Differences in the thermal conductance of tropical and temperate bovid horns

Abstract

Bovid horns consist of a highly vascularized bone core covered by a keratin sheath which seems to offer limited resistance to heat flow. Based on dynamic cooling curves measured for inverted horns filled with warm water, we developed estimates of the thermal conductance of keratin and the coefficients of convective heat transfer at the water-to-sheath and the sheath-to-air boundaries to allow us to quantify heat flux through the horn sheath. Coupled with measurements of the internal and external horn dimensions, we constructed a simplified conceptual model of sheaths from 68 horns of 14 bovid species to test the prediction that the horns of temperate bovid species offer greater resistance to heat flux than the horns of tropical bovids. The specific heat capacity of the keratin sheath was 1.53 ± 0.07 (SD) J g−1°C−1. The coefficient of conductive heat transfer for keratin was 6.30 ¥ 10−3 ± 0.30 ¥ 10−3 (SD) W cm−1°C−1. We estimated the coefficients of convective heat transfer at the water-to-sheath and the sheath-to-air interfaces to be 8.79 ¥ 10−3 ± 5.20 ¥ 10−3 W cm−2°C−1 and 2.49 ¥ 10−3 ± 1.98 ¥ 10−3 W cm−2°C−1, respectively. A reduction in the size of the bone core and overlying vascular bed and an increase in the thickness of the keratin sheath in temperate bovids acted to reduce the surface area through which heat was lost to the environment. Because the surface-specific thermal conductances of temperate sheaths were lower than those of tropical sheaths, we estimate that a temperate bovid having horns of the same length and external surface as a tropical bovid would experience only 75.7% of the heat loss when facing a thermal gradient of 20°C. We argue that differences in horn morphology between temperate and tropical Bovidae appear to have evolved as adaptations to restrict heat loss in the former while facilitating heat loss in the latter group.