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Humming Fish Reveal Ancient Origin of Vocalization

Ker Than
for National Geographic News
July 17, 2008
 
Human ballads, bird songs, and croaky frog serenades are all controlled by a common brain circuit that first appeared in animals hundreds of millions of years ago, a new paper reveals.

The finding is based on a study of toadfish and their close relatives midshipman fish, which both use their air-filled swim bladders to grunt, growl, and hum to attract mates and defend territories. (See photos of toadfish and midshipman fish.)

A group of rhythmically firing nerve cells in the fishes' brains determines the contraction rate of muscles attached to their swim bladders and controls the pitch and duration of the calls.

Andrew Bass of Cornell University and colleagues mapped this neuronal network in larval toadfish and midshipman fish. The team found that the circuit for vocalizing develops across a specific brain region that includes the base of the hindbrain and the upper spinal cord.

This is precisely the same pattern of brain development seen in other vocalizing vertebrates, including birds, amphibians, and primates such as humans.

The last time all of these creatures shared a common ancestor was more than 400 million years ago, when the evolutionary line that led to toadfish split from the line that eventually led to land vertebrates.

Because the vocalization circuit is found in different groups of animals belonging to both of these lineages, scientists reason that the brain region must have evolved before the lineages split.

"This one circuit in the brain is very ancient, and it's been retained throughout the course of evolution by all animals that vocalize," Bass said.

(Watch video of a male midshipman fish grunting to defend its nest.)




Bass and colleagues speculate that the primitive fish in which the brain circuit first appeared may have been able to communicate vocally.

But it's also possible that the circuit first developed to serve an entirely different purpose and was recruited for vocalization much later in evolutionary history.

The discovery suggests that all amphibians, reptiles, birds, and mammals currently have—or had in their past—the brain machinery necessary for vocalizing.

Sharks and their close relatives, for example, are thought to be silent, but they might still have the brain wiring that controls vocal noises.

What's more, Bass speculates, many more animals might vocalize for communication than we realize.

"How carefully have all animals been looked at for people to know that in fact they don't make sounds?" he said.

"If you don't study animals at the right time of year, you might not pick up that they're making any sounds."

The research is detailed in this week's issue of the journal Science.

"Beautiful Diversity"

Melina Hale, a biologist at the University of Chicago, expressed astonishment at the deep evolutionary history of the brain circuit for vocalizing, and she suspects many other scientists will be surprised as well.

"When we think about vocalization and our own ability to talk or birds' ability to sing, we think of it as very specialized for terrestrial vertebrates," Hale said. "But this isn't a key innovation of ours."

Hale added that even though the different vertebrate groups share the same brain mechanism, each lineage exploits it differently to produce sounds.

Toadfish vocalize using their swim bladders, birds use an organ called the syrinx, and mammals use their larynxes. (Related: "Fastest Known Muscles Found in Songbirds' Throats" [July 10, 2008].)

"There's this fundamental similarity in brain circuits," Hale said, "but then there's this beautiful diversity on top of it."
 

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