Seahorses are known for making little "click" sounds while they're feeding, but a group of Austrian and Brazilian researchers have discovered a completely new, never before heard type of seahorse vocalization, which they describe as a "growl."
While it has been known since at least the late 1800s that seahorses could make sounds, studies investigating the functions of those sounds have been rare. To begin to address that dearth of research, scientist T. P. R. Oliveira rounded up a group of longsnout seahorses, Hippocampus reidi, a species which is found along the eastern coast of the Americas from Cape Hatteras to Brazil, and in the Gulf of Mexico.
Then, having equipped an aquarium tank with a hydrophone, the researchers took audio recordings of seahorses in three different situations: feeding, courtship, and stress. Feeding and courtship are straightforward enough. To induce a stressful situation, the researchers simply handled the seahorses. Oliveira explains that a researcher held onto each seahorse's body at a distance of two centimeters from the hydrophone. "Although handling has a level of artificiality," write the researchers, "it does provoke fish to produce sounds as if they were captured by a predator. Seahorses are frequently grabbed and held by predators such as frogfish before being swallowed."
As expected, the seahorses "clicked" during feeding. Their clicks, which are audible to humans, were also emitted during courtship especially on the third day immediately before copulation. While it isn't clear what the function of clicking during feeding is, it is thought that it helps couples to know when its time to get busy and to help maintain the pair bond. In some species, males also use clicks in competition over females. The acoustic properties of feeding clicks and courtship clicks are different, though, which leads Oliveira and colleagues to suspect that they may be addressed to receivers at different distances, perhaps, or to different audiences.
What was surprising, however, was the stress-induced "growl." This was the first study to ever describe this type of seahorse sound. Growls were never emitted during social interactions with other seahorses, only when being handled by a human experimenter. If the seahorses indeed perceived that experience as similar to being captured by a predator, then it is reasonable to assume that the growls are part of their natural vocal repertoire.
Why make noises when captured? For fish, nobody really knows.
The predominant assumption would be that they warn other seahorses about the presence of a predator, like a monkey alarm call. Or perhaps they work to attract a second predator which would then attack the original predator, allowing the seahorse time to escape. The problem with that, given the acoustic information that Oliveira collected, is that the growls aren't audible enough at large distances for either of those possibilities. But Oliveira and colleagues have another idea: "growls may constitute an additional escape mechanism because sound production is accompanied by body vibrations, which might startle predators." The hypothesis isn't completely bonkers, since catfish, lizards, and some birds also vibrate while making noise to escape becoming somebody else's dinner.
I just wish the researchers had included a recording of all these sounds as a supplement to their paper. Because what could be more adorable than tiny angry seahorse growls?
Header image: Cliff/Wikimedia Commons.