2. One-male groups: Competition to gain access to females is intensified Males compete for access to groups of females Outsiders exert constant pressure on resident males Tenure of resident males often short
3. Infanticide is a sexually-selected male reproductive strategy Females nurse infants for many months If unweaned infant dies, female resumes cycling immediately Death of infant makes females available for mating sooner Infanticidal males gain immediate mating opportunities If male tenure is short, infanticide enhances male mating opportunities
4. If infanticide is a sexually-selected male reproductive strategy, we predict: Infanticide will be linked to changes in male residence or status Males will kill unweaned infants Males won’t kill their own infants Infanticidal males will gain reproductive benefits Evidence supports all four predictions
5. 1. Infanticide is associated with changes in male status:Males don’t kill unless they GAIN reproductive access they did not have before
6. 1. Males begin to kill infants soon after they join group Hanuman langurs, Borries & Koenig 2000
7. 2. Males kill unweaned infants Probability of surviving presence of infanticidal male
9. 4. Infanticidal males gain reproductive benefits Infanticide brings females back into estrus Infanticidal males often mate with mother of dead infant
11. Counterstrategies to thwart infanticide Defend victims of attack Mothers Female kin Males present at conception Fathers Confuse paternity Estrus swellings Mate with many males Mate with newcomers
12. In baboons, male-female ties may be response to infanticide In some populations, infanticide is common when new males join group or males rise in status New mothers form associations with particular males possible father of current infant Males protect females’ infants Males provide care and attention preferentially to infants who are actually theirs (they can tell somehow!)
13. Sexually-selected infanticide has now been documented in a number of taxa All the major groups of primates Prosimians New World monkeys Old World monkeys Apes Lions Rodents Birds Many still think (incorrectly) that it is pathological and not adaptive
14. Controversy about whether infanticide is a sexually selected strategy persists because people confuse “is” and “ought” This is called the “naturalistic fallacy” assume that natural phenomena are right, just, unchangeable, good Worry that if infanticide is adaptive for langurs or lions, it would be justified in humans But this reasoning is wrong we can’t extract moral meaning from behavior of other animals or what is natural
