Published online 9 July 2010 | Nature | doi:10.1038/news.2010.344


Parental care linked to homosexuality

Birds that devote less time to their offspring engage in more same-sex behaviour.

Laysan AlbatrossFemale Laysan albatrosses often pair up.Enrique Aguirre / Photolibrary

Birds that spend less time parenting engage more frequently in homosexual behaviour, according to a study published this week. The findings offer a possible explanation for the evolution of homosexuality: parents that devote less time to their offspring have more time and energy to interact with members of the same sex while still producing offspring.

Biologists had thought that homosexuality is disadvantageous on an evolutionary level because it distracts animals from pursuing sexual encounters that could result in offspring. Yet more than 130 species of birds participate in homosexual activity — and sometimes a lot of it. In the Laysan albatross (Phoebastria immutabilis), for example, up to 31% of pairs are female–female in some populations, and up to 20% of pairings in graylag geese (Anser anser) are male–male. Scientists have struggled to explain such patterns.

But homosexuality may not be costly for birds that have plenty of mating opportunities because of lower parenting demands, says Geoff MacFarlane, an ecologist at the University of Newcastle in Callaghan, Australia. The less effort that females or males put into parental care, the more they participate in homosexual activities, according to a survey of the literature his team published this week in the journal Animal Behaviour1.

Vincent Savolainen, a biologist at Imperial College London, says homosexual behaviour is sometimes considered a Darwinian paradox because it does not result in offspring. "This is one of the few studies that explains homosexual behaviour from the evolutionary point of view," he says.

After scouring dozens of books, journals and databases, MacFarlane's team analysed records of 93 bird species that have exhibited homosexual behaviour in the wild. They found that same-sex courtship, mounting and pair-bonding are prevalent: 38% of these species display female–female sexual behaviour and 82% participate in male–male behaviour. In some species, nearly one-third of all sexual endeavours are female–female, and up to two-thirds are male–male. But overall, homosexual activity accounts for less than 5% of sexual encounters in the species they studied.

Free as a bird

Bird species show a range of parental strategies, from male-dominated care to female-dominated care. The team scored each species based on the relative contribution of males and females to parental chores, such as building nests, and feeding and defending chicks. Females that provide most of the care-taking show little to no homosexual behaviour. By contrast, females that give less care show higher rates of same-sex courtship and pair bonding. Similarly, males that contribute less care exhibit more male–male courtship and pair bonding compared to males that devote more time to their offspring.


The authors suggest that a release from parental duties affords individuals a greater opportunity to interact sexually with multiple partners, including those of the same sex. But they cannot determine the causes of homosexuality based on their analysis. Birds may engage in homosexual behaviour to practice courtship displays, reduce social tension or solidify dominance. Or the behaviour could help them to form alliances, share care-taking responsibilities or gain access to resources. Despite the wealth of explanations, it is not clear whether homosexuality is a neutral by-product of evolution or whether it serves an adaptive function.

"This study suggests that when there's no cost, homosexuality can persist, which isn't the same as saying it's adaptive," says Allen Moore, an evolutionary geneticist at the University of Exeter in Penryn, UK. "It may be that when there's no parental care involved, it's like having a hobby."

It's also not clear whether these results can be extrapolated to other classes of animals, such as fish and mammals. But MacFarlane's team is finding similar results in primates: those with multiple partners engage in more homosexual activity, adding fuel to his hypothesis that polygamy allows homosexuality to occur without impacting reproductive success. "The next logical step is to see if similar patterns occur across other vertebrate species," he says. 

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