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Gannets Sula bassanus, very large altricial birds, exhibit well developed thermolytic behaviour (panting, gular fluttering, posturing, etc.) by two days posthatch. In contrast, interactive mechanisms of heat conservation and thermogenesis developed gradually during the first three weeks posthatch, with a transitional phase from thermal dependence to independence during posthatch weeks 2 to 3. From hatching through 12 d slight increases in shivering capacity and down growth and an approxi- mate 25% decrease in relative surface area were thermally ineffective. Shivering thermogenesis improved markedly from 13 to 24 d and was the primary determinant of the emergence of thermal independence. The ontogenetic contrast between early onset of heat tolerance and gradual development of heat conservation and thermo- genic capacities suggests that heat stress is a greater threat to nestling survival than cold and that parental behaviour can cope more effectively with cold. Adaptive protection against hyperthermia in newly hatched young is widespread, as indicated by the onset of thermolysis before thermogenesis and heat conservation in most open-nesting seabirds. Insulation associated with down growth complements thermogenic development for many large altricial nonpasserines and more precocious species but not for passerines. The former nest in the open and are exposed to the cooling effects of wind, whereas the latter tend to be buffered from the wind by nest-sites and nests. Comparatively, leg musculature functionally matures before flight muscles and is the primary source of thermogenesis for most large altricial nonpasserines, whereas pectoral muscles assume this role for smaller nonpasserines, passerines, and precocial species.
|Department(s):||Science, Faculty of > Biology|
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