Image: Horse and colt
Tom Gannam  /  AP file
Scientists discovered that healthier mothers invested more in sons over daughters, with the investment consisting of more food — milk in this case — protection and direct contact.
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updated 10/8/2008 1:56:11 PM ET 2008-10-08T17:56:11

Do mothers inherently favor sons over daughters? They do if the mom is healthy and she is a wild horse, according to a new study on literal horseplay, but researchers suggest the equine findings could carry over to other polygynous animals, including humans.

"Polygynous" refers to species with males that can mate with more than one individual over a relatively short period of time, with the pairings all possibly resulting in pregnancies.

"Mothers are advantaged differently by investing in sons or daughters in relation to their own condition and the future reproduction of their offspring," lead author Elissa Cameron told Discovery News.

"Sons have the highest potential payoff," she explained, "as sons can leave you many more grand-offspring than daughters can."

Cameron, director of the Mammal Research Institute at the University of Pretoria in South Africa, and her colleagues made the determination after studying wild horses in New Zealand.

Known as Kaimanawa, these horses live in year-round bands consisting of multiple mares, at least one stallion and their foals. Given the difficulties of studying fast-moving, feral animals over wide territories, the study involved tracking by horseback as well as by helicopter.

Cameron and her team focused on the interactions between mothers and their young, and particularly on the foals' play behavior. Horses play by manipulating objects, simulating courtship and mounting, running alone or with a partner, and by play fighting.

"Males tend to be more boisterous than females, which reflects the greater competitive role they play in later life," she said.

The scientists discovered that healthier mothers invested more in sons over daughters, with the investment consisting of more food — milk in this case — protection and direct contact. These sons then played more, even at the expense of the moms, who temporarily lost weight and strength taking care of their boys.

Mothers in overall poor condition, however, invested more time and resources in their daughters according to the study, which has been accepted for publication in the journal Animal Behavior.

Cameron explained that while males in general breed more, not all male horses will turn out to be busy breeders, whereas almost all female horses that reach adulthood will breed at least once.

"Therefore, if you have a lot of extra resources and can turn your son into a highly competitive male, he will leave you more grand-offspring," she said. "Alternatively, if you have few resources to invest, a son would be unlikely to ever breed, whereas your daughter would probably breed, thereby leaving you more grand-offspring."

© 2012 Discovery Channel

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