Bird parents typically raise their young in seemingly peaceful cooperation and were therefore seen as reflecting the ideal of monogamous partnership. This view has changed dramatically over the last decade: since molecular 'fingerprinting' techniques became available, behavioural ecologists have routinely used paternity analyses to study mating systems.

We now know that unfaithful behaviour referred to as extra-pair copulations is more the rule than the exception, in particular in socially monogamous songbirds. A team of scientists from the Max Planck Research Centre for Ornithology in Seewiesen, Germany, and from the Zoological Museum in Oslo, Norway, now show the evolutionary advantages of this behaviour, using data from a long term study on the blue tit, a small hole-nesting songbird (Nature, October 16 2003). Through extra-pair copulations with distantly breeding males, females produce offspring that are more outbred and therefore genetically more diverse, compared to their half-siblings sired by the social father.

Monogamy is rare among animals. Even species that form socially monogamous pairs of one male and one female are often not monogamous in the strict sense. Males and females commonly copulate with more than one partner during a single reproductive event. Therefore, young raised in the same brood are often sired by different males. The evolutionary advantage of unfaithful behaviour is obvious for males: the clutch- or litter size of the partner limits the maximum number of offspring that can be produced. Extra-pair copulations with other females provide the only opportunity for a male to sire additional young. For females, the significance of promiscuity is less obvious. If evolution only favours high numbers of offspring, then females would not benefit from mating with multiple males. However, evolutionary biol
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Contact: Dr. Bart Kempenaers
b.kempenaers@erl.ornithol.mpg.de
49-815-793-2334
Max-Planck-Gesellschaft
16-Oct-2003