Mammals
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 analysing 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 hypothesised that
mammals are selfish creatures, manipulating the sex of their offspring
in order to maximise 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.
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