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Something fishy in the nest: in many fish species, dad does the caregiving. But some sneaky bluegill males have ways of avoiding the responsibilities of fatherhood

Natural History, Feb, 2004 by Bryan D. Neff

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.

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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 for 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." The male showers the eggs with his sperm. A female may dip many times into one male's nest, eventually leaving hundreds or even thousands of eggs. Then the females depart for deeper water and leave the males in charge.

I do much of my research in southeastern Ontario's Lake Opinicon, where the temperature-dependent spawning season is short. Colonies of bluegills typically spawn on eight distinct days, between five and seven days apart, from late May until early July. For the first two or three days after spawning, a parental male must oxygenate the eggs by fanning freshwater across them with his pectoral fins; any slacking off and the eggs will fail to hatch. By the fourth day the young fry emerge, and the male's main duty becomes protecting them from predators: bass, catfish, perch, and other sunfish. By the tenth day the fry leave the nesting area and fend for themselves. Tending the young takes a toll; males do not forage while caretaking, and so they lose about 15 percent of their body weight.

The sacrifices of parenting are not for all bluegill males, however. About 20 percent of them follow an entirely different reproductive course. Aptly called cuckolders (a term coined by Mart R. Gross, a conservation biologist at the University of Toronto), the blue gill males of this minority group mature at two years of age, several years earlier than the parentals. In the first phase of their sexual lives the cuckolders are referred to as "sneakers." After locating a spawning colony, they lurk behind rocks or debris near a nest until a female enters the nest depression. Just as she releases her eggs, the sneaker darts into the nest, discharges his sperm, and then hightails it out before the parental male can catch him. The tactic is risky, because the parental male has the edge in size; if he does manage to nab the intruder, he can easily kill him.

Something fishy in the nest: in many fish species, dad does the caregiving. But some sneaky bluegill males have ways of avoiding the responsibilities of fatherhood

Natural History, Feb, 2004 by Bryan D. Neff

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