Alloparenting is associated with reduced maternal lactation effort and faster weaning in wild chimpanzees

Abstract

Alloparenting, when individuals other than the mother assist with infant care, can vary between and within populations and has potential fitness costs and benefits for individuals involved. We investigated the effects of alloparenting on the speed with which infants were weaned, a potential component of maternal fitness because of how it can affect inter-birth intervals, in wild chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Uganda. We also provide, to our knowledge, the first description of alloparenting in this population and present a novel measure of the contribution of milk to infant diets through faecal stable nitrogen isotopes (δ¹⁵N). Using 42 mother-infant pairs, we tested associations of two alloparenting dimensions, natal attraction (interest in infants) and infant handling (holding, carrying), to the proportion of time mothers spent feeding and to maternal lactation effort (mean nursing rates and mother-infant δ¹⁵N differences). Neither natal attraction nor infant handling was significantly associated with feeding time. Infant handling was inversely associated with both measures of lactation effort, although natal attraction showed no association. Alloparenting may benefit mothers by enabling females to invest in their next offspring sooner through accelerated weaning. Our findings emphasize the significance of alloparenting as a flexible component of female reproductive strategies in some species.

Keywords: allocare; alloparenting; lactation; nursing; stable isotopes; weaning.

Figure 1.

GEE β-coefficient estimates (circles) and their 95% confidence intervals (CIs; bars) for the effects of natal attraction on each dependent variable and covariate. Nursing rate, mean mother–infant δ¹⁵N difference and the proportion of time the mother was feeding were continuous variables, infant age was an ordinal variable, while maternal parity (primiparous, multiparous) and infant sex (female, male) were binary variables. **p < 0.01.

Figure 2.

GEE β-coefficient estimates (circles) and their 95% CIs (bars) for the effects of infant handling on each dependent variable and covariate. Nursing rate, mean mother–infant δ¹⁵N difference and proportion of time the mother was feeding were continuous variables, infant age was an ordinal variable, while maternal parity (primiparous, multiparous) and infant sex (female, male) were binary variables. *p < 0.05; **p < 0.01.