Mantled howler monkeys’ responses to infanticide risk
A study into the behaviour of mantled howler monkeys in Honduras has found that females with infants are significantly more likely to be vigilant in response to loud calls from extra-group males, than females without infants.
Infanticide risk among howler monkeys is high due to the presence of extra-group males that do not have breeding access to females. These extra-group males gain breeding access to females by aggressive takeovers of existing groups in which resident males are ejected from the group and existing infants are killed.
Infanticide by extra-group males serves to increase male reproductive success because when females stop lactating after the death of an infant they soon begin cycling and can be fertilized by the newly resident males.
Howler monkeys are named after their distinctive loud calls. These loud calls are reported to function as a territorial behaviour between groups, which provide honest signals of group location and the number of males making the calls.
Infanticide risk among howler monkeys is high due to the presence of extra-group males that do not have breeding access to femalesThe likely function of these territorial displays by male howler monkeys is to reduce the rate of group takeovers by potentially infanticidal males. Playback experiments with black howler monkeys (Alouatta pigra) in Belize have indicated that female behaviour is influenced by the presence and outcome of male calling, particularly when females have young infants.
Since 2008, Operation Wallacea has been investigating female responses to loud calls in a hyper-dense population of mantled howler monkeys (Alouatta palliata) at Rancho Manacal, Honduras. Unlike black howler monkeys, mantled howler monkeys live in multi-male, multi-female groups of up to 40 individuals; when food supply is limited, these groups are reported to split into temporary sub-groups for the purpose of foraging.
We predicted that the ‘fission-fusion’ dynamics of mantled howler monkey societies would affect female responses to loud calls because the number of resident males available to protect them from potentially infanticidal males will change according to sub-group size and composition.
Responding to loud calls
Preliminary analysis of our dataset has indicated that, as with black howler monkeys, female mantled howler monkeys with infants are significantly more likely to be vigilant in response to loud calls from extra-group males than females without infants. The number of extra-group males calling relative to the number of resident males in the group also affected female vigilance, but this effect was only statistically significant when the calling males-to-resident male ratio was calculated based on the current sub-group size and composition rather than the number of males in the group as a whole.
Thus, when mantled howler monkeys are in a cohesive group containing many males and females, females with infants appear less concerned about loud calls from extra-group males. However, when mantled howler monkey groups split into smaller foraging sub-groups, females with infants are significantly more likely to be vigilant in response to loud calls when the number of males calling equals or exceeds the number of protector resident males in their subgroup.