Toadfish Halobatrachus Didactylus 2014 05 21
It might not be much of a looker, but the Lusitanian toadfish (Halobatrachus didactylus) sure can hold a tune. Image: Joao Pedro Silva, Flickr

Do fish sing? Sounds like a silly question. Birds sing, whales sing and frogs sing (in a manner of speaking) ... but fish? Yes, it's no joke. Scientists are discovering that fish are vocalists too.

To human eyes, the Lusitanian toadfish (Halobatrachus didactylus) is no beauty. But to female toadfish, the songs of the males are apparently beautiful music. Dr Clara Amorim at ISPA (Instituto Superior de Psicologia Aplicada - University Institute) in Lisbon, Portugal, is fascinated by fish vocal communication, and it's the main focus of her research. The toadfish she studies live in the shallow, murky waters of Portugal's Tagus River estuary.

Toadfish don't just sing solos, "they form choruses like frogs", says Amorim. In this species, both males and females make sounds, but it's the males that have a broader "musical" repertoire. Researchers including Amorim have described the range of toadfish sounds as grunts, croaks, double croaks and boatwhistles.

Not unlike some human teenagers, juvenile fish make sounds "mostly of single grunts", explain Raquel Vasconcelos and Friedrich Ladich, authors of a 2007 study on these homely-looking fish. Recording fish sounds from wild-captured toadfish held in tanks, they found that grown-up toadfish use more sophisticated sounds than the youngsters. Adults emit a whole series or "train" of grunts.

Sounds are produced when the fish rapidly contract muscles embedded in their swim bladder (an expandable air-filled sac that helps fish control their buoyancy). "They actually use the swim bladder as a drum, with each muscle contraction producing a sound pulse," says Amorim. So toadfish, it seems, are not only singers, but also percussionists. 

“So toadfish, it seems, are not only singers, but also percussionists.”

Male toadfish ramp up boatwhistle "arias" when they are trying to interest females in entering their nest, she explains. Males continue to boatwhistle once females are inside their nest, a behaviour Amorim hypothesizes might stimulate females to release their eggs. In this species, females lay eggs and then abandon them, leaving all the baby duties to the males. So singing males, thinks Amorim, might be advertising their credentials as good dads.

"Males that sing more have a higher lipid [fat] content in their body,” Amorim has found. That's important to being a good dad because males tending to the nest are too busy babysitting and warding off intruders to find much food for themselves – so a nice store of fat is good to have. Toadfish are one of the many fish dads that sometimes cannibalize their own eggs (more on that here) when they’re low on energy, so Amorim suspects that fatter males provide better care, in part because they eat fewer of their own eggs.

Singing has also been discovered in cichlids, as investigated by Karen Maruska in the social cichlid fish Astatotilapia burtoni. In this fish too, vocalising is a means for matchmaking. Dominant males produce sounds while performing quivering or tail-waggling displays for the female. 

Curious to know how fish females perceive these sounds, Muruska placed males and females in tanks, used hydrophones to record fish sounds, and tested their sensitivity to sound by placing electrodes into their brain case (like a human EEG). She found a drastic difference in female hearing ability depending on their breeding stage. When ready to spawn, females were all ears, but when already mouth-broo/all-articles/2014/may/07/honey-i-ate-the-kids-infanticide-in-the-animal-kingdom/ding their eggs, they were no longer so sensitive to crooning males. (In this species moms do all of the childcare). 

Courtship is a multi-sensory experience in these cichlids, and songs are not the only cue females use to assess potential mates. Males of this species urinate to signal their dominance to other males and to advertise to females, and cichlids are also highly visual. "Acoustic cues probably provide some additional information used during mate choice decisions," says Maruska.

The number of fish known to sing, or at least make sounds, is growing. And they are not just singing for love. Sometimes they sing for their supper (to improve their success at finding food), to defend a territory, or to provide a warning signal to other fish. So when it comes to what we know about animal vocalists, it's not over until a toadfish sings.

Video: Maruska KP, Ung US, Fernald RD (2012) 

Top header image: Gavin Clabaugh, Flickr