Differences in the thermal conductance of tropical and temperate bovid horns

Bovid horns consist of a highly vascularized bone core covered by a keratin sheath which seems to offer limited resistance to heat now. Based on dynam ic cooling curves measured for inverted hems filled with warm water, we dev eloped estimates of the thermal conductance of keratin and the coefficients of convective heat transfer at the water-to-sheath and the sheath-to-air b oundaries to allow us to quantify heat flux through the horn sheath. Couple d with measurements of the internal and external horn dimensions, we constr ucted a simplified conceptual model of sheaths from 68 hams of 14 bovid spe cies to test the prediction that the horns of temperate bovid species offer greater resistance to heat flux than the horns of tropical bovids. The spe cific heat capacity of the keratin sheath was 1.53 +/- 0.07 (SD) J g(-1)deg rees C-1. The coefficient of conductive heat transfer for keratin was 6.30 x 10(-3) +/- 0.30 x 10(-3) tSDI W cm(-1)degrees C-1. We estimated the coeff icients of convective heat transfer at the water-to-sheath and the sheath-t o-air interfaces to be 8.79 x 10(-3) +/- 5.20 x 10(-3) W cm(-2)degrees C-1 and 2.49 x 10.3 +/- 1.98 x 10(-3) W cm(2)degrees C-1, respectively. A reduc tion in the size of the bone core and overlying vascular bed and an increas e in the thickness of the keratin sheath in temperate bovids acted to reduc e the surface area through which heat was lost to the environment. Because the surface-specific thermal conductances of temperate sheaths were lower t han those of tropical sheaths, we estimate that a temperate bovid having ho rns of the same length and external surface as a tropical bovid would exper ience only 75.7% of the heat loss when facing a thermal gradient of 20 degr ees C. We argue that differences in horn morphology between temperate and t ropical Bovidae appear to have evolved as adaptations to restrict heat loss in the former while facilitating heat loss in the latter group.