Males who stick around to care for offspring that may not belong to them are not 'hapless dupes' but are playing the odds in a game of evolutionary poker, according to Oxford University research.
Previous studies have assumed that 'cuckolded' males, whose mates have offspring fathered by another male, should adjust their behaviour to care less for babies they believe are not theirs. This is because the cost of parental care is so high: the effort a typical garden bird puts into bringing up a clutch of chicks is, in human terms, the equivalent of cycling the Tour de France.
However, in many species males will often care for offspring that are not their own, even when they are aware that their females are unfaithful.
To investigate why cuckolded males stick around researchers from Oxford University, Lund University and Yale University analysed 62 studies across 48 species of insects, fish, birds, and mammals, including humans. They found that whilst males may appear tolerant of cuckoldry in fact they adjust their care according to how likely it is that females are unfaithful and calculating whether caring will reduce the number of offspring they father in the future.
Across all species the team found that promiscuous mating by females reduced male parental care by 12%, with males of most species making some adjustment.
A report of the research is published in this week's PLOS Biology.
'It may seem illogical that individual males should put so much effort into bringing up offspring that are very likely not to be carrying their genes,' said Dr Ashleigh Griffin of Oxford University’s Department of Zoology, a Royal Society University Research Fellow who led the research. 'But in fact males are constantly weighing up the costs and benefits of the care they give in the context of their reproductive success over an entire lifetime. Where either the risks of cuckoldry or the costs of raising offspring are low it may make sense for males to be tolerant and turn a blind eye to female unfaithfulness.'
One example of the calculations males make might be that a male bird coming to the end of his reproductive life, who will die before the next breeding season, might care for the chicks of a promiscuous mate because they have 'nothing to lose'. Another would be that in a species with a very low risk of cuckoldry it does not make sense for males to abandon even obviously promiscuous females as, overall, being primed for this behaviour would lead to too many males abandoning nests with offspring that are theirs.
'We like to think our morals make humans different from other animals, but in fact when you draw a graph of how males of all species adjust their parental care in response to cuckoldry humans are where you'd expect; somewhere in the middle, making little or no adjustment,' Dr Griffin said. 'One conclusion you might draw from this is that in this area at least our morals and ethics enshrine our evolved instincts, finely honed by natural selection, rather than overriding them.'
The analysis suggests how further research in this area could be targeted to test the team's findings: for instance examining species where the cost of caring is very low and males would not be expected to adjust their level of parental care even if females are promiscuous.
A report of the research, entitled 'Why Do Cuckolded Males Provide Paternal Care?', is published in PLOS Biology.