Fig. 1. Changes of body mass during the course of lactation and cold-exposure. Values are means ± s.e.m. (N=8). The lactating voles had higher body mass compared to non-reproductive (NR) voles, and body mass decreased significantly during the 15-day lactation compared to the NR voles, which had constant body mass throughout. Cold exposure did not influence body mass in either the NR or lactating voles. NW, non-reproductive voles in the warm; NC, non-reproductive voles in the cold; LW, lactating voles in the warm; LC, lactating voles in the cold.