22 December 1998
Seduced by a domineering, two-timing female and then largely abandoned to raise young that aren't his own, the male wattled jacana would seem to be a loser in the genetic lottery. But the tropical shorebird has some consolations, at least some of the young he cares for carry his genes. Sometimes other males raise young that he has sired.
Reporting in the Proceedings of the Royal Society of London Series, Cornell University biologists Stephen T. Emlen and Peter H. Wrege, and Michael S. Webster of the University of Buffalo, explain the costs and benefits behind one of the animal kingdom's most extreme examples of sex-role reversal and cuckoldry.
The scientists observed wattled jacanas (Jacana jacana) over a six-year period on Panama's Chagres River, witnessing more than 1 400 noisy copulations between dominant females and much smaller males. The males were left for weeks to incubate eggs and then, as single parents, to raise chicks of uncertain paternity for two more months.
The females, meanwhile, were copulating with other males. Jacanas are practically unique among vertebrate animals, in that females pair simultaneously with a harem of males (a mating system called polyandry) and the males perform essentially all of the care of the young.
The biologists made DNA fingerprints of 465 adult and juvenile birds and found that the males were tending chicks from a different male in over 40 per cent of the broods. Those nonrelated chicks in the males' nests were the result of what biologists call extra-pair fertilizations.
Extra-pair fertilization rates rose above 70 per cent where the researchers observed the females mating promiscuously. Jacanas copulate at a high rate compared with most bird species. While a single copulation would provide enough sperm to fertilize a clutch of four eggs, the researchers found jacanas averaging 65 copulations for each clutch produced.
When no extra males were available in a harem, all chicks were sired by the care-taking male. Thus a male's risk of being cuckolded comes entirely from other males that are simultaneously paired to the same female.
"These results are surprising," comments Emlen, the Jacob Gould Schurman Professor of Behavioral Ecology at Cornell. "Biologists have assumed that males would only bear full parental responsibility for incubating eggs and raising the young under one condition: When there is certainty that the young are genetically his own - that is, when the male's assurance of paternity is high. Instead, a male jacana sits on the nest, watching the mother of the chicks he will raise, while she continues to mate with other males nearby."
But why does the male jacana tolerate this behavior by his mate? Simultaneous polyandry is beneficial for female jacanas, notes Emlen, an evolutionary biologist who tries to explain the advantage of inherited behaviors for various animal species and for individuals in each species. "If a large, dominant female can maintain a harem," Emlen says, "her reproductive success is enhanced by having lots of males raising young for her. But the males may pay the cost. There thus is sexual conflict over the preferred mating system. Females benefit from pairing polyandrously; males, however, would avoid the cost of cuckoldry if they paired monogamously."
Another field observation by the biologists helps explain why male jacanas put up with such an unfair system: In heavily populated and highly competitive habitats with limited space for nests some males never get the chance to reproduce at all.
"I guess you could say the males are lucky to be seduced and abandoned, considering the alternative," Wrege says. "At least they're adding something of their own to the gene pool."