Among mammals, the order Primates is exceptional in having a high taxonomic richness in which the taxa are arboreal, semiterrestrial, or terrestrial. Although habitual terrestriality is pervasive among the apes and African and Asian monkeys (catarrhines), it is largely absent among monkeys of the Americas (platyrrhines), as well as galagos, lemurs, and lorises (strepsirrhines), which are mostly arboreal. Numerous ecological drivers and species-specific factors are suggested to set the conditions for an evolutionary shift from arboreality to terrestriality, and current environmental conditions may provide analogous scenarios to those transitional periods. Therefore, we investigated predominantly arboreal, diurnal primate genera from the Americas and Madagascar that lack fully terrestrial taxa, to determine whether ecological drivers (habitat canopy cover, predation risk, maximum temperature, precipitation, primate species richness, human population density, and distance to roads) or species-specific traits (body mass, group size, and degree of frugivory) associate with increased terrestriality. We collated 150,961 observation hours across 2,227 months from 47 species at 20 sites in Madagascar and 48 sites in the Americas. Multiple factors were associated with ground use in these otherwise arboreal species, including increased temperature, a decrease in canopy cover, a dietary shift away from frugivory, and larger group size. These factors mostly explain intraspecific differences in terrestriality. As humanity modifies habitats and causes climate change, our results suggest that species already inhabiting hot, sparsely canopied sites, and exhibiting more generalized diets, are more likely to shift toward greater ground use.

Infanticide is considered a conspicuous expression of sexual conflict amongst mammals, including at least 35 primate species. Here we describe two suspected and one attempted case of intragroup infanticide in spider monkeys that augment five prior cases of observed or suspected infanticide in this genus. Contrary to the typical pattern of infanticide seen in most primate societies, where infants are killed by conspecifics independent of their sex, all eight cases of observed or suspected infanticide in spider monkeys have been directed toward male infants within their first weeks of life. Moreover, although data are still scant, infanticides seem to be perpetrated exclusively by adult males against infants from their own social groups and are not associated with male takeovers or a sudden rise in male dominance rank. Although the slow reproductive cycles of spider monkeys might favor the presence of infanticide because of the potential to shorten females' interbirth intervals, infanticide is nonetheless uncommon among spider monkeys, and patterns of male-directed infanticide are not yet understood. We suggest that given the potentially close genetic relationships among adult males within spider monkey groups, and the need for males to cooperate with one another in territorial interactions with other groups of related males, infanticide may be expected to occur primarily where the level of intragroup competition among males outweighs that of competition between social groups. Finally, we suggest that infanticide in spider monkeys may be more prevalent than previously thought, given that it may be difficult for observers to witness cases of infanticide or suspected infanticide that occur soon after birth in taxa that are characterized by high levels of fission-fusion dynamics. Early, undetected, male-biased infanticide could influence the composition of spider monkey groups and contribute to the female-biased adult sex ratios often reported for this genus.

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