Male short-tailed field voles (Microtus agrestis) build better insulated nests than females

Nest construction is an extremely widespread behaviour. III small endotherm s the nest serves primarily to provide insulation, and thereby retard heat loss of the constructor, of its offspring. In arctic and temperate regions many small mammals build nests to protect themselves from low ambient tempe ratures. We measured the physical properties of nests built by short-tailed field voles Microtus agrestis that were kept in captivity under cold condi tions. The most important factor influencing nest insulation was nest wall thickness; however, nests with thick walls also contained more nesting mate rial. Insulative capacity of the nest did not reach an asymptote up to nest s containing 20 g of material. Nest insulation was not correlated with rest ing metabolic rate, body mass or body composition of the vole that construc ted the nest. However, nests built by males had greater insulation than tho se made by females; males also had significantly lower food intake rates wh en compared to females with nests. No significant difference was observed i n either fat mass or whole animal thermal conductance between males and fem ales. Thermal conductance did increase significantly with increasing body m ass, although not with resting metabolic rate. Voles with nests for prolong ed periods had lower food intakes than voles without nests. The absolute sa ving averaged 1.9 g and was independent of body mass. This was a 28 % savin g on intake for a 22-g vole but only an 18% saving for a 40-g individual. W hen voles had nests for short periods they used the energy they saved to re duce food intake and increase body mass.