To test the hypothesis that large, well-built, nests are an important component of fitness, we kept 12 mating pairs of two high-selected, two control, and two low-selected lines, selected for thermoregulatory nest-building behavior, at 22 and 4 degrees C with access to 10 g of cotton to build a nest, for a period of 180 days. Measurements included number of litters born per family, number of young per litter born and surviving up to 40 days of age, nest type built by the parents, and weight gain of the young from weaning (20 days of age) to 40 days of age. In all lines the production and survival of offspring was substantially decreased at 4 degrees C compared to 22 degrees C, but the high-selected lines produced more and better-quality offspring, surviving up to 40 days of age at both temperatures compared to the control and low-selected lines. This indicates that thermoregulatory nest-building behavior and evolutionary fitness are closely associated.