In other words, the fancier the fishs' communication kit, the more likely it was to come up with new electric discharges and new species that identified one another by those discharges.
Putting it to the test
It all worked out statistically and logically but was it what the fish actually experienced?
"After all," says Carlson, "this sensory world is totally foreign to us. I've worked with these fish a long time, so I can tell a few of the discharges apart by ear. But for the most part I need an oscilloscope to see the differences.
Can the Clade A fish tell the difference between discharges? To test them, Carlson ran behavioral playback experiments on fish caught in Gabon.
"A fish would be going pop, pop, pop and we'd pulse it. Depending on the fish, it would either discharge more rapidly, brrrrrrrrr, or stop discharging altogether.
"But if we repeated the stimulus again and again the fish would stop responding. Once it stopped responding, we hit it with a phase-shifted version of the same pulse. If the fish could tell the difference, the discharge rate or pause duration would increase. If it couldn't tell the difference, there would be no change.
The experiments showed that mormyrid fish in Clade A were able to distinguish among pulses, but other mormyrids (those with the EL brain) were not.
Did the evolution of a fancy signal-processing brain drive speciation in the Mormyridae? "It's always difficult with evolutionary studies to say that any one trait is the cause or the trigger for another," Carlson says. "But in this case we were able to show that the complex signal-processing brain evolved before a burst of speciation, that
|Contact: Diana Lutz|
Washington University in St. Louis