The study, published today (July 10) in the journal PLOS ONE, confirms a long-held theory that animals can influence the sex of their young in response to environmental conditions and other factors. The results come from about 90 years' worth of records for 40,000 mammals, ranging from primates to rhinoceroses, at the San Diego Zoo.

Females that produced the most males went on to have up to 2.7 times the number of grandchildren from those sons as those who had even numbers of male and female offspring.

"When mothers produced predominantly male offspring, those male offspring outcompeted their peers," said study co-author Joseph Garner, an ethologist at Stanford University.

Parents that produced more females also tended to produce more offspring from those daughters than those with an even gender split, though the effect was less pronounced.