The effect of pregnancy and lactation on the body temperatures of free-living vervet monkeys

Abstract

Reproduction is physiologically challenging for female mammals, requiring an increase in food and water intake to meet the demands of gestation and lactation. One way to assess how free-living female mammals cope with these increased demands in changing environments (e.g., fluctuating environmental temperatures and resource availability) is by assessing their body temperature patterns through biologging. As part of a long-term project to investigate the behavioural and physiological mechanisms employed by free-living vervet monkeys (Chlorocebus pygerythrus) in the semi-arid Nama Karoo in the Eastern Cape, South Africa, I investigated how well female monkeys regulated their body temperatures during gestation and lactation.

Body temperature (Tb) data were collected from 30 female vervet monkeys over a seven-year period. During that period, we recorded 27 pregnancies. Mean air temperature (Ta) was 23.0 ± 3.1 °C in summer and 11.4 ± 4.6 °C in winter. Early stages of pregnancy coincided with mid-winter conditions, with pregnancy progressing through spring when mean Ta was 17.3 ± 4.1 °C. Lactation generally started in mid-summer and lasted through autumn when mean Ta was 17.4 ± 4.8 °C. In one year, the vervet monkeys were exposed to drought with total annual rainfall of only 132 mm, compared to the average annual rainfall of 292 ± 99 mm per year. Rain fell predominantly in the summer months peaking between December and March, while the winter months were drier months. Normalized difference vegetation index (NDVI) (i.e., vegetation greenness) was used as a proxy for food availability.

As shown for other mammals, pregnant vervet monkeys displayed gestational hypothermia with a progressive decline in overall 24-h Tb (i.e., 24-h minimum, 24-h maximum and 24-h mean) as gestation progressed, such that mean 24-h Tb was 0.4 °C lower at the end of pregnancy compared to the start of pregnancy. Despite later stages of pregnancy coinciding with warmer conditions and increased food abundance, mean 24-h Tb progressively declined up until the day of parturition. Gestational hypothermia in free-living vervet monkeys was therefore independent of environmental conditions. Pregnant females also displayed improved homeothermy compared to lactating and non-lactating females, where 24-h amplitude of the Tb rhythm was 0.1 - 0.2 °C lower in pregnancy.

During lactation, minimum 24-h Tb of females was slightly higher (0.1 °C) than that of non lactating females but there was no significant difference in maximum 24-h Tb between lactating and non-lactating females. The similar Tb patterns in lactating females suggest that they received adequate water and energy intake to meet the demands of lactation. The 24-h amplitude of the Tb rhythm of lactating females was on average 0.1 °C lower than non lactating females. Over the course of lactation minimum 24-h Tb and mean 24-h Tb did not change but maximum 24-h Tb increased slightly by approximately 0.2 °C and the 24-h amplitude of the Tb rhythm increased by 0.1 °C. The small increase in maximum 24-h Tb and the 24-h amplitude of the Tb rhythm over the course of lactation indicates that lactating females generally maintained Tb very well, especially considering the increasing energetic and water demands of caring for a growing infant.

During drought conditions, lactating females experienced a decrease in minimum 24-h Tb associated with poor energy availability, and an increase in maximum 24-h Tb associated with water stress, compared to normal years. As a result, they exhibited an increase in the 24-h amplitude of the Tb rhythm, likely reflecting a trade-off in allocating limited food and water supplies to other homeostatic systems rather than to thermoregulation. In general, lactating females regulated body temperature relatively well, without severe hyperthermia or hypothermia, indicating that they were able to cope with the energetic and water demands of lactation during a period of poor resource availability.

My study has contributed valuable insights into understanding how female vervet monkeys respond to environmental (e.g., fluctuating environmental temperatures and resource availability) and physiological (e.g., reproduction) stressors through investigating body temperature patterns. Although female vervet monkeys coped very well with the energetic and water demands of reproduction in their semi-arid environment, future climate change predictions are likely to exacerbate the seasonal characteristics of the Karoo, which may result in increased environmental stress (e.g., higher ambient temperatures and more frequent droughts). Biologging of body temperature provides insights on reproductive phenology and the physiological welfare of mothers and may be a valuable tool to assess how various free living mammals cope with changes in their environments.