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 (T-b) across 24 births in a population of wild vervet monkeys using biologgers. We identified distinct perturbations in T-b during the birth period, including declining T-b during labour and the rapid recovery of T-b post-parturition. Vervet monkeys typically gave birth during their inactive phase in synchrony with the nadir of the maternal nychthemeral T-b rhythm but also showed remarkable inter-individual variability in their absolute T-b during birth. Our findings support the view that selection may have favoured a nocturnal timing of primate birth to coincide with lower night-time T-b and environmental temperatures, which improve thermal efficiency during birth.

Automated summary

Most primates, including humans, give birth during the inactive phase of the daily cycle. However, our understanding of the timing of nocturnal birth in wild diurnal primates is limited. This study measured maternal core body temperature during 24 births in wild vervet monkeys and found distinct changes in temperature during the birth process. Vervet monkeys typically give birth during their inactive phase, in sync with the lowest point of the maternal daily temperature rhythm, but individual variations in temperature during birth were observed. These findings suggest that the timing of primate birth may be influenced by the lower night-time temperatures and improve thermal efficiency during birth.