Evolution of primate social systems

Three key factors determine primate social systems. 1.) [blank_start]Defensibility[blank_end] of resources determine [blank_start]female[blank_end] distribution. 2.) The [blank_start]spatial[blank_end] and [blank_start]temporal[blank_end] distribution of food patches determines how [blank_start]large[blank_end] a group can be, and whether groups must periodically undergo [blank_start]fission[blank_end]. 3.) [blank_start]Predator[blank_end] pressures influence whether animals can travel alone, whether [blank_start]males[blank_end] must protect [blank_start]young[blank_end]. If animals have to travel in [blank_start]large[blank_end] groups for protection, an individual's rate of [blank_start]food[blank_end] intake will be [blank_start]decreased[blank_end].

These ecological pressures affect a wide range of [blank_start]mammals[blank_end] though - but primates are unusual in their year-round [blank_start]association[blank_end] between [blank_start]males[blank_end] and [blank_start]females[blank_end]. Females have high levels of [blank_start]infant[blank_end] [blank_start]care[blank_end], so there might be selection on males for strategies to return females to [blank_start]oestrous[blank_end].

Infanticide has been inferred or observed in 51 species of primates. It has been observed in l[blank_start]emurs[blank_end], p[blank_start]latyrrhines[blank_end], co[blank_start]lobines[blank_end], ce[blank_start]rcopithecines[blank_end] and h[blank_start]ominoids[blank_end].

The early evolution of [blank_start]infanticide[blank_end] probably went a bit like this: 1.) Ancestral primate probably had [blank_start]litters[blank_end] of several young, who were left in the [blank_start]nest[blank_end] whilst she foraged. Lactation was [blank_start]shorter[blank_end]. 2.) Female life-history selected for [blank_start]slower reproduction[blank_end] and a [blank_start]longer lactation[blank_end] period. This increased the risk of [blank_start]intanticide[blank_end] by males. 3.) Males may have [blank_start]guarded[blank_end] nests at first whilst the the female foraged. This is still seen in [blank_start]ruffed lemurs[blank_end].

Infanticide then progressed from males guarding nests whilst females foraged. 1.) [blank_start]Litter sizes[blank_end] declined and infants become less [blank_start]altricial[blank_end]. Females carried young, which increased [blank_start]foraging efficiency[blank_end]. 2.) An association between [blank_start]male[blank_end] and i[blank_start]nfant[blank_end] may have been an important step in moving between [blank_start]solitary[blank_end] life to constant [blank_start]intersexual[blank_end] association.