Infanticide pressure accelerates infant development in a wild primate

The rate at which infants develop can vary within species. This variation may be due to differences between infants in their nutritional intake and physiology, or the ability of females to adjust the amount and timing of maternal investment to maximize their lifetime reproductive success. This is the first primate study that uses a large sample size and multivariate analyses to investigate whether variation in early infant development (measured visually using durations of natal coat stages) is explained by differences in infanticide pressure, predation pressure or feeding competition among mothers. We recorded the number of days that infants took to transition through each of the two natal coat stages (white to grey: N = 32; grey to black-and-white: N = 22), as well as through their entire natal coats (white to black-and-white: N = 38) in a population of wild ursine colobus, Colobus vellerosus. Infant males, which are at greater risk of infanticidal attacks, transitioned coat colours earlier than females, and infants in multimale groups, where infanticide occurs more frequently, transitioned earlier than infants in unimale groups. Variation in group size did not affect natal coat durations, which suggests that the intensity of predation risk and feeding competition do not influence development. Instead of terminating investment in offspring before birth, as in the 'Bruce effect', females may invest more heavily in infants after birth in order to speed up infant development and reduce the time period during which offspring are the most vulnerable to infanticide. Mothers may therefore have flexible means of exerting choice over maternal investment in relation to infanticide risk. However, the extent to which mothers and infants are responsible for adjusting the speed of development is unknown.