The thermal consequences of primate birth hour and its evolutionary implications.

Most primates, including humans, give birth during the inactive phase of the daily cycle. Practical constraints therefore limit our knowledge of the precise timing of nocturnal birth in wild diurnal primates and so limit our understanding of selective pressures and consequences. We measured maternal core body temperature (Tb) across 24 births in a population of wild vervet monkeys using biologgers. We identified distinct perturbations in Tb during the birth period, including declining Tb during labour and the rapid recovery of Tb post-parturition. Vervet monkeys typically gave birth during their inactive phase in synchrony with the nadir of the maternal nychthemeral Tb rhythm but also showed remarkable inter-individual variability in their absolute Tb during birth. Our findings support the view that selection may have favoured a nocturnal timing of primate birth to coincide with lower night-time Tb and environmental temperatures, which improve thermal efficiency during birth.