Data and R script from: Females prioritize future over current offspring in wild seasonally breeding Assamese macaques

Classical work on birds by 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 invested 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.