Mammals can 'choose' sex of offspring, study finds Jul 10 2013
Steve Fisch
Joseph Garner analyzed decades of breeding records and found that mammalian species can "choose" the sex of their offspring in order to produce extra grandchildren.
A new study led by a researcher at the Stanford University School of Medicine shows that mammalian species can "choose" the sex of their offspring in order to beat the odds and produce extra grandchildren.
In analyzing 90 years of breeding records from the San Diego Zoo, the researchers were able to prove for the first time what has been a fundamental theory of evolutionary biology: that mammals rely on some unknown physiologic mechanism to manipulate the sex ratios of their offspring as part of a highly adaptive evolutionary strategy.
"This is one of the holy grails of modern evolutionary biology — finding the data which definitively show that when females choose the sex of their offspring, they are doing so strategically to produce more grandchildren," said Joseph Garner, PhD, associate professor of comparative medicine and senior author of the study, published July 10 in PLOS ONE. The results applied across 198 different species.
The scientists assembled three-generation pedigrees of more than 2,300 animals and found that grandmothers and grandfathers were able to strategically choose to give birth to sons, if those sons would be high-quality and in turn reward them with more grandchildren. The process is believed to be largely controlled by the females, Garner said.
"You can think of this as being girl power at work in the animal kingdom," he said. "We like to think of reproduction as being all about the males competing for females, with females dutifully picking the winner. But in reality females have much more invested than males, and they are making highly strategic decisions about their reproduction based on the environment, their condition and the quality of their mate. Amazingly, the female is somehow picking the sperm that will produce the sex that will serve her interests the most: The sperm are really just pawns in a game that plays out over generations."
The study builds on a classic theory first proposed in a 1973 paper by scientists Robert Trivers and Dan Willard, founders of the field of evolutionary sociobiology. They challenged the conventional wisdom that sex determination in mammals is random, with parents investing equally in their offspring to generate a 50-50 sex ratio in the population. Instead, they hypothesized that mammals are selfish creatures, manipulating the sex of their offspring in order to maximize their own reproductive success. Thus, parents in good condition, based on health, size, dominance or other traits, would invest more in producing sons, whose inherited strength and bulk could help them better compete in the mating market and give them greater opportunities to produce more offspring. Conversely, mothers in poor condition would likely play it safe, producing more daughters, whose productivity is physiologically limited. Other hypotheses make similar predictions — that females who choose mates with particularly "good genes" (e.g. for attractiveness) should produce so called "sexy sons" as a result, Garner said.
The hypothesis was reinforced in 1984 in a seminal Nature paper by T.H. Clutton-Brock at the University of Cambridge, who found that among wild red deer, dominant mothers produced significantly more sons than deer who held a subordinate position within the herd.
"This paper was a huge leap forward, providing the first suggestion that the idea might work in mammals," Garner said. "But because it relied on data from only two generations, it couldn't show whether females that produced more sons also gained more grandchildren from those sons." In fact, this key prediction of the hypothesis has remained untested, because complete three-generation pedigrees are so hard to obtain in the wild, Garner said.
Yet Garner and his colleagues were able to advance the research by reconstructing three-generation pedigrees of multiple species. They turned to the San Diego Zoo, enlisting the help of animal-care supervisor Greg Vicino in combing through decades of records on more than 38,000 animals from 678 species. The project was labor-intensive, requiring years of work to reconstruct the pedigrees and breeding histories of the animals, Garner said.
The researchers ended up with a pool of 1,627 granddams (female grandparents) and 703 grandsires (male grandparents) for whom they had a complete record of three generations. Major mammal groups were represented, including primates; carnivores, such as lions, bears and wolves; cloven-hoofed animals, such as cows, buffalo and deer; and odd-toed grazing animals, such as horses and rhinos.
They found that when females produced mostly sons, those sons had 2.7 times more children per capita than those whose mothers bore equal numbers of male and female offspring.
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Poster Comment:
This reminds me of the movie, "Planet of the Apes" in which the humans were treat as animals in the zoo.