Mammal mums can alter their offspring's sex
14:04 23 February 04

Knowing whether a pregnancy will produce a boy or girl is not left up to
chance for some mammals - UK biologists claim they have conclusive proof
that zebras, bison and certain other mammals actively adjust the sex of
their offspring.

It has long been known that many insects, birds and fish are capable of
influencing the sex of their offspring, but the idea of gender adjustment in
mammals has been controversial. American biologist Robert Trivers first
suggested that female red deer alter the sex of their offspring according to
their physical condition at the time of conception, and the idea has been
debated for the 30 years since.

"We looked at all 73 of the different studies carried out into sex
adjustment in ungulates - herbivorous mammals with hoofed feet - and found a
consistent pattern across the different species. Sons were produced in
higher numbers by mothers who were in good condition and daughters were
preferentially produced by mothers in poor condition," explains Stuart West
at the University of Edinburgh, who carried out the study with Ben Sheldon
from Oxford University.

"In ungulate species, a few strong males control a large number of females,
so only a small proportion of males get to mate, whereas most of the females
will mate.

"Therefore high quality females were more likely to undergo the greater
demands of producing sons, as the sons were more likely to be of high
quality and therefore have a chance of mating. Conversely, if maternal
quality was poor, the ungulates produced daughters, since poor quality
daughters had a greater chance of mating than poor quality sons," he told
New Scientist.

Read the rest at NewScientists
http://www.newscientist.com/news/news.jsp?id=ns99994708

Comment:
The reason for gender selection is not, as the article seems to imply (but
the paper being reported probably doesn't), by a conscious decision by the
mother but by a process of natural selection (of the trait to choose) that
has a benefit to the herd.  The mother receives no personal benefit
whatsoever by the breeding success or failure of her offspring.

Where the fittest females produce males and the rest produce females the
herd's survival fitness is maximised.  Mothers that are not maximally fit
and produce males anyway, those males are less likely to breed and so less
like to father females similar to their mothers.

This seems to be an example of herd or group fitness with a single natural
selection component acting on individuals.  Note that there is no selection
pressure on fit females that produce females.

Of course the pivotal word in this article is 'quality'.  By what criteria
is 'quality' judged?

Posted by
Robert Karl Stonjek.
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