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Adaptive variation in litter size and sex ratio at birth in a sexually dimorphic ungulate

In the Saharan arrui litter size and sex ratio at birth are strongly influenced by parity and maternal dominance rank at the time of conception. When females give birth for the first time they always produce single offspring, while multiparous females produce both singletons and twins. As maternal rank increases females tend to produce the following sequence: F, FF > M > MF > MM. This sequence differs from Williams’s (1979) prediction, in that FF are produced by females of lower rank than M. This may be so, because the Saharan arrui is a strongly sexually dimorphic species in which the differential costs of sons and daughters may be greater than in Williams’s model, and in which dominant females have much to gain from producing exceptionally good males. Single males are born heavier than other types of calve; among twins, investment in male—male twins seems particularly high because they are as heavy as female singletons and heavier than other twins. In addition, investment in males increases with maternal rank, while investment in females seems unaffected. Despite lower levels of investment in females, single females are more likely to survive than single males and twins.