Bovid horns play a prominent role in social and sexual interactions. In cold climates, however, heat loss through the horn surface may represent a major energy cost. We measured surface temperatures of horns (Th) for two Barbary sheep (Ammotragus lervia; one male and one female) at the Granby Zoo, Québec, using temperature-sensitive radiotrans-mitters. Over ambient temperatures (Ta) of 5 to — 19°C, Th never dropped below 3°C. At Ta ≈ −10°C, the difference between Th and Ta was 17°C for the male and 21°C for the female. Using empirical models to predict heat loss through horns and resting metabolic rates, we estimate that, at Ta of ≈−10°C, heat loss through the horn surface is 20% of resting metabolic rate for females and 29% for males which have larger horns than females. We argue that the metabolic costs of possessing large horns in cold climates may impose constraints on morphology and sexual selection.