The thermoregulatory potential of Ovis horn cores

Cranial measurements were taken from 378 Ovis skulls (258 male, 120 female) , covering most subspecies of wild sheep. Horn core length and circumferenc e data were used to estimate the core surface. This highly vascularized ple xus constitutes the radiating area. A simple index of heat-exchange capacit y 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 diffe rent types of sheep could be compared, as well as allowing the detection of correlations with environmental gradients, which would point to a thermore gulatory role for horn cores. It can be assumed that, for sheep living in c old climates, heat conservation is important, while for those living in hot environments, enhanced heat dissipation would be advantageous. Our data co nfirm 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 cm(2)/kg, while desert-dwe lling 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 cm(2)/kg. Other sheep types have indices of intermediate size s. It is our position that this evolutionary trend to vary core size in res ponse to ambient temperature is independent of a parallel trend to increase horn size for the benefit of enhancing reproductive success.