Introduction

Life history and male infanticide risk

Infanticide by males unlikely or unable to have fathered a female's current dependent offspring is an adaptive male reproductive strategy if the mother can soon be fertilised again and the infanticidal male is in a position to be the likely sire of her next infant (Hrdy, 1979; Hrdy, Janson and van Schaik, 1995). It is remarkably common among primates (Hausfater and Hrdy, 1984; Struhsaker and Leland, 1987; Hiraiwa-Hasegawa, 1988). Its importance as a source of infant mortality probably varies widely, but it is estimated to be responsible for 31–64% of all infant mortality in some well-studied species (hanuman langurs: Sommer, 1994; Borries, 1997; mountain gorillas: Watts, 1989; red howlers: Crockett and Sekulic, 1984). Several detailed studies have demonstrated that it is an adaptive behaviour for males (op. cit.; Hrdy, 1979).

Male infanticide is potentially common in primates because many species have prolonged lactational amenorrhoea. In species without lactational amenorrhoea, early resumption of mating activity (‘postpartum oestrus’) means that killing infants will not advance the female's next birth. The incidences of postpartum mating and lactational amenorrhoea, in turn, are determined by the relative length of gestation and lactation. Where lactation is longer than gestation, we find lactational amenorrhoea, in both primates and other mammals (van Schaik, in press).

Among primates, there is a strong relationship between the mode of infant care, the incidence of postpartum mating and the incidence of male infanticide.