Females prioritize future over current offspring in wild seasonally breeding Assamese macaques

Classical work on birds by David Lack was foundational to life history theory when it uncovered a trade-off between offspring quantity and quality. Evidence for a similar trade-off was later found in singleton-bearing mammals, but its extent and underlying mechanisms are not fully understood. Here, we explore the role of adaptive reproductive scheduling and maternal energy depletion as the basis of the trade-off with data on 410 births by 104 mothers recorded over 18 years in a wild Assamese macaque (Macaca assamensis) population with seasonal reproduction. In any given mating season and after controlling for maternal age effects, the probability for a female to conceive was strongly predicted by the presence of a dependent offspring. The younger the current infant was, the less likely mothers were to invest in a new reproductive event, possibly to avoid stacked investment into nursing and unborn offspring. An inverse relationship between current infant survival and the conception of a new sibling points toward a shift in maternal resource allocation to future offspring. However, to avoid the energetic drain of shorter birth intervals, mothers delayed their reproductive timing within the mating season by 49 days with negative downstream effects for the next reproductive opportunity.