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 i nfluenced by parity and maternal dominance rank at the time of concept ion. When females give birth for the first time they always produce si ngle offspring, while multiparous females produce both singletons and twins. As maternal rank increases females tend to produce the followin g sequence: F, FF > M > MF > MM. This sequence differs from Williams's (1979) prediction, in that FF are produced by females of lower rank t han M. This may be so, because the Saharan arrui is a strongly sexuall y dimorphic species in which the differential costs of sons and daught ers may be greater than in Williams's model, and in which dominant fem ales have much to gain from producing exceptionally good males. Single males are born heavier than other types of calve; among twins, invest ment in male-male twins seems particularly high because they are as he avy as female singletons and heavier than other twins. In addition, in vestment in males increases with maternal rank, while investment in fe males seems unaffected. Despite lower levels of investment in females, single females are more likely to survive than single males and twins .