ANT333 Lecture #16 – Primate Infanticide: Adaptation or Pathology?
What to “Get” Today
In evolutionary biology, selection works at level of the individual.
Infanticide is rare and so difficult to observe, but important fitness event for all involved.
Difference between adaptive vs. maladaptive hypotheses.
Each evolutionary strategy can result in a counter-strategy.
Operational Definition of Infanticide
Infanticide involves killing of young offspring by a mature animal of its own species.
Observed in insects, birds, fish, and many mammal species; such as lions, prairie dogs, and
Infanticide Typically by Males
1st Primate Infanticide Study: Hanuman Langurs in Jodphur, India
Studied by Dr. Sara Hrdy in 1970’s in Jodphur.
Question: Why high infant mortality rates in 1-male troops?
Observed: Infant deaths peak near times of new male affiliating with stable female troop core.
Conclusion: new males killing infants of previous male in attempt to maximize their own
Infanticide Makes Sense When:
1. Population densities are high, making more male competitors available.
2. Males have a short tenure with troop, limiting reproductive opportunities.
3. Breeding is not highly seasonal so a mother losing an infant rapidly comes into estrus. – sexually
receptive at any time of the year
Three Hypotheses on Infanticide
Social Pathology Hypothesis
Generalized Aggression Hypothesis
Sexual Selection Hypothesis
Mutually exclusive – don’t overlap with each other
I. Social Pathology Hypothesis
Infanticide aberrant, dysfunctional behavior due to high population densities.
Not part of normal “makeup” of species.
Jodphur Langurs: Makes sense because: (1) one of study groups was being crowded into urban
areas by deforestation, and (2) in other study areas animals were provisioned, which often
increases levels of aggressive behavior.
Female Infanticide in Gombe Stream Chimps
Passion began killing & eating several of babies in her community with her daughter Pom
Seen to eat 3, chase 3 others, and there were 8 others who disappeared under mysterious
circumstances. In this period, there were almost no infants weaned successfully in the group.
Behavior ceased when Passion died. –no advantage for passion
1 II. Generalized Aggression Hypothesis
Infants killed cannot avoid aggression of adult males.
Infanticide: epiphenomenon of heightened male aggression directed at all group members.
Common when researcher assumes rather than sees infanticide.
III. Sexual Selection Hypothesis
Infanticide male reproductive tactic: loss of infant leads to early onset of estrous in mother.
Reproductive advantage to target lactating females
By killing an infant, male gains reproductive advantage provided that:
1. Death of unweaned infant shortens subsequent interbirth interval of mother
2. Killer increases his chances of mating with mother & siring her next infant
3. Male not related to infant he kills.
#1: Interbirth Interval
Infanticidal males gain reproductive advantage through earlier conception by females.
The female, after having lost their dependent infant, may resume sexual activity earlier and bear
next infant significantly sooner than females with surviving infants.
Has been documented in some infanticidal species.
Issue: Seasonal breeding.
Remember: this change can reflect loss of infant to infanticide or other causes. So, point #1
seems well founded.
Has been documented in females who have lost their offspring due to other causes
Does not occur to primates who have seasonal breeding
#2: Increased Chances of Mating
Argued for purely on observational & anecdotal information.
Infanticidal males remained residents in group where they committed infanticide; they may sire
subsequent infant & might have benefitted by infanticide.
Researchers sometimes observed infanticidal male(s) to mate with victim’s mother.
#3: Male not Related to Infant
Most researchers only in field 1-2 years
Paternity studies on adult & infant males in 18 wild groups of Hanuman langurs.
Of 16 presumed cases of infanticide by a male, none involved male related to missing infant.
Thus, researchers concluded that paternity excludes infanticide in langurs.
Infanticide in Patas Monkeys
Found from Western Ethiopia to Senegal.
Mainly a terrestrial species.
Group sizes 5-74 inds.
Diet of grass seeds, new shoots, & acacia gums
1-male, multifemale groups. – males leave, female stays in natal group
Adult males’ temporary tenure in group.
2 Infanticide in Patas Monkeys
During 7-year period, infanticidal male only one of 13 resident males not present during actual
conception season but present during following birth season.
Mothers differentially targeted for male aggression; increased sevenfold (7x) during days
surrounding infanticide & then decreased to baseline levels after infanticide.
Infanticide in Patas Monkeys
As predicted by sexual selection hypothesis,