Why males stray more than females

Do males have more to gain than females from mating with additional partners?

The theory that they do, and that this can help to explain different sex roles observed in the males and females of many species, is known as 'Bateman's principles', named after the work of English geneticist Angus John Bateman.

In a recent study reported in Proceedings of the Royal Society B a team, led by Oxford University researchers, investigated Bateman's principles in relation to populations of red junglefowl (Gallus gallus), the wild ancestor of the domestic chicken.

'Bateman's principles state that males are more variable than females in the number of offspring they produce and number of sexual partners,' explains Dr Tom Pizzari of Oxford University's Department of Zoology, one of the research team. 'This leads to a stronger relationship between number of offspring and number of partners in males than in females. In other words, males gain more reproductive success by mating with additional partners than females do.

'This difference is explained by the fact that males produce orders of magnitude more sperm than there are eggs available for fertilisation, so their reproductive success is strongly limited by female (egg) availability. Females on the other hand tend to produce a smaller number of larger eggs, and generally mating with additional males does not influence the number of eggs that a female can afford to produce.'

To test the principles the team studied groups of red junglefowl and carefully recorded all mating events and assigned parentage to every offspring produced. They then ran experiments to test the relationship between reproductive and mating success.

'Studying Bateman's principles properly presents many challenges,' Tom tells me. 'First, detailed information on mating success (who mates with whom) is required. Previous studies did not measure mating behaviours, but simply inferred who mates with whom based on parentage of the offspring. This approach however, misses out all those mating events which failed to result in fertilisation.

'Second, Bateman's principles are concerned with how males increase their reproductive success by mating with additional females. However, there are other pathways through which males can increase the number of offspring sired: mating with particularly fecund females, and defending their paternity in sperm competition. Again, most studies so far have explored Bateman's principles without controlling for these alternative pathways.

'Finally, one must be very careful about how to interpret a positive relationship between reproductive success and mating success in females. One possibility is that females genuinely increase the number of offspring produced by mating with additional males, another is that females that are inherently more fecund are more attractive to males and so end up with more partners.' 

The new study showed that in failing to address these challenges traditional approaches can lead to very drastic biases in estimating Bateman's principles and that future research in this area should combine independent data on mating behaviour, multivariate statistics, and experimental tests.

'Our results suggest that once these biases are controlled for, Bateman was essentially correct: males gain more reproductive success by mating with additional partners than females, however these sex differences are much smaller than estimated by traditional methods,' Tom comments.

'This means that males are more strongly selected to compete over access to mates than females, explaining why sexual selection is typically more intense in males, providing an answer to Darwin's original question of why it is males that often display more exaggerated traits in a species.'