The thermal consequences of primate birth hour and its evolutionary implications

Abstract

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.

Keywords: hyperthermia; hypothermia; parturition; reproduction; thermoregulation; vervet monkey.

Figure 1.

Maternal 72 h body temperature (T_b) rhythms across two diurnal (orange lines) and two nocturnal (blue lines) vervet monkey births. Asterisks denote time of birth. Black lines denote the average non-birthing female time-matched 72 h T_b, and grey lines denote ambient air temperature. Top-left birth was photographed and video recorded (electronic supplementary material, Video).

Figure 2.

Maternal body temperature at the time of vervet monkey birth (dots) and the average female nychthemeral 24 h body temperature rhythm during the birth season (black line). Blue dots represent inactive-phase nocturnal births, purple dots the inactive-phase diurnal births and orange dots the observed active-phase diurnal births. Blue line denotes the average timing of typical inactive-phase births.