Comparative assessment of the thermal insulation properties of hair in foals of different populations of the Yakut horse

Abstract

This study aims to conduct a comparative analysis of thermoregulatory characteristics in two populations of Yakut horse foals — the Momsky (Arctic region) and Amginsky (Central region) — during the first six months of life. The thermal insulation properties of the coat were assessed using an insulation index (II) based on coat mass, hair density, and hair length. In winter, the II for Momsky foals was 7270.65 (density: 1812 ± 127 hairs/cm²; mass: 0.107 ± 0.0058 g/cm²; hair length: 37.5 ± 0.5 mm; diameter: 57.9 ± 1.31 µm), while the Amginsky foals had an II of 4646.34 (density: 1443 ± 108 hairs/cm²; mass: 0.070 ± 0.0043 g/cm²; hair length: 46.0 ± 0.68 mm). To approximate thermal conductivity (k), values of thermal resistance (R) in the range of 0.8–0.9 ° C·m²·W⁻¹ were used, consistent with data for cold-adapted mammals (Walsberg, 1991; Kvadsheim et al., 1994, among others). Using the formula k = L / R, approximate conductivity values were calculated. The thermal conductivity of the Momsky foals (k = 0.042–0.047 W/m·K) was lower than that of the Amginsky foals (k = 0.051–0.058 W/ m·K), which is attributed to the higher hair density that reduces heat loss. Although Amginsky foals had longer hair, the lower density increased convective loss. Both II and k were adjusted for seasonal variation and hair diameter, confirming that the higher density and optimal diameter in Momsky foals ensure effective thermoregulation even with shorter hair length. Morphologically, Momsky foals were more compact (girth index: 118.9% vs. 112.7%), minimizing heat loss in accordance with Bergmann’s rule. Overall, the findings indicate that the Mom sky population of Yakut horse foals displays more pronounced thermoadaptive traits during the early stages of postnatal development.