Evolutionary determinants of non-seasonal breeding in wild chacma baboons

Animal reproductive phenology varies from strongly seasonal to
non-seasonal, sometimes among closely related or sympatric species. While
the extent of reproductive seasonality is often attributed to
environmental seasonality, this fails to explain many cases of
non-seasonal breeding in seasonal environments. We investigated the
evolutionary determinants of non-seasonal breeding in a wild primate, the
chacma baboon (Papio ursinus), living in a seasonal environment with high
climatic unpredictability. We tested three hypotheses proposing that
non-seasonal breeding has evolved in response to (1) climatic
unpredictability, (2) reproductive competition between females favoring
birth asynchrony, and (3) individual, rank-dependent variations in optimal
reproductive timing. We found strong support for an effect of reproductive
asynchrony modulated by rank: (i) birth synchrony is costly to subordinate
females, lengthening their interbirth intervals, (ii) females alter their
reproductive timings (fertility periods and conceptions) in relation to
previous conceptions in the group, and (iii) the reported effect of birth
synchrony on interbirth intervals weakens the intensity of reproductive
seasonality at the population level. This study emphasizes the importance
of sociality in mediating the evolution of reproductive phenology in
group-living organisms, a result of broad significance for understanding
key demographic parameters driving population responses to increase
climatic fluctuations.