Seasonal Patterns of Body Temperature Daily Rhythms in Group-Living Cape Ground Squirrels Xerus inauris

Michael Scantlebury,Nigel C Bennett,Kenneth E Joubert,Mary-Beth Manjerovic,Philip W Bateman,Marine Danek-Gontard,Jane M Waterman,Mark Briffa

doi:10.1371/journal.pone.0036053

Abstract

Organisms respond to cyclical environmental conditions by entraining their endogenous biological rhythms. Such physiological responses are expected to be substantial for species inhabiting arid environments which incur large variations in daily and seasonal ambient temperature (Ta). We measured core body temperature (Tb) daily rhythms of Cape ground squirrels Xerus inauris inhabiting an area of Kalahari grassland for six months from the Austral winter through to the summer. Squirrels inhabited two different areas: an exposed flood plain and a nearby wooded, shady area, and occurred in different social group sizes, defined by the number of individuals that shared a sleeping burrow. Of a suite of environmental variables measured, maximal daily Ta provided the greatest explanatory power for mean Tb whereas sunrise had greatest power for Tb acrophase. There were significant changes in mean Tb and Tb acrophase over time with mean Tb increasing and Tb acrophase becoming earlier as the season progressed. Squirrels also emerged from their burrows earlier and returned to them later over the measurement period. Greater increases in Tb, sometimes in excess of 5°C, were noted during the first hour post emergence, after which Tb remained relatively constant. This is consistent with observations that squirrels entered their burrows during the day to ‘offload’ heat. In addition, greater Tb amplitude values were noted in individuals inhabiting the flood plain compared with the woodland suggesting that squirrels dealt with increased environmental variability by attempting to reduce their Ta-Tb gradient. Finally, there were significant effects of age and group size on Tb with a lower and less variable Tb in younger individuals and those from larger group sizes. These data indicate that Cape ground squirrels have a labile Tb which is sensitive to a number of abiotic and biotic factors and which enables them to be active in a harsh and variable environment.

Highlights

Organisms respond to cyclical variation in environmental conditions by entraining their endogenous biological rhythms [1,2]
One such rhythm in endothermic species is that of body temperature (Tb), which is considered to be a consequence of the balance between heat production and heat dissipation [3]
It is unclear whether one should examine the effects of environmental variation on raw Tb data or use some index which can be comparable across species (e.g. ‘Heterothermy Index’, ‘heterothermy index (HI)’ [12])

Summary

Introduction

Organisms respond to cyclical variation in environmental conditions by entraining their endogenous biological rhythms [1,2]. Angilletta et al (2010) [13] suggest that future empirical work should examine the potential ‘‘selective pressures imposed by regional and temporal heterothermy’’ They identify several potential candidates which might cause Tb variations to evolve which include food and water availability, Ta and social huddling. Restricted food and water supplies and low Ta values should favor energy-saving reductions in Tb and temporal heterothermy Implicit in their arguments is the fact that extremes of variation in Ta and in particular cyclical variations in Ta may result in adaptive variation in Tb daily rhythms [13,14,15,16]. Tb daily rhythms are likely to be affected by group size in social animals

Methods

Results

Conclusion