Faithful females key to evolution of bird societies

Science 19 Aug 10 Females with fewer sexual partners can explain where bird species have evolved to cooperate in the rearing of their young, according to Oxford University research. In a study of 267 bird species, the researchers found that promiscuity rates overall were three times higher among species that show no social cooperation than in species where adult birds other than the parents help in raising chicks, for example by assisting in providing food, incubation or defence. 'We found that across bird species, whether or not a species is cooperative depends upon the sex lives of females,' says Dr Ashleigh Griffin of the Department of Zoology at Oxford University, who led the research. 'The more promiscuous a female is - the more mates she has - the less help there is likely to be from the rest of the family in raising the young. She explains: 'When females are faithful and mate with only a single male, her offspring will be full siblings and hence closely related to each other. This favours cooperation, because helping a close relative reproduce can be an efficient way of transmitting genes to the next generation. 'In contrast, if a female is promiscuous and mates with multiple males, her offspring will be half siblings and so less related.