Dr. Taborsky and his colleagues have studied the social lives of African cichlids, colorful freshwater fish from Lake Tanganyika. The cichlids live in relatively large groups of 10 or so individuals, a dominant breeding pair and a retinue of adult and adolescent helpers. The helpers share in all duties, Dr. Taborsky said. They defend territory, they help keep the nests tidy and they clean, fan and oxygenate the breeding pair’s eggs. When the eggs hatch into larvae, the helpers take up the babies in their mouths for cleaning  all the while forgoing their own breeding efforts.

Significantly, the helper fish are often unrelated to the royal pair over whose spawn they so officiously fawn. What’s in it for the helpers? “We call it pay to stay,” Dr. Taborsky said. “Helpers are allowed to stay in the territory and gain security and protection against predators. But they have to pay rent, so to speak, or they risk being expelled.”

In laboratory experiments, the researchers have shown that when subordinate cichlids are temporarily prevented from performing their duties, the fish compensate at the first chance by ostentatiously redoubling displays of helpful behaviors.

Researchers have identified many other surprising analogies between humans and fish. Dr. David Reznick of the University of California, Riverside, has discovered that female guppies go through a kind of menopause, surviving well beyond their reproductive life span, a finding that may bear on the evolution of menopause among women.

Catherine L. Peichel of the Fred Hutchinson Cancer Research Center in Seattle has determined that the fish are awfully human, particularly in their migratory prowess.

“Sticklebacks migrated out of their ancestral marine habitat and invaded lots of new environments over an evolutionary time frame of about 10,000 generations,” Dr. Peichel said. “That’s roughly the same number of generations since humans migrated out of Africa and adapted to habitats all over the world. It’s a parallel process.”

Even some of the same genes that shifted format in human migrations, like those responsible for skin pigmentation, also changed as sticklebacks ventured from salt water to fresh.