The Johns Hopkins Gazette: February 24, 2003
February 24, 2003
VOL. 32, NO. 23


Funny Fish, Serious Science

Behavior of odd-looking species opens window on evolutionary processes

By Michael Purdy

Johns Hopkins Gazette Online Edition

No doubt about it, Eric Fortune runs with a colorful crowd. The young assistant professor of psychological and brain sciences enthusiastically peppers his conversation with strange and entertaining anecdotes about professional colleagues like the scientist who smeared praying mantises with Vaseline, or the researcher who identified eye muscles in swordfish that gave up acting like muscles and went to work as heaters instead.

And he's got more than a few stories to tell about his own research in odd-looking species of fish known collectively as weakly electric fish.

Eric Fortune, an evolutionary biologist, studies several species collectively known as weakly electric fish to probe the evolution of basic nervous system mechanisms.

"See that little ball of gold behind the fish's eyes?" he asks. "That's its brain."

The odd and the unexpected should come as no surprise around Fortune because he is by trade an evolutionary biologist, an avid student of the ever-expanding bag of tricks, re-engineering, creativity, misdirection and out- and-out bizarreness that life uses to try and get a leg, fin or follicle up in the never-ending competition for the resources it needs to reproduce.

What's a biologist doing in the Psychological and Brain Sciences Department of the Krieger School? Fortune uses behavioral studies to probe the evolution of very basic nervous system mechanisms.

He fondly remembers the exact moment when he began to realize what an outstanding window behavior could be for evolutionary study. It was during a class on primates in his undergraduate days at the University of Chicago.

"The instructor told a story about how sometimes when powerful, high-status male baboons were off defending the group against some predator or perceived threat, a lower-status male would sneak around and mate with the females," recalls Fortune, who still clearly relishes the irony of that bit of baboon deviousness. "And I thought to myself at that moment that behavior was such a powerful force in evolution that I couldn't ignore it."

Fortune had been intent for some time on spending his life studying evolutionary biology, but there'd always been a single sticking point in that prospect--the fact that so many evolutionary processes were rooted in events that took place long ago and over the course of many generations.

After another college course on the neurobiology of song production in songbirds, he became convinced that "looking at behavior could give me a way of looking at evolution that didn't require speculation about past processes."

His path to behavioral studies reveals an aspect of Fortune that's less than obvious. He clearly enjoys the creativity and bizarreness so prevalent in evolutionary biology, but he is doggedly insistent on removing speculation and the potential for ambiguity from his science. He doesn't have any personal interest in fish but chose to begin his research program in weakly electric fish because they are "an ideal system for studying the questions that I'm interested in."

Several species of weakly electric fish are found in South America and in Africa. The two groups evolved independently, and in recent evolutionary time each has undergone a "species explosion," a sudden, dramatic increase in the number of closely related species of the fish.

"There's been a huge radiation of species, all based on maybe just a few founder populations," Fortune explains. "And there are distinct neural and behavioral differences between the various animals that scientists can compare in order to understand how particular neural implementations control new behaviors."

South American weakly electric fish have long, flat, triangular bodies shaped like a knife blade, with a single fin running along the bottom of the body. Some species are sold in pet stores as "glass knife fish," others as "brown ghosts" or "black ghosts." For a look at one, pay a visit to the Fortune lab's Web site:

Like many aquatic species, they can produce and detect electric fields to help locate objects in their environment and characterize their shapes. The fish also use the fields in mating and social communications, so their perception of the fields affects a wide range of behaviors.

Using artificially generated electric fields and direct recording of the activity of neurons in the fish's brains, Fortune works to see how different behavioral responses are encoded at the level of individual nerve cells. The fish work well for him because their nervous system is relatively simple and because of a number of other characteristics.

"Recording the activity of neurons from awake, behaving animals is a difficult task because most behavior that we're interested in requires that the animal move--that is, have muscle contractions," Fortune explains. "Not so in these fish! We give them curare [a paralyzing agent], and although the fish is immobilized, the electric behavior is completely normal. As a result, we can make specialized recordings, called intracellular recordings, from the neurons that control the behavior while the behavior is occurring."

Fortune finds the South American species of weakly electric fish particularly fun to study because they don't distinguish their own electrical signals from those of nearby fish.

"There is a concept known as 'corollary discharge'--if you're going to touch a table, a motor program is sent not only to your arm, hand and finger but also to brain areas that process sensory information," he explains.

This additional signal--the corollary discharge--alerts the sensory system that sensory input should be coming in soon from the finger, and if the input comes at the right time, the sensation is perceived as being self-generated.

"This corollary discharge occurs in well-understood brain circuits in the African weakly electric fish," Fortune says. "The fish I study--the South American Gymnotiforms--don't have a corollary discharge and therefore do not recognize whether their electrosensory input is from another fish's electric field or from their own. That's cool because when the fish are in the experimental recording tank, they will perceive artificial electric fields as their own."

Fortune's initial research on the fish has been centered mostly on jamming avoidance, a behavior the fish use when they get close to each other and their electric fields combine to create an interference pattern.

"If the interference occurs at a certain set of low rates, the pattern impairs the ability of both fish to electrolocate," Fortune says. "The fish have a behavior to avoid this problem: The fish with the higher frequency raises its frequency, and the fish with the lower frequency lowers its frequency."

This adjustment raises the frequency of the interference pattern to levels that no longer impair the perceptions of either fish.

"So, how can we relate this to human experience? Luckily, you can relate it to watching TV," Fortune says. "As everyone knows, television is actually a series of still images flickering rapidly. Now imagine a TV that flickered at a slow rate, like two or three times a second--like a strobe light at a disco. Your perception of movements would be greatly altered and impaired. We avoid this problem by using a higher flicker rate in movies and on TV. Remarkably, it appears that both fish and humans use the same neural mechanisms for these perceptual processes."

Fortune likes to point out that inserting a $10 probe from Radio Shack into the tank with the fish allows him to record their electric fields as tones or "song." The effect is somewhat reminiscent of whale song--if whales were taught to sing by Philip Glass or some other modern composer fond of low dissonance. (These tones can also be heard on the group's lab site.)

He is currently laying plans to expand his studies to include a somewhat more colorful group of singers: songbirds.

"I have no intrinsic interest in birds, either," Fortune notes, "but they have an absolutely fabulous system of behaviors associated with singing that is directly tied to evolutionary processes. We already know a lot about the brain structures involved, in part due to the interesting work of my departmental colleague Greg Ball, and there is a great diversity of singing behaviors among songbird species, so there's a lot of material to work with. Comparisons between these animals can be used as 'natural experiments' to understand the neural basis and evolution of these complex behaviors."