Close to the forest floor in the mountains of Brazil, a tiny spot of neon orange makes a soft, cricket-like chirp. This male pumpkin toadlet — a frog only about the size of the tip of an adult human’s finger — is hoping to find love. But it had better not rely on its conversational skills. Both the males and females are deaf to their own calls, a new study shows. Instead, the throat swellings that accompany their chirps might be what catches a lady’s’ eye. The soft calls themselves may simply be leftovers from a long ago ancestor — and an example of evolution in action.
In many species of frogs, males grab the attention of would-be mates with a serenade. But frogs don’t have visible outer ears, as humans do. Instead, “there’s a disc behind the eye,” notes Sandra Goutte. She works at the University of Campinas in Brazil. As a herpetologist, she studies reptiles and amphibians. That disc, she notes, is the tympanum, or ear drum, of its middle ear. Sound waves hit the frog’s ear drum directly and get transmitted to the inner ear. This membrane helps to increase the amount of sound energy transmitted to the inner ear.
In that inner ear, tiny hair cells are arrayed in organized ranks. They aren’t real hairs. These just resemble them as they bend and sway in response to sound waves. Their movement transmits sound signals to the brain. In people, high frequency sounds — such as the pumpkin toadlet’s call — will bend hair cells at the base of the inner ear. Low frequency ones, such as the thud of a scientist’s foot on the forest floor, tickle hair cells farther inside.
But frogs don’t have these well-organized ears. That includes two species of pumpkin toadlet — Brachycephalus ephippium (BRAK-ee-she-faal-us Eh-FIF-ee-um) and B. pitanga. “The weird part about these frogs [is that] they don’t have a middle ear that seems to be important,” Goutte says. “But they have a call."
The toadlets are very tiny, and so is their chirp. “This is the softest call I’ve ever heard,” Goutte says. To detect it, he says, “You need to know what you’re listening for.” Indeed, she had to strain to pick up their chirps. That led them to question whether the toadlets, which lack a middle ear, could hear their own calls.
To find out, Goutte spent two years in the laboratory and the forest working with the tiny animals. They were not always happy to have her around. “They do this angry arm-waving,” she says. “It’s supposed to be scary, but it’s not because they’re so small.”
Inner ear investigations
If pumpkin toadlets could hear their calls, Goutte and her colleagues reasoned, they should respond in some way. The males might turn toward the sounds (supposedly from their rivals) or change their own calling behavior. Females, in contrast, should walk toward the sound of their suitor’s songs.
In fact, nothing happened.
“We played their own calls in the lab or in the field,” Goutte notes. And no matter how many calls they played, the target frogs did nothing. “It never worked,” she says. “They really didn’t care. I was desperate. I wondered what I was doing wrong.”
But perhaps the frogs instead detected the sounds through their bodies. Sounds travel through the air in waves. Those vibrations might wiggle the sensitive skin of a tiny frog enough to travel through their bodies to the inner ear.
To test that, Goutte brought more pumpkin toadlets to the lab. She placed them on a vibrating plate, which she shook at the same frequency as the frogs’ calls. The frogs’ chests did vibrate. But again, the animals showed no response.
By looking carefully at the tiny ears of the pumpkin toadlets, Goutte found that the frogs weren’t just playing hard to get — they truly were hard of hearing. Their middle and inner ears were underdeveloped. These animals had no eardrums. What’s more, their inner ears had no membranes to help transmit sound. And the tiny hair cells that should sway in response to sound waves were sloppy and unorganized. While the frogs’ inner ears could respond to louder, low sounds, they never sensed the soft, high pitches of their own voices.
Even if the calls could vibrate the frog’s bodies enough to reach the inner ear, Goutte now notes, it wouldn’t matter. Without a strong inner ear to process the high, soft sounds, those calls would always go unheard. Goutte and her colleagues published their findings September 21 in the journal Scientific Reports.
“I actually think it’s a very cool and interesting study,” says Eva Ringler. She studies behavioral ecology in Austria at the University of Veterinary Medicine Vienna.
Many would have given up
Walter Wilczynski is a neuroscientist — someone who studies the brain — at Georgia State University in Atlanta. “When I first heard about [these findings] I was skeptical,” he says. After all, it’s much easier to prove that something does exist than to prove that a stimulus has no effect. “But [the team] looked at a lot of different evidence. And it’s difficult to argue with the fact that the frogs are insensitive to the frequencies of their call.”
“What makes this good science is they got a negative result, basically,” he says. “They looked at the calls and the auditory system and couldn’t find a response.” Many scientists might have given up, Wilczynski notes. Instead, Goutte’s team persisted until it found an answer. He concludes that this was “good science because it’s very thorough.”
Producing a call that you can’t even hear seems to make no sense. But the call may still serve a purpose. The toadlets inflate their bright orange throats when they chirp, Ringler notes. Frog ladies may now be on the lookout for those neon orange chin sacs. That vocal sac, she says, “is the main visual the frogs seem to need.”
This means the call itself could be vestigial — something that used to have a purpose, but doesn’t anymore. “You don’t see vestigial behaviors generally,” says Wilczynski. Vestigial body features are more common. For example, the human tailbone is thought to be one. Behaviors lacking any purpose are harder to find.
With the toadlets, the visual appeal of the throat might have replaced the sound the frogs’ ancestors once used to flirt, Goutte says. “It looks like we’re witnessing evolution in the making,” she concludes, “with one communication system taking over from another one.” As time goes on, the calls may get softer and softer. Eventually, they might fade out of the forest entirely.
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amphibians A group of animals that includes frogs, salamanders and caecilians. Amphibians have backbones and can breathe through their skin. Unlike reptiles, birds and mammals, unborn or unhatched amphibians do not develop in a special protective sac called an amniotic sac.
behavior The way something, often a person or other organism, acts towards others, or conducts itself.
behavioral ecology The study of the evolutionary basis of animal behavior. It looks at the environmental conditions animals must deal with in order to survive and reproduce. Scientists who work in this field are called behavioral ecologists.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
colleague Someone who works with another; a co-worker or team member.
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
evolution (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. Or the term can refer to changes that occur as some natural progression within the non-living world (such as computer chips evolving to smaller devices which operate at an ever faster speed).
field An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory. (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).
forest An area of land covered mostly with trees and other woody plants.
frequency The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.
hair cells The sensory receptors inside the ears of vertebrates that allow them to hear. These actually resemble stubby hairs.
herpetology The biology of reptiles and amphibians. Scientists who work in this field are known as herpetologists.
journal (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.
matter Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."
membrane A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.
neuroscientist Someone who studies the structure or function of the brain and other parts of the nervous system.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
reptile Cold-blooded vertebrate animals, whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles.
sound wave A wave that transmits sound. Sound waves have alternating swaths of high and low pressure.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
transmit (n. transmission) To send or pass along.
tympanum Also called the ear drum, it’s a membrane that vibrates in response to sound. In mammals and people with visible, outer ears, the tympanum is located out of sight. In species such as frogs, however, the tympanum is sometimes visible as a round spot behind the animal’s eye.
vestigial An adjective for some existing structure (such as body part or organ) that over many, many generations seems to have lost any useful function. It may be much smaller than a functional organ in another species, or may be only partially developed.
veterinary Having to do with animal medicine or health care.
vibrate To rhythmically shake or to move continuously and rapidly back and forth.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
Journal: S. Goutte et al. Evidence of auditory insensitivity of vocalization frequencies in two frogs. Scientific Reports. Vol. 7, published online September 21, 2017. doi: 10.1038/s41598-017-12145-5.