Something Fishy in the Nest
Neff, Bryan D., Natural History
In many fish species, dad does the caregiving. But some sneaky bluegill males have ways of avoiding the responsibilities of fatherhood.
Questions of fatherhood are a staple of dramatic conflict, whether in Greek tragedy, soap opera, or divorce court. In the natural world, too, conflicts brought on by uncertain paternity have opened up rich veins of phenomena for scientific investigation. Behavioral ecologists, who study how natural selection shapes animal behavior in light of ecological and social conditions, have long grappled with the role that parentage-genetic relatedness-plays in how much care fathers provide their young.
A fundamental principle of adaptation, growing out of Darwin's theory and elaborated in recent times by several prominent evolutionary biologists, is that individuals promote the spread of their own genes over the genes of competitors. The degree to which a male is sure he has sired offspring, then, should influence how much time and energy he invests in caring for them. To what extent that prediction might apply to human beings is a matter of debate; even tor other animals, confirming it proves to be tricky.
DNA testing can determine who sired whom, but does the outcome of a DNA test really overwhelm other factors, such as parental age or number of offspring, that affect the time and energy an adult animal devotes to caretaking? Geneticists can determine family relationships via DNA fingerprinting, but can the males of animal species identify their young, and if so, how? And how much does care of the young by the biological parent matter anyway, so long as the genes of the parent survive? Those questions have intrigued me for more than a decade. Recently some answers have been emerging from work with a common freshwater fish I've found to be a finned exemplar of Shakespeare's line from The Merchant of Venice: "It is a wise father that knows his own child."
The bluegill sunfish (Lepomis macrochirus), a social, schooling species popular with anglers, is endemic to the freshwater lakes, ponds, and rivers of North America. The fish are particularly prolific in lakes with relatively shallow, warm water and plenty of weedy vegetation. Males, which are about 25 percent larger than females, range from eight inches long in southern Canada to fourteen inches in the southern United States. Young bluegills eat mainly zooplankton, but as they grow older, they become opportunistic and devour almost anything that fits into their mouths.
In bluegills, as in many other fish species, only the male cares for the offspring. During the summer breeding season, adult males called parentals venture near the shore of a lake, where they collect in colonies that may have hundreds of members, all ready to reproduce. The parental males fight fiercely for nesting sites in the center of the colony-in part because those locations provide the best available protection against such egg-feeding predators as catfish. Not surprisingly, the largest males usually secure the most central positions.
After staking out a spot, parental males construct nests by sweeping a bowl-shaped depression in the lake bed with their tail fins. Nests are about the size of large dinner plates and are positioned right next to one another in the crowded colony site. It takes a few days for the colonies to form. Once all the males have selected sites and built their nests, the general commotion subsides and the parentals await the females.
Then they arrive. A school of females that can number in the hundreds swims in, above the colony. What they look for in a mate is not yet known. But they do seem choosy, passing up numerous nests and potential mates before accepting one. Evolutionarily, of course, selecting a mate is critical: the offspring will not survive without proper care from a parental male. Yet once spawning begins, it lasts just a day-though it can recur several times in a season. During the day of spawning a female tilts her body and releases a spurt of about thirty eggs into a male's nest, a behavior known as a "dip. …