Fishy Paternity Defense: Bluegill dads: Not mine? Why bother?

Bluegill sunfish have provided an unusually tidy test of the much-discussed prediction that animal dads’ diligence in child care depends on how certain they are that the offspring really are their own.

DAD’S NEST. Among a crowd of protective fathers, the big bluegill male in the foreground hovers over his egg-filled depression. M.R. Gross/U. Toronto

When researchers presented male bluegills with phony evidence of cuckoldry, the dads slacked off on nest defense, says Bryan Neff of the University of Western Ontario in London, Canada. Later, reassured of their paternity, the fish grew fiercely protective, Neff reports in the April 17 Nature.

The idea that genetic relatedness affects how liberally parents invest in their offspring makes sense theoretically, but it’s been tricky to test, says David Westneat of the University of Kentucky in Lexington, who also studies parental care. However, he calls the new study “a really focused, strong experiment” and “the best evidence to date.”

The bluegill (Lepomis macrochirus) occurs across much of North America. In the Ontario lake that Neff studies, most males take 7 years to mature. Then, 100 to 200 hefty nest-builders gather, swishing their tails so that each sweeps out a depression, virtually rim-to-rim with his neighbors’. Schools of females show up for a day of egg laying. The dads fertilize the eggs, and the females swim off.

The male spends the next week guarding his 12,000 to 60,000 offspring without any break for foraging.

About 20 percent of males mature when only 2 years old and spend their lives siring offspring in other males’ nests. When small, these cuckolders hide nearby and zoom in at a strategic moment to fertilize eggs. When the cuckolders grow bigger, they develop femalelike coloration and mannerisms and “enter the parental male’s nest in drag,” says Neff.

To see how males react to questionable paternity, Neff placed four young cuckolder males in clear containers around each of 34 randomly selected nests. The containers kept the sneakers from siring offspring but gave the nest builders an eyeful of apparent rivals. The day after the bluegills spawned, Neff dangled a container with a pumpkinseed sunfish, which preys on eggs and fry, and scored the ferocity of the dads’ defense.

After the eggs hatched, Neff repeated the defense test. His earlier work had shown that males can’t judge their relatedness to eggs but can sniff out the paternity of fry. Neff predicted that after the males recognized that the apparent cuckolding hadn’t done any damage, they would change from lackadaisical defenders to ferocious ones. His tests showed that, indeed, they did.

In another experiment, Neff swapped out about a third of the eggs from 20 males’ nests and measured defensive behavior before and after the eggs hatched. After a male sniffed the unrelated fry, his ferocity declined.

The trouble with other experiments, says Westneat, has been that researchers often didn’t know the cues that the animals use to judge paternity. The beauty of the new work, he says, is that Neff figured out and manipulated the cues.


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Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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