Better brains make one fish, two fish, into lots and lots of fish.
After upgrading their ability to communicate using electrical signals, a group of African fish exploded into dozens of species. This may be the first study to show a link between central brain evolution and increasing species diversity, researchers report in the April 29 Science.
“The brain structure triggered an explosion of signals and an explosion of species as a result,” says Carl Hopkins, who studies neurobiology and animal behavior at Cornell University and was not involved in the new study.
Among mormyrid fish, conversation is literally buzzing. Using specialized electricity-emitting organs in their tails, these African natives string together short shocks into a primitive analog to Morse code, says study coauthor Bruce Carlson, a neuroecologist at Washington University in St. Louis. Mormyrids can’t discuss philosophy, but they can employ this rat-a-tat to send out some basic signals — for instance, “I’m interested in mating with you.” But not every electric fish species has the same grasp of the language, Carlson says.
From behavioral studies, the team demonstrated that some mormyrid species can sense and respond to small shifts in electric pulses. Others can’t. The difference appears to lie in a brain region called the exterolateral nucleus, which interprets electric communication. At some point — Carlson suspects about 50 million to 60 million years ago — this brain region in one group of electric fish got bigger and split in two. And for the most part, the researchers discovered, the descendants of these brain-boosted fish happen to be those with the better vocabulary. “It gives them the ability to detect very subtle changes in the shape of the electrical signal,” Carlson says.
Like humankind after the fall of the Tower of Babel, these new fish took their wider vocabulary and spread. In fact, they burst forth. Since they now understood a broader range of electric pulses, smarty-pants fish could have more varied conversations. The fish themselves diversified to take advantage of those new conversations, Carlson suspects. Based on an analysis of a subset of living mormyrids, the team concluded that fish with the upgrade diversified at a statistically faster rate. In all, 175 living species of electric fish carry the upgrade, while only 32 don’t.
But well-versed electric fish have more to them than just brains, says Curtis Bell, a neuroscientist at the Oregon Health & Science University in Portland. Less-literate fish have electricity sensors just on their heads, for instance, while well-versed fish have them all over. The sensors could have evolved first, and the brain just grew to catch up with the flow of information, Bell says. Carlson acknowledges that it would be difficult to determine which came first — the change in the sensors or in the brain. Regardless, he says the two systems evolved closely together, and the brain was a big part of the electric fish’s shocking expansion.