that this response could be due to the novelty of hearing any type of vocalization.

Discriminating Bat

So the team did a second set of experiments in which they had a bat listen to the call of their “friend” until the call didn’t create any type of behavioral response, such as turning the head. This means the listening bat had become habituated to the call, according to the study, published recently in the journal Animal Cognition.

Then, the scientists alternated playing a vocalization of the bat friend with that of an unfamiliar bat. The listening bats were significantly more likely to turn their heads toward the call of their friend—indicating both that they recognized their friend and that they could distinguish between individual vocalizations. (Also see “‘Talking’ Whale Could Imitate Human Voice.”)

“In our study, we found that the … false vampire bat is able to discriminate between different voices, including both known or unknown individuals,” Kastein noted.

“However, to what extent bats are able to label an unknown bat as unknown, we cannot say.” She suspects that in real life, recognizing other bats by their voices is aided by smell and, to a lesser extent, vision.

The zoo was looking for females of the Mangarahara cichlid, a species of fish so rare that none are thought to exist in the wild, and one that is in critical danger of going extinct entirely if the zoo’s two males and a further bachelor fish at a zoo in Berlin don’t find mates soon.

Dateless: A male Mangarahara cichlid. Photograph from ZSL/AP

Many creatures are in this situation because they are endemic (i.e. unique) to specific areas and habitats and therefore at increased risk for extinction, or else because their numbers have been reduced by habitat destruction or hunting and poaching.

The Mangarahara cichlid isn’t alone. Here are some other lonely creatures looking for a friend.

Lonesome George

Lonesome George was the last of his kind. Photograph by Michael Melford, National Geographic

One of the most well-known members of the Forlorn Creatures Club was the aptly named Lonesome George, a Pinta Island tortoise from the Galapagos who sadly passed away last year after several failed attempts to successfully mate him with another tortoise and thereby continue the subspecies. At over 100 years old Lonesome George at least had a good run, but it was a long and solitary one (see “Lonesome George Not the Last of His Kind, After All?“).

Here are a few other lonesome creatures who we hope can find love and avoid a similar fate.

Beauty Spots

A young Amur leopard, one of the world’s rarest felines. Photograph by Luca Barovier, Your Shot

The forests of Eastern Russia may not seem like big cat country but they are in fact home to the Amur leopard, a quick, clever, and striking feline of which only roughly 30 are thought to exist. Their eye-catching appearance is actually part of the problem. Despite their critically low numbers, the leopards are still being killed by poachers for their valuable skins.

Further compounding the Amur’s population problem is a food problem: It’s typical prey of deer and hare are often hunted by the residents of nearby villages, making it difficult for the population to grow. Luckily for the Amur leopard the Russian government took action in 2012 to create a 650,000-acre (263,046-hectare) protected zone called Land of the Leopard National Park, giving the species a fighting (and loving) chance.

Big Love

Then there’s the Javan rhinoceros. Once common throughout Southeast Asia, these lonely hulks are now isolated entirely to Ujung Kulon National Park in western Java, with fewer than 100 individuals thought to be remaining. As perhaps the rarest large mammal on Earth, needless to say their situation is critical and it’s in the interest of biodiversity that their love lives be as exciting as possible.

Lonely Mountains

Mountain gorillas continue to face pressures. Photograph by Michael Nichols, National Geographic

Now spare a thought for a lonely heart from our own great ape family: the mountain gorilla of Central and East Africa. As humans have encroached on their habitat, mountain gorillas have been driven to higher, more unforgiving altitudes. The main factor putting the squeeze on the mountain gorilla is habitat loss, as displaced humans from conflict zones move in and in turn displace the gorillas, clearing flora for agriculture and to make charcoal.

Thanks to conservation efforts populations have stabilized somewhat in the past few years, though with a population estimated to be under 900 they remain critically endangered.

Small Porpoise, Big Pond

A vaquita porpoise caught in a fishing net. Photograph by Flip Nicklin, National Geographic

The vaquita is a mini-porpoise that makes its home in Mexico’s Gulf of California. It has suffered as a casualty of the fishing industry, getting unintentionally caught and killed in nets (a phenomenon known as “bycatch”). Because of this there are estimated to be only 200 individuals remaining.

Since their dolphin cousins are famously frisky animals, with prehensile penises, who even copulate face-to-face, populations would hopefully rebound if the porpoises were left to their own devices and free of human ones. One can only hope that efforts to eliminate bycatch incidents will be sucessful and that the vaquita will be left to get busy getting busy. (See video of restoration in the Colorado River Delta.)

These insects, whose ears are each only about the size of a pinhead, can recognize sounds between 30 and 300 kilohertz (kHz)—a range never before seen in the animal kingdom.

A greater wax moth rests in England in 2007. Photograph by Andrew Darrington, Alamy

People speak at about 3 kHz. In youth we can hear up to about 20 kHz, but we lose our ability to recognize higher frequencies as we age. Dogs generally hear frequencies of about 30 kHz. But nothing compares to the moth’s 300-kHz extreme.

Since there aren’t any sounds in nature that come close, scientists hypothesize that greater wax moths evolved supersensitive ears to evade bats, their main predators. (See “‘Whispering’ Bat Evolved to Trick Prey.”)

“I think it’s a really amazing study,” said Damian Elias, a biologist at the University of California, Berkeley, who wasn’t involved in the experiment.

“Biologists make assumptions as to what types of sounds and frequencies animals should hear,” he said, but ”when you actually measure it, you’re often surprised.”

No Words to Explain It

To test the moths’ hearing, the scientists built special speakers that played increasingly higher frequencies. The team then used a laser to measure the moths’ ear movements in response to each frequency. They also assessed the electrical nerve signals sent from the moths’ ears to their brains.

These two calculations indicated whether the moths’ ears were responding to individual sounds. The results showed that the moths ended up topping out at about 300 kHz—a pitch so high that even researchers have a difficult time understanding the pinnacle.

“The problem is that because humans don’t hear that sort of frequency, we don’t have any words for it,” said study co-author James Windmill, an acoustical engineer at the University of Strathclyde in Scotland.

“There’s not that much that happens in nature that happens at those frequencies.”

Bat vs. Moth

Bats are the only animals that come close, with a hearing capability of about 212 kHz. Windmill and his team hypothesize that moths—found throughout much of the world—use their highly acute hearing to avoid bats that see them as tasty snacks.

Listen to a silver-haired bat attack an insect.

The two groups have been locked in an evolutionary arms race for generations, each trying to best the other by evolving new traits to hunt and evade. Right now, the moths are winning, said Windmill, whose study was published May 8 in Biology Letters. (Related: “Moths Jam Bat Sonar, Throw the Predators Off Course.”)

 The advantage is a matter of physics, where response time and frequency are closely tied. Since moths are able to hear such high pitches, they have the ability to react to lower pitches much faster. Moths listen for a bat’s ultrasound pulses and take evasive action. (Interactive: Hear tropical bat calls.)

Moth Ears Could Inspire New Tech

Compared with a human ear, the moth ear is incredibly simple, consisting of just a few nerve cells that feed directly into the brain. The natural system is so unique that Windmill says he’s working with the military and hopes to develop microphones based on the design.

More sensitive microphones could be adapted for hearing aids and cell phones, which have what’s called the “cocktail-party problem.”

Whereas human ears are able to pick out and focus on one conversation even when there’s a lot of background noise, the microphones on most devices aren’t as targeted—meaning that Windmill’s moth-ear study could be the catalyst for technological breakthroughs. (Also see “Urban Grasshoppers Sing Louder.”)

“Evolution is a great inspiration for scientists,” noted the University of California’s Elias.

“Animals have remarkable capabilities to sense things,” he said. “They’re able to do things that human hearing and human-engineered devices are very poor at doing.”

We’ve known for years that female black widow spiders and other arachnids eat males during mating.

Now, new research shows that males of a type of ground spider known as Micaria sociabilis also eat females, and scientists are trying to figure out what motivates this behavior.

A male M. sociabilis attacks his mate. Photograph courtesy Lenka Sentenská

More than just a first date from hell, sexual cannibalism happens when one member of a species kills and eats a member of the opposite sex immediately before, after, or during mating.

This behavior is most common in arachnids like the black widow, as well as other invertebrates like insects, gastropods, and copepods. Most commonly, the female eats the male—but occasionally, the reverse is true. Male sexual cannibalism has been observed in another species of spider, Allocosa brasiliensis, and in crustaceans, but previously researchers had no idea what factors drove this behavior. (Watch a video of a male spider attacking a female one.)

Ultimate Sacrifice

On the surface, this seems like a pretty weird way to pass along your genes to the next generation. To scientists, however, sexual cannibalism can make a lot of sense. When prey is scarce and males are abundant, males become extremely valuable as a food source to females, noted study co-author Lenka Sentenská, a biologist at Masaryk University in Brno, Czech Republic.

Females invest much more energy into egg production than males do in sperm production, which tends to make them pickier about who they mate with. (See “Male Spiders Give ‘Back Rubs’ to Seduce Their Mates.”)

As well, not all males seem to fight being cannibalized, said Sentenská, whose study appeared recently in the journal Behavioral Ecology and Sociobiology.

“Males of some species voluntarily sacrifice themselves to a female, because such behavior enables them to copulate longer and pass more sperm. Males who are trying to escape usually copulate for a shorter time. Sometimes, this behavior is also viewed as an extreme paternal investment of the male, who sacrifices his body to provide nutrients to his future offspring via the female,” she said.

Deadly Dinner Date

Researchers have traditionally focused on female sexual cannibalism, both because of its relative frequency and perhaps because of humanity’s attraction to the femme fatale.

A female M. sociabilis. Photograph courtesy Stano Pekar

But in some species, such as M. sociabilis, females are also in danger of being eaten by males. Sentenská and her colleague Stano Pekár examined what factors drove male sexual cannibalism in this particular species. (Watch a video of the world’s largest spider.)

These small brown spiders commonly live in trees in Central Europe. M. sociabilis has two generations of offspring each year—one in spring and one in early summer. Whereas females born in the springtime frequently survive to see the arrival of the second generation in June or July, male numbers drop precipitously in May. This creates some significant factors that the authors believe could lead to sexual cannibalism.

Males can choose among females of different size and quality—for instance, by mating with older females from the spring generation or with virgin females from the summer generation. (See pictures of spiders up close.)

To test whether these factors actually affect whether males ate females, Sentenská and Pekár paired adult male M. sociabilis with females of various ages and sizes. They found that larger males were more aggressive and much more likely to attack a female regardless of her age or size. Overall, however, males of any size were more likely to attack and eat females from the older generation.

Picky Males

Between 44 and 52 percent of the time, the males and females mated. In 20 percent of all of these trials, the male attacked and ate the female during copulation. However, the frequency of cannibalism varied depending on time of year. (See “‘Castrated’ Spiders Are Better Fighters, Study Says.”)

“Males are usually viewed as nondiscriminatory machines eager to impress and mate with every female encountered. Our study shows that males can be choosy and that they can present their choice in a quite extreme way—by killing unpreferred females. Moreover, despite the rarity of such behavior, this strategy seems to be advantageous for them,” Sentenská said.

These results might help explain male sexual cannibalism in other species, the researchers concluded.

However, Sentenská quipped, “I definitely would not encourage readers to try this strategy in their personal life.”

That’s what a male seadevil does. Is your honey 50 times your size and liable to eat you after a snuggle? Let’s hope not, else she’d be a garden spider.

A great bustard male displays in front of two smaller females in Badajoz, Spain. Photograph by Ramon Navarro, Foto Natura/Minden Pictures/Corbis

The animal kingdom is full of amatory pairs whose extreme physical differences would give a matchmaker pause. But many of these dimorphic differences make good evolutionary sense, Daphne J. Fairbairn explains in her book Odd Couples: Extraordinary Differences between the Sexes in the Animal Kingdom.

National Geographic Senior Writer Rachel Hartigan Shea spoke with Fairbairn, a biologist at the University of California, Riverside, about why in nature, love isn’t always one size fits all.

How did you become interested in sexual differences in animals?

For my PhD dissertation, I was going out day after day capturing mice in Vancouver, Canada. I discovered that males and females died at the same rates in the spring but for completely different reasons. The young male mice were getting into fights, but what was killing the females was getting pregnant. It struck me that no one cares what sex a mouse is, but for the mice it matters enormously to how they experience their lives. I never forgot that lesson.

Why are the differences between the sexes in some animals so extreme?

If you are coming into the world as a male, the way you get your genes into the next generation is getting your sperm to meet up with the eggs of females. So whatever it takes to do that is how the males are going to turn out. (Related Q&A: “Unlikely Animal Friendships.”)

So what does it take for male elephant seals, who are seven to eight times larger than females?

The way a male elephant seal passes on his genes is by having physical contests with other male elephant seals to keep them away from females so he can mate with them.

A male southern elephant seal dwarfs a female in South Georgia Island. Photograph by Yva Momatiuk and John Eastcott, Minden Pictures/Corbis

His whole life history is structured so that he will be big, aggressive, and politically savvy on the breeding grounds. That means that he’s born larger [than females], he takes more milk from his mother than he would as a female, and he grows at a faster rate.

In order to sustain that growth rate, he ends up having to go foraging for a different kind of food that is found in a different part of the ocean [from where the females are foraging]. His whole life is different.

Male orb spiders, on the other hand, are much smaller than females. How does that affect their lives?

If you are a male orb spider, your biggest threat is that you won’t ever find a female to mate with. You have to leave your web and search for the trail of a female orb spider. A tiny body and long legs makes it more efficient to move through tangled vegetation. (See “Male Spiders Give ‘Back Rubs’ to Seduce Their Mates.”)

The male spot orb weaver (right) treads carefully around the female, which may eat him. Photograph by Urak Istvan, My Shot

Across the animal kingdom, who’s bigger?

Females. If you think about a spider or a worm or an insect, they make a whole bunch of eggs, their bodies swell up with eggs, and they extrude the eggs in a big batch. It helps to be big.

What’s the strangest animal you studied for your book?

Bone worms. When scientists found these bone-eating worms [on whale carcasses at the bottom of the ocean], they couldn’t find the males until they looked inside the females. The males look like they’re single cells, just yolk and sperm. (See bone-worm pictures.)

Has your research given you a new perspective on the differences between men and women?

On the one hand, males and females are different from the get-go in their physiology, their behavior, their morphology. So to one branch of radical feminists I’d say, stop trying to say there’s no difference between men and women. On the other hand, compared with most animals, human males and females are hardly different at all.

This unique organism got us thinking about what other creatures are transparent or translucent, so we put together this list.

1. Transparent Amazonian Fish

Cyanogaster, a recently discovered translucent fish. Photograph courtesy Natural History Museum

It’s speculated that the combination of its nearly invisible nature and nocturnal ways may be the reason why Cyanogaster noctivaga wasn’t discovered until now.  The fish is also tiny, measuring an estimated maximum of 0.7 inches (17 millimeters) long and makes its home in the notoriously murky Rio Negro, which may have contributed to its elusiveness.

Being that hard to see confers obvious survival benefits as a form of camouflage, so it’s not surprising that when it comes to transparency it *ahem* clearly isn’t alone.

(See Photos of a Fish With a Transparent Head)

2. Golden Tortoise Beetle

Golden tortoise beetles can shine metallic gold or change color to red. Photograph by George Grall, National Geographic

The golden tortoise beetle (Charidotella sexpunctata) is high in the running for the tiniest and most beguiling of nature’s translucent creatures. At 0.2 to 0.3 inches (5 to 8 millimeters) long it resembles a metallic ladybug and is something of a chameleon, shifting in color from gold to a reddish bronze throughout the year.

Also known as the “goldbug,” the lovely leaf-eater manages this transformation by reflecting light through liquid stored below a transparent outer cuticle.

3. Glass Frogs

The translucent glass frog Hyalinobatrachium pellucidum. Photograph by Joel Sartore, National Geographic

Hyalinobatrachium pellucidum, also known as the “glass frog,” is native to the cloud forests and rivers of Ecuador. Hyalinobatrachium pellucidum’s pale green skin is translucent to the point that the majority of its vital organs are clearly visible. Unfortunately, the species is endangered due to continuing habitat destruction.

4. Sea Angels

A sea butterflyfish swims in waters off Enderbury Island. Photograph by Brian Skerry, National Geographic

Sea angels are mollusks native to the Arctic Ocean whose scientific name Gymnosomata is Greek for “naked body” and whose appearance is both ghostly and beautiful (see video below). Sea angels are hermaphroditic and feed on another, possibly related, species of “winged” mollusk known as the sea butterfly (Thecosomata). Since they appear to flutter through the water on tiny translucent wings it’s easy to see how sea angels got their common name, although they also have pointy protrusions that resemble horns…

5. Barton Springs Salamander

An endangered Barton Springs salamander, Eurycea sosorum. Photograph by Joel Sartore, National Geographic

The Barton Springs salamander (Eurycea sosorum) is a small lungless salamander unique to Barton Springs, a group of natural springs in Austin, Texas. Its speckled, reddish-brown skin is translucent and eggs or even the contents of the salamander’s last meal are often visible.

Since the Barton Springs salamander is found nowhere else in the world and relies on fresh, unpolluted spring water it’s been listed as an endangered species since 1997.

While for many animals translucency is a way of life, some creatures exhibit it only in certain phases… 

6. Translucent Tadpoles

Note the tadpoles’ coiled intestines. Photograph by Paul A. Zahl, National Geographic

These Costa Rican tadpoles have see-through skin that reveals neatly coiled intestines.

7. Larval Squid

A transparent larval squid. Photograph by David Liittschwager, National Geographic

This larval squid shows off translucency and iridescence in equal measure.

8. Monarch Butterfly Pupa

A translucent monarch butterfly pupa. Photograph by Paul A. Zahl, National Geographic

This monarch butterfly, seen at a tender age, offers a preview of its famous brilliant orange hue peeking out from the transparent coating of its pupa.

 What are some transparent creatures we’ve left out? Have you seen (through) any other examples?

The species, named Tinkerbella nana after Peter Pan’s miniscule assistant, averages 250 micrometers long. That’s 0.01 of an inch—little wider than the diameter of a human hair.

The new fairyfly is named after Tinkerbell. Photograph courtesy Jennifer Read

T. nana is a type of wasp known as a fairyfly (Mymaridae). Don’t be fooled by its sweet-sounding name, small size, or the delicate fringe on its wings—T. nana and other fairyflies are parasites that live on the eggs of other insect species. (Also see “Parasitic Wasp Swarm Unleashed to Fight Pests.”)

Finding such small insects is no easy task—researcher John Huber of Natural Resources Canada and entomologist John Noyes of the Natural History Museum in London, England, found the fairyflies only by sifting through leaf litter and debris, looking for eggs. Huber and Noyes recently published their find in the Journal of Hymenoptera Research.

Despite its microscopic size, T. nana isn’t the smallest known insect. That particular honor goes to Kikiki huna, a species of fairyfly found on Hawaii. This fly is only half as long as T. nana, and scientists don’t know whether it’s possible for insects to get any smaller.

Meet some other members of the animal kingdom known for their remarkably petite physiques.

Smallest primate. Southeast Asia’s pygmy tarsiers (Tarsius) and Madagascar‘s mouse lemurs (Microcebus) seem to be in a running competition for world’s smallest primate.

A tarsier munches on an insect in the Philippines. Photograph by Erik Sellgren, Your Shot

At approximately 4 inches (10 centimeters) long, the pygmy tarsier can fit inside a person’s fist, and full-grown adults frequently weigh in at less than two ounces (57 grams). Mouse lemurs have a combined head, body, and tail length of less than 10.6 inches (27 centimeters), according to the National Primate Research Center at the University of Wisconsin.  The smallest mouse lemur, M. berthae, weighs about 1.1 ounces (31 grams). (Read about new mouse lemurs discovered in 2006.)

A new species of mouse lemur found in Madagascar. Photograph by Mark Thiessen, National Geographic

Primatologist Mireya Mayor holding a newfound mouse lemur in Madagascar. Photograph by Mark Thiessen, National Geographic

Smallest vertebrate. The world’s smallest vertebrate, a frog known as Paedophryne amanuensis, was discovered in 2009 in Papua New Guinea. The frog measures only 0.3 of an inch (7.7 millimeters), according to a 2012 study in the journal PLoS ONE.

Dwarfed by a dime, Paedophryne amauensis inhabits New Guinean rain forests. Photograph courtesy Christopher Austin

Scientists found the frog the way they found T. nana—by sorting through leaf litter, although no one currently knows exactly what it eats or how it lives.

Smallest mammal. The Kitti’s hog-nosed bat (Craseonycteris thonglongyai), a.k.a. the bumblebee bat, has been dubbed one of the world’s smallest mammals. A threatened native of Thailand and Myanmar (Burma), the bumblebee bat weighs in at 0.071 of an ounce (2 grams) and measures only 1.1 to 1.3 inches (2.8 to 3.3 centimeters) in length.

Kitti’s hognosed bat, or the bumblebee bat, is the world’s smallest bat. Photograph by Merlin D. Tuttle, Science Source

Smallest bird. Continuing with the bee theme, meet Mellisuga helenae, the bee hummingbird, which lives on Cuba and nearby islands. Weighing 0.056 to 0.071 of an ounce (less than 2 grams) and measuring 2.0 to 2.4 inches (5 to 6 centimeters) long, M. helenae has won the title of world’s smallest bird. Not only is it bee-size, M. helenae enjoys feeding on nectar like its namesake. Unlike a bee, however, M. helenae has bright, iridescent feathers.

A bee hummingbird perches on a twig in Cuba. Photograph by Steve Winter, National Geographic

Tell us—what small critters have you come across?

And when its prey dives into cracks and crevasses within a coral reef, the grouper uses its own version of sign language to get help, a new study says.

The fish enlists the assistance of two other predators, the giant moray eel and the Napoleon wrasse, waiting up to 25 minutes for one to come into sight.

A coral grouper near a scuba diver in the Red Sea. Photograph by Reinhard Dirscherl, WaterFrame/Getty Images

When one does, the grouper points its nose toward the concealed prey and starts to shake its body from side to side. This signal is the equivalent of ringing a dinner bell—food is here!

That’s when the interspecies killing team goes to work. The wrasse is the strongman, smashing into the reef and breaking it apart—forcing its prey to flee or get pulverized.

“[Wrasse] have a very powerful jaw, and they can destroy holes that aren’t well constructed,” said study co-author Redouan Bshary, a behavioral ecologist at the Université de Neuchâtel in Switzerland. “They can break coral.”

“Prey will evacuate holes just to avoid getting smashed together with their hiding place,” added Bshary, who observed the behavior during scuba diving research trips to the Red Sea.

While less destructive, morays are no less deadly. Their slim bodies allow them to squeeze into the crevasse to track the prey within. If the fish manages to escape both both wrasse and moray, then the grouper gets one more shot at a meal. (Also see “Herring Break Wind to Communicate, Study Suggests.”)

“Now, while they’ve learned to cooperate, fish don’t share,” Bshary noted. “Whoever gets the prey, swallows it whole.”

Even with multiple parties competing for one food source, groupers are more successful in a group. (See a picture of a goliath grouper.)

When hunting alone, groupers only catch their prey about 1 out of every 20 attempts, Bshary said. When they have help, the ratio is significantly better—about one out of seven, he added.

Group Hunt

Groupers can also use sign language as a call to action. Sometimes before prey has been sighted, groupers will approach a wrasse and moray and shimmy, which translates into a request for a team hunt. The trio will scour the ocean, each utilizing their own unique skill sets. (See pictures of ocean animals’ survival skills.)

“They all go hunting together,” said Bshary, whose study appeared April 23 in Nature Communications. “It looks quite impressive when they come all together and start inspecting.”

Scientists still haven’t figured out why groupers are able to communicate with other species. While people, apes, and some birds are adept at signaling, the scientific community previously thought a fish’s tiny brain wasn’t up to the task.

Bshary and his team have logged a lot of hours underwater to study the grouper’s odd disco, braving pruny fingers and wicked sunburns.

“There’s this idea that you need a large brain to use referential gestures, [but] even a fish with a rather standard brain shows this ability to produce referential gestures,” Bshary said. “This is important. It’s decoupling cognitive abilities from brain size.”

The next step, he said, is to repeat the experiment with the species within the lab to see what other secrets the grouper’s strange signs may unlock.

Researchers have known since the 1950s that Tetrahymena—a freshwater ciliate covered in hair-like projections—has seven sexes, Types I through VII, and each type can mate with every sex but its own (such that Type I can mate with individuals that have Types II through VII, and so on). But exactly how Tetrahymena determines its sexes remained a mystery—until now. (Get a genetics overview.)

The freshwater critter Tetrahymena is covered in hair-like projections. Image by Aaron J. Bell, Science Source

Scientists have discovered that a random process of DNA splicing and rearrangement during development determines which sexes Tetrahymena will become.

Tetrahymena mostly reproduces asexually, dividing into two identical daughter cells. When Tetrahymena reproduces, one of its two nuclei—known as the germline nucleus—doesn’t do anything, and only the genes on its other nucleus—called the somatic nucleus—are activated. (See “Synthetic DNA Created, Evolves on Its Own.”)

But when food is scarce, the organism can opt for sexual reproduction, which creates novel genetic combinations that may give daughter cells a better chance of surviving in a harsh environment.

Such tough conditions also drive the organisms to develop more sexes—and thus more mating options, said study leader Eduardo Orias, a biologist at the University of California, Santa Barbara.

“If you only have two sexes, there’s only a 50 percent chance that someone you meet will be of the right type,” Orias said. With seven different sexes, however, a ciliate’s chances of meeting Mr. or Mrs. Right increase to about 85 percent.

Seven Genes for Seven Sexes

But when scientists sequenced the Tetrahymena genome in 2006, they didn’t find any genes that jumped out at them as determining sex.

So in the lab, Orias and colleagues first deprived a strain of Tetrahymena of food to induce it to start mating sexually. To make things even more confusing, the strain used by the researchers had only six sexes, not seven.

Catching the ciliates in flagrante delicto, the researchers compared genes that were expressed during mating and those that were expressed during normal growth. The researchers identified one pair of genes that was active only during mating. When they inactivated this pair of genes, Tetrahymena no longer mated, making those genes good candidates for those that determine sex. (Also see “Heat Triggers Sex Change in Lizards by ‘Turning Off’ Key Gene.”)

The researchers then scoured the genome sequence of Tetrahymena, and they found six copies of these mating-type genes in the germline nucleus, one for each sex.

These six sexes were clustered together on a small segment of DNA, one right after the other. This array is bookended by the standard start and finish of a protein-coding gene. The next question the scientists had to answer was how Tetrahymena got from six or seven pairs of mating-type genes down to one.

Mating Roulette

In experiments, the team showed that, during mating, Tetrahymena randomly sort, snip, and rearrange the six or seven pairs of the mating-type genes until the organism is left with just one pair of genes on its somatic nucleus.

“It’s as if they had a roulette wheel with numbers on it and just gave it a spin,” Orias said.

Tetrahymena retains the full array of mating-type gene pairs on its germline nucleus for the next time it finds itself hungry and hears the soft sounds of Barry White playing in the background, so to speak.

What’s more, this one gene pair is likely expressed on the membrane of the cell. This makes sense, Orias explained, because Tetrahymena frequently touch before mating. It could be that this touch lets Tetrahymena determine the sex of the individual, not unlike how parents frequently dress their babies in either pink or blue to indicate gender. (See “Behind the Scenes at a NatGeo Baby Cover Shoot.”)

Understanding Tetrahymena‘s strange sexual behaviors has a larger impact than just figuring out how one particular group of species finds a mate, the team added. The research also can help researchers better understand how DNA is spliced and recombined in other species, which is important in the development of immune function and even potentially cancer cells.

If Tetrahymena ever needed a dating service, however, these seven-sexed critters would likely pay a fortune.

Female Ulidiid flies (Euxesta bilimeki) expel and eat ejaculate because it allows them to decide who will be the best father of their offspring, a new study says. (Also see “Female Flies Put Up a Fight to Keep Sex Short.”)

Just as lovers, partners, and spouses frequently exchange gifts and share meals during courtship and beyond, so do many animal species.

Usually, the male builds a nest or provides a food gift for the female, in order to help convince her that his genes are high quality and that he would make a good father. In these cases, the female is directly choosing her mate, and the offspring benefit both from the good genes of the parents and from the food or housing provided by the male. (Read about weird mating and courtship rituals.)

But for some species of mammals, birds, and insects—in which fertilization is internal—the effects of female choice are less obvious to the male. In a process called cryptic female choice, a female can exert control over her baby daddy by expelling a male’s sperm after copulation.

This method is especially effective in species like E. bilimeki, in which the female can store the male’s sperm before using it to fertilize her eggs, according to the study authors.

The Mating Game

Researchers knew that female E. bilimeki would expel and eat sperm, but didn’t know why.

To find out, Christian Luis Rodriguez-Enriquez and colleagues at the Institute for Ecology in Vera Cruz, Mexico, watched 74 pairs of E. bilimeki court and mate. They found that all of the females expelled and ate at least some of the ejaculate that they’d stored in specialized sperm-storage organs. (See “Sperm Tracked in 3-D—A First.”)

When the team looked closer, they found that one-quarter of the females purged all of the ejaculate from their bodies. This meant that the males with whom they recently mated would have no chance of fathering their offspring, according to the study, published recently in the journal Behavioral Ecology and Sociology.

Since females mate multiple times, the amount of ejaculate she expels can help increase or decrease the likelihood that a male will be the father of her offspring. The more sperm she expels, the less likely the male will pass on his genes.

So what makes an undesirable suitor? A male fly that’s too aggressive, the study found: The longer a male pursued a female before mating made the female more likely to expel and consume the ejaculate.

Rodriguez-Enriquez and colleagues hypothesize that this may be because the female grows tired of evading the male and copulates simply to avoid his amorous pursuits. Expelling the sperm means that she doesn’t have to worry about making babies with males who don’t know the meaning of “back off!”

Plus, she just might get some nutritious liquid in exchange for her hassle.

Sperm: It’s Not What’s For Dinner

Ulidiid flies live in the deserts of Mexico and the U.S. Southwest, where water and food are scarce. That made researchers initially suspect that the female flies might be eating the ejaculate for sustenance. (Also see “Beetles Are Thirsty for Sex.”)

To test this theory, the team raised females on one of several different diets: a very high-quality diet containing protein, sugar, and water; a high-quality diet containing sugar and water; water only; and neither water nor food.

The females that were given no food or water for two days did live longer if they consumed expelled sperm after mating, but this consumption had no effect on the survival or longevity of the well-fed flies.

But the starved and thirsty flies were no more likely than the well-fed flies to consume the ejaculate—showing that they don’t eat it for food.

Although mysteries remain about the sperm-eating flies, the results show that females continue to play a crucial role in selecting the father of their offspring—even after mating.