Lecture 13

How do members of a group benefit from group size? 1.) Dominant males benefit most from [blank_start]single-male[blank_end] groups, followed by [blank_start]multi-male groups[blank_end] with takeovers by [blank_start]outside[blank_end] males. 2.) Females prefer [blank_start]multi-male[blank_end] groups with takeovers from [blank_start]inside[blank_end] the group. However they prefer [blank_start]single-male[blank_end] groups if there is a very [blank_start]powerful[blank_end] male. 3.) [blank_start]Multi-male[blank_end] groups will only happen when presence of additional males brings benefits to [blank_start]females[blank_end] or where groups are too [blank_start]large[blank_end] for one male to [blank_start]monopolize[blank_end] [blank_start]fertilisations[blank_end]. 4.) Field data suggest that females can control the [blank_start]composition[blank_end] of the [blank_start]adults[blank_end] in a group, but have less control over the [blank_start]source[blank_end] of [blank_start]takeovers[blank_end] or relative [blank_start]male[blank_end] [blank_start]strength[blank_end].

Arguments against the importance of infanticide in primates: 1.) If infanticidal behaviour was [blank_start]selected[blank_end] [blank_start]for[blank_end], we would expect to see increasingly [blank_start]aggressive[blank_end] [blank_start]males[blank_end]: this would not be [blank_start]adaptive[blank_end]. 2.) Relatively few cases have been [blank_start]empirically[blank_end] demonstrated: more have been [blank_start]inferred[blank_end]. 3.) Examples of [blank_start]counterstrategies[blank_end] to [blank_start]infanticide[blank_end] are relatively [blank_start]rare[blank_end].

Perhaps some of the failings of infanticide to explain certain phenomena is because infanticide is becoming a [blank_start]megapardigm[blank_end]. It is often being used as a [blank_start]stock[blank_end] [blank_start]explanation[blank_end].

Important analysis of [blank_start]23[blank_end] cases of infanticide found support predictions that it's a male reproductive tactic. 1.) Infanticide is a lot more common in [blank_start]single-male[blank_end] groups. This support the [blank_start]sexual[blank_end] [blank_start]selection[blank_end] [blank_start]hypothesis[blank_end]. 2.) Almost all of the cases were by [blank_start]unrelated[blank_end] and [blank_start]immigrant[blank_end] males. Only [blank_start]2[blank_end] cases came from [blank_start]inside[blank_end] the group. A large portion came from after a [blank_start]male[blank_end] [blank_start]takeover[blank_end]. 3.) All the victims were [blank_start]unweaned[blank_end] and infanticide reduced the [blank_start]birth[blank_end] [blank_start]interval[blank_end] by [blank_start]66%[blank_end] in [blank_start]4[blank_end] cases. A large portion of the males [blank_start]mated[blank_end] with the female after the killing.

If a female [blank_start]mates[blank_end] with an [blank_start]infanticidal[blank_end] male, she will increase her own [blank_start]reproductive[blank_end] [blank_start]success[blank_end]. Mating with an infanticidal male means [blank_start]male[blank_end] [blank_start]offspring[blank_end] will have these good [blank_start]genes[blank_end] and be successful. When usurper kills a [blank_start]baby[blank_end] [blank_start]silverback[blank_end] [blank_start]gorilla[blank_end], the female is more likely to mate with the [blank_start]usurper[blank_end], even if the [blank_start]father[blank_end] tried to protect the infant.

Infanticide counterstrategies: 1.) [blank_start]Obscur[blank_end]ing [blank_start]paternity[blank_end]: Females will [blank_start]mate[blank_end] with [blank_start]multiple[blank_end] males. 2.) [blank_start]Self-protection[blank_end] strategies by [blank_start]infants[blank_end]: e.g. [blank_start]age[blank_end] [blank_start]deception[blank_end], [blank_start]concealment[blank_end], [blank_start]avoid[blank_end]ing [blank_start]strangers[blank_end]. 3.) Strong [blank_start]affilative[blank_end] bonds between males and females. 4.) Female [blank_start]dispersal[blank_end], female [blank_start]alliance[blank_end]s and relationships between [blank_start]males[blank_end] and [blank_start]infants[blank_end] may all be shaped by infanticidal risk.

Questions to address: 1.) What the [blank_start]lifetime[blank_end] [blank_start]cost[blank_end]/[blank_start]benefit[blank_end] balance for infanticidal males? 2.) How does male presence in [blank_start]pair[blank_end] [blank_start]bonds[blank_end] influence risk of infanticide? 3.) How effective are infanticide [blank_start]counterstrategies[blank_end]: are infanticidal/ [blank_start]sexual[blank_end] [blank_start]coercian[blank_end] hypotheses most parsimonius explanation?