OLSON, J.M.; RICKLEFS, R.E.: Ontogenic changes in metabolic capabilities and shivering thermogenesis in the willow ptarmigan
The willow ptarmigan is a resident of arctic and subarctic regions routinely subjected to cold, wet conditions, even during the short breeding season. As a precocial species, newly hatched and young ptarmigan chicks forage on their own soon after hatching. Thus, it is reasonable to hypothesize that even the youngest chicks possess substantial capabilities for regulatory thermogenesis. Metabolic rate and body temperature (Tb) of individual chicks (1-day to 24-days old) and adults were measured continuously during exposure first to gradually decreasing ambient temperature (Ta) followed by gradually increasing Ta. Ta was decreased until the bird started to go hypothermic and/or the metabolic rate decreased persistently. Resting metabolic rates at thermoneutral temperatures (RMRs) were higher than those predicted for precocial species. Younger chicks maintained lower Tb at thermoneutrality (~38.5�-40.5�C) than older chicks and adults (~41�-42�C). All birds measured were capable of regulatory thermogenesis. Maximal metabolic rate of the youngest chicks ranged ~2- to 3.5-fold higher than RMR. Surprisingly, this factorial scope did not change significantly through day 24, and was only slightly higher in adults. However, the range of Tas tolerated increased during development, owing primarily to lower conductance and lower critical temperatures in older birds. Often, increases in metabolic rate during cooling, especially in younger (1- 16-day) chicks, demonstrated bouts of higher metabolic activity; independent measurements of electomyographical activity in selected skeletal muscles substantiated this bursting pattern of thermogenesis. These thermoregulatory capabilities allow ptarmigan chicks to forage independently from an early age.
