Seasonal variation in cold hardiness and peak rates of cold-induced thermogenesis in the Dark-eyed Junco (Junco hyemalis)

Abstract

-I used helium-oxygen (heliox) gas mixtures (approximately 79.5% He: 20.5% 02) to study seasonal variation in cold resistance and peak rates of thermogenesis (Vo2 max = maximum oxygen consumption maintained over a 10-min period) in Dark-eyed Juncos (Junco hyemalis). Cold tolerance increased markedly in winter-acclimatized juncos. Heliox temperatures required to induce hypothermia within 90 min in more than 50% of the birds tested were 4?C in summer and -9?C in winter. In winter, Vo2 m.. increased significantly on both a per-bird and mass-specific basis. In summer, Vo2mm,, was 5.78 ml 02/min (20.75 ml 02 g-1 h-'), and 7.39 ml 02/min (23.42 ml 02 gh-1) in winter. Vo2m,, exceeded standard metabolic rate by 6.6 times, and by 6.7-7.2 times in winter. Maximal instantaneous oxygen consumption (Vo2 i = maximal 1-min Vo2) under cold stress did not vary seasonally on either a per-bird or mass-specific basis. Mass loss rates during cold stress did not vary seasonally. Winter increases in V02 ma, and cold tolerance characterize metabolic acclimatization in Dark-eyed Juncos. Received 2 May 1989, accepted 29 January 1990. SMALL birds that overwinter in temperate climates show marked seasonal changes in cold tolerance (Hart 1962, Barnett 1970, Pohl and West 1973, Dawson and Carey 1976, Dawson et al. 1983). Seasonal variation in peak rates of cold-induced thermogenesis (Vo2ma., defined here as maximal Vo2 maintained over 10 min) in these birds has received little attention, mostly because of problems associated with generating experimental temperatures low enough to elicit maximum metabolism. High specific heat helium-oxygen gas mixtures (heliox) facilitate heat loss, and induce V02 max at less extreme temperatures (Rosenmann and Morrison 1974). Consequently, heliox gas mixtures have been used to document winter increases in maximal thermogenic capacity in several mammals, including snowshoe hares (Lepus americanus; Feist and Rosenmann 1975), red-backed voles (Clethrionomys rutilus; Rosenmann et al. 1975), and white-footed mice (Peromyscus leucopus; Wickler 1980). In birds, Dawson and Smith (1986) documented an increased Vo2ma in winter-acclimatized American Goldfinches (Carduelis tristis) from Michigan compared with those acclimatized to spring conditions. Higher VO2max in winter may indicate changes in the metabolic machinery involved in thermogenesis that increase thermogenic capacity and enable small birds to prolong cold resistance. However, it remains unclear whether seasonal variation in maximal thermogenic capacity is widespread among small birds wintering in temperate cli-