ИСТИНА

Great Tit males and females clearly differ in body size and social positions in wintering groups. We attempted to find out if short time changes in ambient temperature (Ta) modify the effect of social factors on energetics of males and females. Yearling Great Tits were randomly joined in heterosexual pairs and kept in outdoor aviaries from the beginning of the winter season to July (Moscow region, 2002-2007, N=260). We used one- and two-room aviaries which modeled two gradations of density factor and environment heterogeneity. The food was provided ad libitum. Among tits living in one-room aviaries, the winter basal metabolic rate (BMR) was higher in females than in males. This relation was not pronounced in pairs occupying two-room aviaries. BMR of females was negatively influenced by ambient temperatures during the entire winter period. In males, similar temperature dependence of BMR occurred only in the beginning of winter. The energetic difference between males and females reached maximum in mid-winter (on average, the coldest period in the study area) in pairs staying in one-room aviaries. The superiority of females in BMR was minimal in both types of aviaries in the years of an anomalously warm winter (2006-2007). The results suggest that under conditions of intensive social competition, low temperatures may act as a factor markedly increasing sexual asymmetry in maintaining energetic costs.