Competition between female relatives in a matrilocal mammal

Evolutionary theory predicts that in sexual organisms the average parents should invest equally in their male and female progeny1 except where related males compete with each other to mate, when selection may favour parents that produce a female-biased sex ratio2–4. It has recently been suggested that in species where daughters adopt home ranges overlapping those of their mothers while sons disperse, competition for resources between female siblings may select for mothers who produce male-biased sex ratios5. We show here that in a mammal living in matrilocal, non-territorial groups—the red deer (Cervus elaphus)—the presence of resident female relatives depresses the reproductive success of adult females, providing a possible explanation of the apparent bias in parental investment towards male offspring in this species6.