The chapters in this volume point to four major conclusions. First, infanticide by males in many kinds of animal is most reasonably interpreted as a reproductive strategy, although not every observed case fits neatly all the criteria specified by this hypothesis (van Schaik, Chapter 2; Palombit et al., Chapter 6; Blumstein, Chapter 8; Veiga, Chapter 9). As further evidence accumulates, the patterns appear to reinforce the sexual selection hypothesis (e.g., Borries et al. 1999b) rather than non-adaptive interpretations (see Sommer, Chapter 1). Second, there are predictable correlates of infanticide risk among primates and other mammal species, including life history factors such as long infant dependency relative to gestation, large litter size, altriciality and social factors such as the loss of protectors, in particular the rate of breeding male replacement (van Schaik, Chapters 2 and 3; Borries & Koenig, Chapter 5; Blumstein, Chapter 8; van Noordwijk & van Schaik, Chapter 14; Nunn & van Schaik, Chapter 16). Third, females in species with infanticide by males are not just passive recipients of male aggression, but have developed a broad array of behavioral and physiological strategies to reduce infanticide risk. These traits are a major focus of this volume and will be developed more fully later.
A fourth conclusion to emerge is that females may also kill infants, in some species more frequently than do males (Blumstein, Chapter 8; Veiga, Chapter 9; Digby, Chapter 17; Voland & Stephan, Chapter 18).