#bioacoustics

Meet Gephyromantis (Phylacomantis) pseudoasper

Peak calling activity? Between 3 and 4 am 😡

Its preferred perch? The canvas of my tent 🙃

The decibels of its call? Jet-engine levels 🫨

The sound? Fuckin' Star Wars lasers.

Over 64 years ago, the ocean quacked. Specifically, sailors aboard an Oberon-class submarine in 1960 detected a series of short, odd noises that resembled an agitated waterfowl. But they weren’t the only people to hear the sounds. In the ensuing decades, multiple instances of the strange audio—nicknamed the “Bio-Duck”—were documented, particularly in the waters surrounding Australia and Antarctica, including a soundscape analysis of the South Fiji Basin in 1982. And while the jury is still out on the quack’s exact culprit–or culprits–there’s a new theory. According to experts, what researchers have described as one of the “largest still unresolved mysteries of the Southern Ocean” may be a conversation between underwater inhabitants.

Hello fish, this is a really interesting topic because fish bioacoustics is actually a whole research field, and there are many different mechanical solutions fish have evolved to generate sound. I will focus on how the sounds are physically produced as we already talked about what they are mostly used for in communications.

How fish produce sounds

Fish do not have vocal cords. All fish sounds are produced through mechanical vibration of tissues or hard structures, which then transmit pressure waves into the surrounding water.

Most known mechanisms fall into three main categories:

swim bladder drumming

stridulation (rubbing structures together)

hydrodynamic or skeletal clicks

Some species also combine multiple mechanisms.

Swim bladder drumming

This is probably the most widespread mechanism in fishes that produce tonal or humming sounds.

The swim bladder is a gas-filled organ normally used for buoyancy regulation. Because it is filled with gas, it is also very good at transmitting and amplifying vibrations, which makes it useful as a resonating structure.

In sound-producing species, specialized sonic muscles attach to the swim bladder wall. These muscles can contract extremely rapidly.

When the muscles contract repeatedly, they cause the swim bladder to vibrate like a drum membrane. The gas inside the bladder transmits those vibrations outward as sound waves.

Important details about this system:

Sonic muscles are often among the fastest contracting muscles in vertebrates.

Contraction rates can reach 100–200 contractions per second in some species.

The frequency of the muscle contractions largely determines the pitch of the sound.

In some fish the muscles attach directly to the swim bladder wall. In others they attach to nearby bones that then transmit vibration to the bladder.

The swim bladder itself often has species-specific shapes or internal structures that affect resonance, which means different fish species produce characteristic sounds.

Well studied examples include fishes in the family Sciaenidae (drums and croakers), where large spawning aggregations can produce very loud collective drumming sounds.

Stridulation

Stridulation is sound production through friction between hard body parts.

The term comes from studies of insects, but the mechanical principle is the same in fish.

A fish produces sound by moving two structures against each other so that repeated contact produces vibration. This vibration then travels through the body and into the water.

Common structures involved include:

pectoral fin spines rubbing against the shoulder girdle

pharyngeal teeth grinding together

specialized ridges on bones or fin rays

Catfish are a well-known example. Many species have pectoral fin spines with serrated edges. When the spine rotates in its socket, the serrations scrape against the pectoral girdle bones.

This produces a series of rapid clicks or squeaks.

The motion can be repeated quickly, producing a sustained rasping sound.

Other fish use pharyngeal teeth (teeth located in the throat rather than the jaw). When these teeth grind against each other or against bony plates, they can generate audible clicking or crunching sounds.

Stridulatory mechanisms are common in reef fishes and benthic fishes that have robust skeletal elements capable of producing frictional vibration.

Tendon plucking and skeletal vibration

In some fish, sound production involves rapid movement of bones or tendons that strike or vibrate against other structures.

One well studied example occurs in toadfishes (family Batrachoididae).

Toadfishes produce a distinctive humming call. In this system, sonic muscles contract rapidly and pull on tendons connected to the swim bladder.

The bladder vibrates in response, but the tendon tension and skeletal attachment points influence the vibration pattern, producing a sustained hum.

Some fish also produce sharp knocks or pulses when skeletal elements briefly collide during rapid movements.

These sounds are usually short-duration pulses rather than continuous tones.

Jaw snapping and rapid mouth movements

Another sound source in some fishes is rapid movement of the jaws or mouth structures.

This can occur when:

the jaws snap shut quickly

teeth collide

mouthparts strike against rigid skeletal structures

These movements produce brief pressure pulses in the water.

In reef environments, some fish species generate audible clicking sounds through repeated jaw movements during social interactions.

In many cases the sound is actually a secondary mechanical consequence of rapid feeding or territorial movements rather than a structure evolved purely for sound production.

Resonance and sound amplification

Regardless of how the initial vibration is produced, the sound must be transmitted into the surrounding water.

Several anatomical features help with this:

the swim bladder acting as a resonator

body tissues conducting vibration

skeletal structures transmitting energy outward

Because water transmits sound very efficiently, even relatively small vibrations can travel significant distances.

In species with specialized sonic systems, the swim bladder often functions as a resonant amplifier, which increases sound intensity and allows the signal to propagate farther.

These mechanisms represent different evolutionary solutions to the same problem: creating vibrations that propagate efficiently through water as sound.

Get ready, the aliens are among us!

These two sperm whales are acting weirdly. What Carl does have onto its head? And why George is wearing a helmet?

Well, because the aliens are among them!

Researchers use those devices, called DTAGs (better shaped in reality, as you may see below). They are equipped with different sensors, including one or more hydrophones, to record the animals' vocalizations. Basically, they are listening to them and discovering interesting facts about their biology and ecology.

This is also part of my career path, as I am shaping my experiences within the bioacoustic world of cetaceans. Of course, it's not possible to really know what they are saying each other, but within decades, researchers managed to find some categories of sounds and associates them with particular behaviours. Thanks to these devices, it has been discovered that the huge head of a sperm whales is used for sound production and how it works (Madsen, P. T. (2002). Sperm Whale Sound Production.). They produce a variety of sounds, including echolocation clicks to feed in the deep ocean, and codas for social communication.

Isn't it so fascinating? For me, this world is irresistible. I found myself always so curious and full of questions. Of course, in particular of sperm whales.

Across human cultures and languages, adults talk to babies in a very particular way. They raise their pitch and broaden its range, while also shortening and repeating their utterances; the latter features occur even in sign language. Mothers use this exaggerated and musical style of speech (which is sometimes called “motherese”), but so do fathers, older children, and other caregivers. Infants prefer listening to it, which might help them bond with adults and learn language faster.

But to truly understand what baby talk is for, and how it evolved, we need to know which other animals use it, if any. The great apes don’t seem to vocally, but might use a gestural equivalent. Squirrel monkeys and rhesus macaques use special calls when talking to youngsters, but they’re very different from human baby talk, which is a modified version of normal speech. Zebra finches are closer to us: When singing in front of juveniles, adults add longer pauses between musical phrases and repeat introductory notes. Greater sac-winged bat mothers also change their pitch and timbre when signaling to pups, but again, it’s hard to tell if they’re using a distinct call or doing something analogous to baby talk. To make an inarguable case for the latter, you’d need to study a species that talks with both infants and older peers using the same standardized, identifiable call. In other words, you’d need a dolphin.

Every bottlenose dolphin produces its own unique signature whistle, which is the closest thing any animal has to a human name. Dolphins can recognize individuals through these whistles and will sometimes copy one another’s, perhaps as a form of address. They use their whistles frequently, to announce their position when separated from their pod, or as an introduction when meeting up with new groups. Calves develop their own signature whistles based on those they hear around them, and once learned, the whistles can go unchanged for at least 12 years.

Laela Sayigh, a zoologist at the Woods Hole Oceanographic Institution, has been studying the signature whistles of bottlenoses in Sarasota Bay, Florida, since 1986 as part of the world’s longest-running study of wild dolphins. She and her colleagues regularly catch these animals, check their health, and record their calls before releasing them. Sometimes, they catch mothers and calves together, and the animals exchange signature whistles throughout the process. By analysing 19 such moments, recorded over 34 years, Sayigh’s student Nicole El Haddad showed that mothers raised and widened the pitch of their signature whistles when calling to their calves, just as humans do when talking to their babies.

“We were just blown away by how consistent the effect was,” Sayigh told me. Between their intelligence and strong personality, dolphins behave unpredictably enough that scientists who study them are used to gleaning faint patterns amid messy data. But in this study, every mom changed its signature whistle around its calf in the same way. “The data are extraordinary and impressive,” Sabine Stoll, who studies language evolution at the University of Zurich, told me.

Dolphin baby talk isn’t exactly the same as ours—dolphin whistles don’t get more repetitive—but it’s certainly “the most convincing case of child-directed communication found in nonhuman animals to date,” Mirjam Knörnschild from the Museum of Natural History in Berlin, who led the study on sac-winged bats, told me. And its existence in a species separated from us by more than 90 million years of history is likely a “stunning” example of convergent evolution, Stoll said.

If both species evolved baby talk independently, perhaps they did so for similar reasons. Human parents can better grab their infants’ attention through high-pitched baby talk than through normal speech, and dolphin mothers might do the same. Keeping her signature whistle but raising its pitch “would be a pretty foolproof way for the mom to say ‘This whistle is meant for you’ to the calf, and for the calf to know My mom is talking to me right now and no one else,” Sayigh said. That specificity would allow both of them to keep close contact in a raucous ocean where many dolphins might be sounding off at once.