Mammalian sex ratios and variation in costs of rearing sons and
M. Gomendio*, T.
H. Clutton-Brock†, S.
D. Albon†, F.
E. Guinness† & M.
Animal Behaviour, Madingley, Cambridge CB38AA,
†Large Animal Research Group, Department of
Zoology, University of Cambridge, Cambridge CB2 3EJ, UK
IN red deer, the sex ratio of calves at birth (calculated as the
proportion of calves born that are male) increases with the dominance rank of
the mother1,2, whereas opposite trends exist in several
populations of macaques and baboons3–7. Here we show that
the subsequent survival and reproductive success of subordinate female red deer
is depressed more by rearing sons than by rearing daughters, whereas the
subsequent fitness of dominant females is unaffected by the sex of their
present offspring. By contrast, among subordinate female macaques, the rearing
of daughters has greater costs to the mother's subsequent fitness than does the
rearing of sons, although again, no difference in the costs of rearing sons and
daughters is found among dominant mothers. These findings indicate that both
differences in the relative fitness of sons and daughters and differences in
the relative costs of rearing male and female offspring can favour variation in
the sex ratio.
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