Dirty Old Birds: Why Do Older Male Birds Father More Illegitimate Children?

They tested two possibilities: that older males are better at coercing females into extra-pair affairs (the ‘male manipulation’ hypothesis), or that females solicit more sex from older males than from younger males (the ‘female choice’ hypothesis).

The team observed more than 450 mating attempts by males, and found that older males did not try to make females cheat any more often than younger males, casting doubt on the male manipulation hypothesis.

In fact, they found that successful affairs were more often solicited by females, suggesting they are the ones to choose when, and who, to cheat with. However, the females did not choose older males more often than younger males, suggesting the ‘female choice’ hypothesis may not be correct either.

Lead author of the study Antje Girndt, from the Department of Life Sciences at Imperial and the Max Planck Institute for Ornithology in Germany, said: “There is a difference between what we observe and what the outcome is: we didn’t observe older males cheating more often than younger males, but they do father more offspring. This suggests there is another factor at work, such as older males having more competitive sperm.”

Credit: Charlotte Wen Ting Chng

The researchers suggest that ‘post-copulation’ mechanisms might be driving the older male’s success. An element of female choice might still be a factor, as female sparrows can store sperm internally for some weeks, and may ‘decide’ which sperm to use for fertilisation – although the authors admit this is hard to study.

The team study sparrows to investigate the evolution of monogamy in animals, and the behavioural and biological factors that drive it.

Lead researcher Dr Julia Schroeder, from the Department of Life Sciences at Imperial, said: “Sparrows are socially monogamous but sexually promiscuous, staying with one partner for the security of raising chicks, but with the males not necessarily raising their own chicks.

“We found that there is likely to be a biological effect, rather than a behavioural one, for why older males are more successful at siring illegitimate children. It has been thought that females might choose older males as they are more ‘genetically fit’, but our research casts some doubt on this.”

Contacts and sources:
Hayley Dunning
Imperial College London

Citation: Male age is associated with extra-pair paternity, but not with extra-pair mating behaviour  Antje Girndt, Charlotte Wen Ting Chng, Terry Burke & Julia Schroeder Scientific Reportsvolume 8, Article number: 8378 (2018) doi:10.1038/s41598-018-26649-1
http://dx.doi.org/10.1038/s41598-018-26649-1