Summary of:
Sarah Blaffer Hrdy (1979). “Infanticide Among Animals: A Review, Classification, and
Examination of the Implications for the Reproductive Strategies of Females.” Ethology and
Sociobiology: 13-40.
Summary by:
Randi Scott, Cat Alvarado, and Sanan Mehserdjian
Fall, 2014
Outline
I. Introduction: Overview of paper
-Perpetrators: adult males, adult females, and other immatures
-By outsiders, members of own group, and parents
-Purpose of paper: explain and categorize instances of infanticide
-Five classes of infanticide
II. Classes of Infanticide
A. Exploitation
-Assumes infanticide is evolved and adaptive for the perpetrators
-When perpetrators benefit from consumption or use of their victim
-Infant becomes a food resource, protective buffer against third parties, or a
prop for gaining maternal experience
-Cannibalism
-Squirrels, lions, some fish and insects
-Evolutionary adaptations for protection against cannibalism
-Starvation
-Female primates “aunting” or kidnapping by non-lactating females
B. Resource Competition
- Assumes infanticide is evolved and adaptive for the perpetrators
-Competition for food or nest sites
-Death of infant  increased access to resources for killer/ killer’s offspring
-Wild dogs, elephant seals, large primates
-Xenophobic infanticide by other troops  hyenas, wolves, chickens, eagles
C. Parental Manipulation
- Assumes infanticide is evolved and adaptive for the perpetrators
-Death of infant  improved survival of mother or other offspring
-Imperfect infant  unlikely to carry on genes
-Human females may abort in poor conditions for childbearing
-Kangaroos may throw babies out of pouch to distract predator
-May increase future breading opportunities for parent
-Norway rats  relates to resource availability
D. Sexual Selection
- Assumes infanticide is evolved and adaptive for the perpetrators
-Competition between one sex for reproductive investment of the other sex
 advantageous to kill another animal’s offspring
-Reduces reproductive success of competitors
-Eliminating infant increases infanticidal male’s chance at reproducing
-Occurs during lactation
-Rarely by females  sex-reversed species like quails
-Study involving lemurs confirmed hypothesis
-Lions, various primates  new dominant male kills other male’s offspring
-Females offer cues whether to attack or tolerate her infant
-Rare human evolved behaviors; small tribe in Brazil; some biblical passages
E. Social Pathology
-Does not assume infanticide is adaptive
-Pathological
-Child abuse in humans
III. Interspecific Variation in Infanticide
-In no species is infanticide a common event
-Most cases fit sexual selection pattern
-Highest rates of infanticide: Hanuman langurs: 83% in one troop particularly
vulnerable to male invasions
-New male leader kills unwanted infants  mothers resume copulating with
usurper
-Next highest rates: chimpanzees and gorillas
-Usually as a food resource
-Occurs mostly in species where male takeover is relatively short
IV. Interspecific Variation in Expression of Infanticide
-Differences at specific sites
-Some groups of langurs have been known to commit infanticide and others
have not
-Very little geographical difference; no behavior difference
-Best guess is the degree of human interference  creates “abnormal”
environment
-Are male takeovers associated with habitat disturbance?
-Hrdy argues that population density has more to do with it
V. The Implications of Infanticide for Female Reproductive Strategies
A. Social Organization
-Female gorillas move between groups depending on how successfully her
offspring was raised in a particular unit
-Males protect females after male takeover
B. Female Reproductive Physiology
-Ability of females to conceive soon after losing an infant  adaptation for
evolution of infanticide
-If females delayed ovulation (penalizing infanticide)  beneficial for the
species
-Evolution doesn’t work like that
-Ovulating immediately after benefits the individual
C. Female Sexual Behavior
-Males in many species involved in infanticide rely on behavior to indicate
ovulation
-Female langurs sometimes travel to all-male troops to mate
-Primates  trend away from hormonal signs of receptivity
-Females benefit from concealing ovulation
Critical Review
Critical Points:
1. Author’s analysis is well thought-out and answers most critical questions. For example, if
infanticide is a male reproductive strategy how would males avoid killing their own
offspring? Hrdy answers with a study on langurs: it is the female, rather than the infant,
who provides males with the cue to attack or tolerate her infant.
2. Difficulties in interpreting captive observations, particularly since infanticide has never
been observed in the wild among some of the most prominent examples, such as macaques.
3. It can be very difficult sometimes to discern whether infanticide is pathological or an
evolved tactic. For example, a scared captive animal may devour her young. Is this
deranged or do some animals respond to danger by eliminating current offspring?
Test Questions
1. Perpetrators of infanticide include
A. Adult males
B. Adult females
C. Other infants
D. All of the above
2. Which of these is not an evolved tactic for infanticide?
A. Exploitation
B. Social Pathology
C. Sexual Selection
D. Parental Manipulation
E. Resource Competition
3. True or False
A female will provide males with the cue to attack or tolerate her infant, based on if the
child is his or not.
KEY
1. D
2. B
3. True