A Tiny Frog That Should Be Deaf Hears With Its Mouth

GardinersFrogFar away on the islands of Mahé and Silhouette in the Seychelles in the western Indian Ocean can be found one of the world’s smallest frog species, Gardiner’s frog, just 10 to 11 millimeters in length. These fogs should be deaf, because they lack a middle ear (in humans, that’s the bit with the eardrum). That part of the ear is thought to be a necessary adaptation to life on land, helping sound to make the transition from air to tissue, where it can be translated into nerve impulses that reach the brain. But Gardiner’s frog has evolved a different method for hearing — it uses its mouth, reports a team of French researchers in a new study in PNAS.

Scientists had been somewhat perplexed by Gardiner’s frogs because, despite lacking the necessary equipment for hearing, the amphibians appear to have no problems communicating. The French team began by recording calls from frogs on Silhouette and playing them back over loudspeakers to frogs in their natural forest habitat. When a male heard the call from one of its own species, it called back a response. But the frogs didn’t respond when they heard sounds from other species. They were definitely hearing and well enough to make species distinctions.

Dissecting the frogs might be a classic technique in the classroom, but here it just wasn’t an option; these frogs are far too small to see anything useful with dissection. Instead, the researchers used a technique called X-ray synchrotron holotomography to image the frog’s inner anatomy. Then they used that data to create a computer simulation of the amphibian and determine how the sound was traveling through the frog’s head.

Previous studies had suggested that sound passed through the lungs on its way to the inner ear, where it would be translated into nerve impulses. Another theory was that sound was conducted through the animal’s bones. But the computer simulations revealed that the mouth (or oral cavity as the scientists name it) is actually the ideal anatomical structure for amplifying sound. The cavity resonates sound at a frequency that nearly matches the frequency most commonly found in the frogs’ calls. The researchers also found that the tissue that separates the inner ear from the inner mouth in Gardiner’s frogs is very thin, and there are fewer layers of tissue, which helps the sound to pass through.

The frog’s mouth might also play a role in determining the direction of sound, the researchers suggest. And, they note, this discovery shows that the middle ear isn’t quite so necessary for life on land as had been thought.

Image credit: R. Boistel/CNRS

Choosing The Closest Guy Sometimes Makes Sense — For A Frog

strawberryfrogThe strawberry poison frog (Oophaga pumilio) is instantly recognizable, with its bright red coloring that announces to potential predators “don’t eat me.” These tiny amphibians — which reach only a couple centimeters in length — are common in their range, stretching from Nicaragua through Costa Rica and into Panama.

In 2004 and 2005, a group of researchers, led by Ivonne Meuche of the Institute of Zoology in Hannover, Germany, tracked strawberry poison frogs in Hitoy Cerere Biological Reserve in Costa Rica during their mating seasons. The scientists wanted to figure out how females chose their male mates, and the results of their study were just published in Frontiers in Zoology. The surprise — females usually just pick the first guy they encounter.

In this species of frogs, females are free to mate with the male of their choice. A female will move into her chosen male’s territory and engage in a little nooky, laying her eggs in the leaf litter. Ten days later, the female will move her newly hatched tadpoles into small pools of water in the leaves of bromeliads or bananas — one tadpole per leaf — and feed them.

When it’s time for the frogs to pair up and make more little frogs, the males call to females. A female could potentially use those calls to judge the males, choosing the best out of the lot. But that’s not what they do, the researchers found. Instead, females mostly just pick the guy that’s closest.

This would probably not be a good strategy if you were a human. Just think about the creeps you meet when out at a bar. Proximity is often not a plus in such a situation. But life is different if you’re a strawberry poison frog. Potential guys are not conveniently grouped together. If a girl frog wants to take a look at multiple males and judge their worthiness, she runs the risk of expending too much energy, which could have gone into producing better, possibly more successful eggs. (Only about a fifth of the eggs laid during the study hatched. Most dried out or were lost to predators.) And if she’s too choosy and doesn’t find a mate, then she might not be able to have any of her eggs fertilized.

Not mating at all is a big risk because there are usually more females than males, at least in the population studied. Under such a situation, it might be better to just pick the first guy you meet. (I imagine there are some universities where the female-male ratio is similar to strawberry poison frogs and thus even the creepy guys get girlfriends.) But while that might mean that some loser males get to pass on their genes, every non-picky female will get to pass on hers. And when you’re a strawberry poison frog, that’s what matters in life.

Image credit: Ivonne Meuche, Oscar Brusa, Karl E. Linsenmair, Alexander Keller and Heike Pröhl, via EurekAlert

Bad News For Animals That Live In The Subnivium

voleYou’ve probably never heard of the “subnivium.” That’s because it’s a term that scientists just made up (they do that). The group of ecologists and biologists, led by Jonathan Pauli of the University of Wisconsin, say in an article recently published in Frontiers in Ecology and the Environment that the subnivium is the seasonal refuge that occurs below the snow where there’s environmental stability. It’s cold there, with temperatures near freezing, but this region serves as a retreat from the harsh, sometimes changeable environmental conditions above.

A healthy subnivium contributes to a healthy ecosystem. Insulated soil lets microbes and fungi breathe and proliferate and process organic matter. Plants benefit from increased carbon dioxide and warmer temperatures, especially during late winter and early spring. Many animals — a list that includes invertebrates, amphibians, reptiles, birds, and small mammals — will use the region to hide out during the winter, feeding off each other or any vegetation they can find. There are whole ecosystems below the snow.

But warming conditions have led to many changes in the patterns of snow in the Northern Hemisphere (that’s where most of the world’s snow is found): The month of maximum snow cover has shifted from February to January. The spring melt is about two weeks earlier than it was decades ago. The extent of land covered in snow in the winter has been shrinking. In many places, the amount of time when snow is more likely than rain has decreased by more than one and a half months. Snow depths are decreasing, as are snow packs. The list goes on and on.

“Snow cover is becoming shorter, thinner and less predictable,” Pauli said in a statement. “We’re seeing a trend. The subnivium is in retreat.”

With all these changes, the stability that the subnivium provides is therefore disappearing. What does that mean for the ecosystems that thrive there? One worry is that organisms, such as plants, that are exposed to cycles of freezing and thawing could experience tissue damage. Others, like voles or insects, that lose the layer of snow that hides them may be subject to predation by birds or other critters.

“Decay of the subnivium will affect species differently, but be especially consequential for those that lack the plasticity to cope with the loss of the subnivium or that possess insufficient dispersal power to track the retreating range boundary of the subnivium,” the researchers write. Those that can adapt to the loss of stability or can move to places where there’s still reliable snow cover will do better than others. Adapt or move — those seems to be the only two options for surviving climate change.

Image of vole courtesy of flickr user musubk

Frogs Take Up Residence In Manmade Caves In Portugal

frogSerra de Estrela Natural Park encompasses a section of Portugal’s highest mountains. The largest natural conservation area in that country, the park is full of wildlife. There are wall lizards, otters, wild cats, water moles, and even wolves, to name a few species.

Back in the 1950s, before the park’s boundaries were delineated, several artificial caves were created in the area. These were drainage galleries — small, horizontal tunnels built to push a few meters into the hillsides, often with a small stream of water running through.

In May 2010, according to a study from Portuguese researchers published in the Journal of Subterranean Biology, an unexpected resident was found in the galleries: the Iberian brown frog (Rana iberica). These frogs are often found the the mountains, and also live in other moist habitats, such as ponds and soaked fields and humid meadows. But they’d never been found in caves.

The researchers returned to the galleries the next year, every three months at first, then every month from December 2011 to December 2012. Adult frogs were found throughout the year, in day and in night, on the ground, tucked into crevices, swimming in water, and even climbing up the walls (as in the photo). Usually the frogs were living deeper into the galleries, more than five meters from the entrance where the daylight had dimmed.

The frogs mated throughout the year, then laid their eggs within the caves. The tadpoles hatched there, and the frogs grew up there. It wasn’t necessarily an easy life, however. Tadpoles would sometimes eat frog eggs — a first for this species — and tadpoles themselves were occasionally eaten by fire salamander larvae.

This may be just be a case of a species moving into a newly available habitat, but it might instead be a consequence of changes humans have made to the frogs’ home. “Nearby breeding sites may have disappeared or experienced disturbance,” the scientists hypothesize in their paper. But they also note that this region of the world is expected to experience dramatic climate change in the near future, with changes that include increasing aridity and greater fire activity. By taking up residence underground, the frogs may be smartly fleeing the changing landscape above. It could be a hopeful sign that the world’s wildlife will do its best to survive the changes we are making to their planet.

Image courtesy of Gonçalo M. Rosa, via EurekAlert

Cow Patties May Have Helped A Toxic Toad Invade Australia


The center of Australia, is a dry, hot, unforgiving landscape, but the edges of the continent are more diverse, with beautiful wooded mountains, lush rainforests and thriving cities. And farms. Lots of farms. About half of Australia is, in fact, covered in land used for grazing livestock (though I should note that some of that grazing does take place in rather harsh areas of the interior as well as nicer bits closer to the coasts). Like many lands around the world, Australia has been incredibly altered by humans.

Altering landscapes often facilitates the spread of non-native species, letting them become invasive pests. The phenomenon is best known among invasive plants, like purple loosestrife, but it seems to hold true for some animals as well, like the cane toad in Australia.

Cane toads, natives of South America, are a big problem in Australia. They first arrived in Queensland in 1935, deliberately brought to the country to help keep beetles in control to improve sugar yields. That plan didn’t pan out, and the toads quickly began to spread across the country. One more amphibian in the landscape might not seem like a huge problem, but cane toads secrete a toxic poison that kills most anything that eats them, even crocodiles. As cane toads have spread, they’ve devastated populations of many native Australian animals.

I first got interested in the cane toad when I blogged about a study in which scientists successfully taught cute critters called quolls to not eat the toads by feeding young quolls young toads (not yet poisonous) laced with a chemical that made the toads taste bad. And then last year, I got a chance to meet the mastermind behind the study, University of Sydney herpetologist Rick Shine, who I profiled in Science magazine. Shine became one of the leading cane toad researchers in Australia after the toads invaded his long-term research site outside Darwin.

In one of his more recent studies, published last November in PLOS ONE, Shine’s group looked at how cow patties might help cane toads survive the hot, dry Australian summer. Members of the group had noticed that toads could often be found on or near cow patties, and they wondered whether the amphibians were indeed more likely to hang out there.

The researchers walked transects set up on a farm located near their research site, recording the locations of both cane toads and cow patties during the dry season. Seventeen of the 26 toads they found were sitting on cow patties, and the remaining nine were fairly close to the piles of poo.

“Toads were found on cowpats more often than expected by chance, and toads that were not on cowpats were closer to them than would be expected by chance,” they write in their paper.

The cow patties, the researchers surmise, provide both a moist environment and food, in the form of dung beetles, which helps the cane toad survive the harsh Australian dry season.

Cane toads are already known to take advantage of water sources, like ponds, set up to supply livestock. Add in the continent’s 300 million cow patties deposited each day — that’s 28.5 million cattle each producing 12 cow patties per 24 hours — and livestock grazing has obviously provided the cane toads with a friendly environment that has helped them spread. Humans have tamed much of Australia to make it more liveable for themselves, but they also made it more liveable for cane toads.

Cane toad image courtesy of flickr user blundershot