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Summary: © 1999 Macmillan Magazines Ltd
differences in the conception sex ratio related to individual pheno-
typic variation may require favourable environmental conditions,
whereas differences in the susceptibility of male and female fetuses
to nutritional stress may generate population-wide trends in annual
birth sex ratios. In the Rum red deer population, the action of one
mechanism swamped the other within about two generations. This
may explain why general trends in sex-ratio variation have been so
difficult to detect.
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Methods
Since 1971, life history data have been collected on individual red deer (Cervus
elaphus) in the North Block of the Isle of Rum, Scotland, an area of about
12 km2
(ref. 18). We take as our measure of density the number of females of
more than one year old in the study area; since the cessation of culling in 1973,
density has risen from 57 to 178. Females produce at most one calf per year
throughout their breeding lifespan, whereas male reproductive success shows
greater variance; adult male weight is 1.7 times that of females, and male calves
are heavier at birth18
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