The thermoregulatory potential of Ovis horn cores

Abstract

Cranial measurements were taken from 378 Ovis skulls (258 male, 120 female), covering most subspecies of wild sheep. Horn core length and circumference data were used to estimate the core surface. This highly vascularized plexus constitutes the radiating area. A simple index of heat-exchange capacity was calculated by dividing the combined surfaces of two horn cores by the mass of the animal. This index provided a standard by means of which different types of sheep could be compared, as well as allowing the detection of correlations with environmental gradients, which would point to a thermoregulatory role for horn cores. It can be assumed that, for sheep living in cold climates, heat conservation is important, while for those living in hot environments, enhanced heat dissipation would be advantageous. Our data confirm this hypothesis. The thinhorn sheep (Ovis dalli and Ovis nivicola) of subarctic and arctic northwestern North America and northern Siberia have the smallest horn cores, with indices of 6.9-7.3 cm2/kg, while desert-dwelling types have indices of more than twice these values. For instance, the desert subspecies of the American bighorns (Ovis canadensis nelsoni, Ovis canadensis mexicana, Ovis canadensis cremnobates) have indices ranging from 15.1 to 16.5 cm2/kg. Other sheep types have indices of intermediate sizes. It is our position that this evolutionary trend to vary core size in response to ambient temperature is independent of a parallel trend to increase horn size for the benefit of enhancing reproductive success.