Infanticide_22

ANP 220

violence

WHO(2002)

• the intentional use of physical force or power, threatened or actual, against oneself, another person, or against a group of community that either results in or has a high likelihood of resulting in injury death, psychological hard, maldevelopment or deprivation.

Infanticide

killing of an infant

• by a conspecific (member of the same species)

infant

• unweaning individual; still nursing (excluding juveniles etc.)

behavioral ecology

infanticide is an evolved behavior; widely accepted

anthopolgy and primatology

infanticide is a highly controversial issue where some critiques argue that by providing an evolutionary explanation researchers justify aggression and killing.

beware of naturalistic fallacy

Use of a naturally occurring phenomenon (adapted behavior) as moral justification (good, desirable)

example:

• biological success = high fitness, adapted

confusion of

• evolutionary explanation (understanding why it occurs)

WITH

• moral justification (=judgement of good or bad)

infanticide _ sussman, cheverus & barlett

not an evolved behavior
◆ The sexual selection hypothesis (SSH) does not work/ Match the predictions

❖ data
◆ few data, rare events (rarer than predation)
◆ No behavioral mechanism to deter infanticide
◆ rarely fit predicted pattern; highly variable
 ◇ infanticidal males do not mateInfanticidal
  ◇ no direct attacks at infants; general male aggression; infants get in the way

❖ theory
◆ no infanticide gene
◆ Selection not demonstrated
◆ likely weak effects on fitness, i.e., weak selection

⇨ Infanticide is a byproduct of aggression

infanticide - Hrdy, Janson, & van Schaik

❖ evolved behavior
◆ sexual selection hypothesis (SSH) works well in many (most) cases

❖ data
◆ rates (N/time) important not N (occurrence),
  which depends on observation time and effort
◆ widespread (many species); effects similar to predation
◆ occurrence variable because conditions variable
◆ counterstrategies by males and females common and successful

❖ theory
◆ more than one explanation
  ◇ not all cases fit SSH
◆ genetic effects demonstrated in mice; not available for primates
◆ though selection coefficients unknown, effects (likely) strong

⇒ SSH primary explanation

infanticide occurring across the natural kingdom

• beetles like the roundneck sexton beetles

• spider like desert spiders

• fish like rainbow kribs

• birds like barn swallows

• mammals like brown bears, horses, lion, bottlenose dolphins

infanticide in nonjuman primates

observed in the wild

genus level

• 21 to 64 genera (excluding homo sapiens

→ common in several apes and many Old World monkeys

occurs occasionally in some New World monkeys and lemurs

main pattern among nonhuman primates

victims infants, females and males

perpetrators - mostly adult males

victim and perpetrator - likely unrelated

circumstances

• chance of top dominant male

• 1 male group : male replacement

• multimale group : new high-ranking male immigration or raise in rank

5 explanatory hypotheses for infanticide

social pathology

general idea

• increases aggression because of unnatural living conditions

  • like living in cities at high densities, frequent male-take overs

• non adaptive benefit, infants as chance victims

• has been abandoned → modified

social pathology by product of male aggression

general idea

• increases aggression after immigration or instability of rank

• aggression aimed at females) perhaps w/o infants or w/ infants

• no adaptive benefits, infants chance victims

• mechanistic explanation

  (proximate); not necessarily alternative to adaptive explanation

background infanticide

starting point

• Males compete with other males over access to females (male-to-male competition is a form of sexual selection)

• pregnant/ lactating females

• “unavailable” for a male; a female invests in another male infant

infanticide

• ends a female’s investment; reduces her and the father’s fitness

  • Mothers and potential fathers, if around, should try to protect the offspring

→ intra- and intersexual conflict

• male and female counterstrategies to reduce the occurrence/ effectiveness of male infanticidal tactics

sexual selection hypothesis

Hrdy (1974)

• early less detailed accounts by others

hypothesis

• killing of infants leads to

• mothers early resumption of cycling

• perpetrator gains time & can sire offspring

predictions

• unrelated infants

• infants (unweaned); interbirth interval shortened

• chances of siring subsequent infant increased

study site species

Nepal gray langurs (Semnopithecus schistaceus)

• 18.3 individuals per group

• 72% multimale, multifemale

• monitored 10 groups: studied 3 groups intensively

• 7 years, more than 40,000 contact hours

habitat and climate

• 300 m (900 ft)

• semi-evergreen forest

• monsoon climate seasonal

infanticide in Nepal gray langurs

infants attacked or killed

• attacks: 24 witnessed and 3 inferred

• infanticide: 1 witnessed & 7 inferred

relatedness male perpetrator to victim

• male presence at conception: 94-98% unrelated

• DNA: 100% excluded from paternity

infant age

• unweaned (all still nursing)

interbirth interval shortened

• yes, in most cases

  • 19.2 months (premature death) versus 32.4 months (survived)

male gain

• DNA: (presumed) male perpetrators sired next infant

primates and the sexual selection hypothesis

general

• majority of cases fit the predictions

• likely explanation for man nonhuman primates

lack of fit

• some cases likely explained by “competition for resources”

  • like competition for helped in marmosets

• some cases unexplained

  • like between-group infanticide in Thomas langurs

Why do males attack females (with infants)?

example: care pattern in Nepal gray langurs

• infant carried / in contact mother

  • 1st month: 100-90% of the time

  • 2nd month: 90-80%

  • 3rd month: 80-70% of time

→ in the first months (when it pays the most, a male who attacks an infant also attacks the mother

Why is infanticide rare and rarely witnessed?

…because of slow life histories*

example: Nepal gray langur female

• If a female survives to age 28 (ca. 20 yrs of reproduction)

  • 8 offspring born

• ca. 50% of infants survive to year 2

  • 4 offspring dead

• Assume 25% die because of infanticide

  • 1 infantocide victim per female over a period of 20 years

→ to witness 1 infanticide one needs to observe 20 females over a period of 1 years with a new male being present

is predation any different?

No..for many primates the impact is similar

example: infanticide and predation in Nepal gray langurs

• in > 40,000 contacts hours

• inferred / witnessed 8 cases of infanticide

  • 15% of infants born

  • 31% of infant mortality

• inferred / witnessed 6 cases of predation

  • 21% of female and immature mortality

→ impact of predation and infanticide is roughly similar (though it is not true for all primates, like not in lemurs)

do females and males try to prevent infanticide

females

• mate with all males in a group

  • paternity confusion

• abortion (“Bruce effect”)

• protective coalitions

  • w/ females or w/ males of both

  • female dispersal of group split

• abrupt (early) weaning

males

• more tolerant to (own) infants

• likely fathers protect infants

genetics of infanticide option 1

• “infanticide gen” (better called “infanticide allele”)

  • A male with the allele is more likely to commit infanticide

Evidence in mice

• Some males are more likely to commit infanticide*

  • Perhaps an allele associated with infanticidal behavior

genetics of infanticide option 2

behavior is genetically influenced but condition dependent

• environmental effects on regulator genes

evidence in primates

• males change their behavior toward infants

• attackers of unrelated infants later become protectors of their offspring

How influential is infanticide?

example: potential loss for a Nepal gray langur female

• if a female survives to age 28 (ca. 20 yrs reproduction)

  • 8 offspring born

• ca. 50% of infants survive to year 2

  • 4 offspring dead

• ca. 31% of infant mortality due to infanticide

  • out of 8 infants born in 1.24 infants will be lost to infanticide

• example: potential gain for Nepal gray langur male

  • impossible to calculate with the available data

  • best guess: impact (fitness increase) of ca. 10-20%

→ for apes and Old World monkeys, the impact is (like) non-trivial

the “infanticide wars”

what’s behind the controversy?

how science works

• different interpretations of the same results

• over time more studies / more results in support of SSH

cherry picking

• some result were (deliberately?) in

naturalistic fallacy and the assumption that the SSH explanation applies to humans

Infanticide in humans is NOT explained by

the sexual selection hypothesis (but there are other possible evolutionary explanations)

infanticide in humans

occurs but is less common than in nonhuman primates

explanations

• competitions for resources

  • step-parents (often males)

  • Avoid parental investment in step-children

parental manipulation

• parents (often mothers)

• Avoid investment in current offspring

  • poor survival chances of offspring