Infanticide: A Sexually Selected Male Strategy

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

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

If infanticide is a sexually-selected male reproductive strategy, we predict:

1. Infanticide will be linked to changes in male residence or status

2. Males will kill unweaned infants

3. Males won’t kill their own infants

4. Infanticidal males will gain reproductive benefits

Evidence supports all four predictions

1. Infanticide is associated with changes in male status:

Males don’t kill unless they GAIN reproductive access they did not have before

1. Males begin to kill infants soon after they join group

Hanuman langurs, Borries & Koenig 2000

2. Males kill unweaned infantsProbability of surviving presence of infanticidal male

3. Males don’t kill own infants

4. Infanticidal males gain reproductive benefits

• Infanticide brings females back into estrus• Infanticidal males often mate with mother of dead infant

Infanticide is a major cause of mortality

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

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!)

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

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