Fibrous fur or fleece coats have an important role in insulating animals and aiding in the maintenance of homeothermy. Alpacas, raised for fibre production, are selected towards the finest fibre to improve the wearability of their fibre in garment form. The thermal consequences of reducing the fibre diameter on the external insulation are unknown, and may have a negative effect for the alpaca's thermal balance. We hypothesised that for a given fibre density, finer fibres would trap more air and provide lower thermal conductivity when exposed to low wind speed, but would be less robust, and so provide less insulation, when exposed to higher wind speed, than thicker fibres. We measured the thermal conductance of eight pelts of similar fibre density but with varying fibre diameter at 0, 1, 2, 4 and 6 m/s wind speeds. Thermal conductivity was similar between pelts of different fibre diameters (P = 0.58) at low wind speed. Conductance increased more in pelts with finer fibres at the high wind speed than in pelts with thicker fibres (P = 0.02). Thus at the same fibre density, finer fibres result in increased heat loss at high wind speed. Increased heat loss at higher wind speed would result in the animal requiring more energy to maintain heat balance below the lower critical temperature, which will reduce fibre production efficiency. (C) 2011 Elsevier B.V. All rights reserved.