Macho Makeover: Fish rapidly ascend social ladder

Some fish really know how to swim to the top. Researchers have found that within minutes of recognizing a social void, a lowly cichlid can alter its looks and behavior to ascend to the dominant spot in its group. Moreover, the same researchers have identified the gene that is primarily responsible for the fish’s changing physiology.

“We had known that social environment controls the reproduction of [cichlids],” says Sabrina Burmeister of the University of North Carolina at Chapel Hill. Other researchers had shown, for example, that a subordinate male, normally sterile, can gain the ability to reproduce within a week of the removal of the dominant male. In the November PLoS Biology, Burmeister shows that the color and behavior of an upwardly mobile fish can change within minutes, reflecting his new status.

Dominant, fertile male cichlids have vibrant yellow as well as blue coloring. Their flashy scales attract females, but they also draw the attention of predators. So, when a predator picks off a dominant fish, one of the less colorful, less aggressive, and infertile subordinate cichlids will undergo physiological changes to take the leader’s place.

Burmeister and her colleagues simulated this situation by establishing tanks with one dominant male, one subordinate male, and numerous females. Cichlids are sensitive to changes in their surroundings, so “we went through some pretty big hoops to make sure the fish were in a comfortable environment,” she notes. Then, the scientists changed things.

The researchers donned night-vision goggles to remove, in darkness, the dominant fish from each tank. An hour later, the team turned on the lights and watched as the once-subordinate male in each tank assessed its new situation. Within 2 minutes, he would begin to acquire brilliant colors and for the first time show interest in the females.

To determine the brain biochemistry that initiated these physiological and behavioral changes, the researchers examined neural tissue from the preoptic area of fish in various states of dominance and subordination. This brain region is known to regulate reproductive behavior in vertebrates, including people.

Burmeister explains that her team searched for active genes known to trigger a cascade of other gene activation. In the brains of fish that ascended to dominance, the researchers found larger numbers of activated egr-1 genes than they did in the brains of subordinate or dominant cichlids not undergoing a status change. The researchers speculate that egr-1 turns on the gene for gonadotropin-releasing hormone 1, which is essential for vertebrate reproduction.

Gregory Ball of Johns Hopkins University in Baltimore comments that the North Carolina team’s finding is significant because it shows a complex social situation quickly triggering gene activity that controls reproduction.

Because the genes involved in the cichlid change are found in all vertebrates, Ball says, “it is quite reasonable to speculate that other species, including humans,” could exhibit such gene expression during social interactions.