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Experimental Evidence for Temperature-Dependent Winter Lipid Storage in the Dark-Eyed Junco (Junco hyemalis oreganus) and Song Sparrow (Melospiza melodia morphna)

1995; University of Chicago Press; Volume: 68; Issue: 2 Linguagem: Inglês

10.1086/physzool.68.2.30166504

1937-4267

Christopher M. Rogers,

Animal Behavior and Reproduction

Stored lipid is the main source of metabolic fuel during winter fasts of small birds, yet relatively little is known about the environmental cues that stimulate lipid storage. The ultimate-temperature hypothesis, well supported for a number of species, suggests that birds fatten in anticipation of average or record low winter conditions on a given day. In contrast, the proximate-temperature hypothesis suggests that small birds respond directly to temporal and spatial variation in winter temperature and deposit an amount of body lipid appropriate to current metabolic demands. Dark-eyed juncos (Junco hyemalis oreganus) and song sparrows (Melospiza melodia morphna) in southwest British Columbia were subjected to warm (20° C) and cold (20° C, gradually reduced to - 12° C over a period of days) temperatures for 10 d in enizironmental chambers allowing free flight and seminatural feeding and roosting conditions. Total lipid content was significantly greater (P < 0.025) in the cold group than in the warm group in both the junco and the sparrow. The same result was obtained for lipid index (g lipid/g lean dry mass) in an analysis of mass-specific fat reserves. In addition, junco populations wintering at 951 m in the coastal mountains of southwest British Columbia showed significantly greater fat reserves than conspecifics wintering at and near sea level in British Columbia and northwest Washington. Finally, juncos in both British Columbia and Washington showed significant inverse correlations between winter fat and proximate temperature. These findings support the proximate-temperature hypothesis and corroborate similar experiments in Wisconsin juncos that repeatedly showed a direct fattening response to low proximate temperatures in environmental chambers (C. M. Weise, personal communication). Although an underlying ultimate component of the winter fat reserve (e.g., an endogenous cycle) in the junco and song sparrow cannot be ruled out by the present experiments, they nonetheless provide strong suggestive evidence for the evolution of a direct fattening response to low proximate temperature in small birds. A caveat is that proximate temperature need not be of paramount importance as a cue and instead may act in parallel or in interaction with environmental indicators of resource predictability, such as snowfall.