Oct. 8, 2008 -- 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."
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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."
She added that horse fathers might affect the process to some degree, as they occasionally groom and play with their foals, but the bulk of the parental care falls on mothers.
The process results in a domino effect that influences the rest of the horse's life. A horse that receives more care from its mother tends to play more. Since playtime appears to enhance both physical and mental health, playful individuals tend to grow into healthier adults, which then start the process all over again.
The health of the mother may even predict if she will give birth to a son or a daughter in the first place.
A separate study on 740 first-time pregnant human moms led by the University of Exeter's Fiona Mathews found that mothers who ate a high-energy diet at the time of conception produced sons more often than moms who took in less nutrition before conceiving.
"Here we have evidence of a 'natural' mechanism that means that women appear to be already controlling the sex of their offspring by their diet," Mathews said. "This research may help to explain why in developed countries, where many young women choose to have low-calorie diets, the proportion of boys born is falling."
While genetics also help to determine an individual's sex, health, playfulness and more, the two studies reveal how mothers can play a major role in controlling the destinies of their children.
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