The Responses to Cold Stress Are Peripheral Vasoconstriction, Piloerection, and Increased Metabolic Heat Production by Shivering and Nonshivering Thermogenesis
As the ambient temperature decreases, homeotherms initially conserve heat by peripheral vasoconstriction. This sets up a temperature gradient along the limbs and reduces skin temperature, so there is only a narrow temperature gradient for radiation and convective heat loss (see Figure 53-3).
Piloerection provides insulation and also decreases heat loss. Further cold stress initiates increases in metabolic heat production by shivering or nonshivering thermogenesis. All adult mammals can shiver, and neonates born in an advanced state of development, such as lambs and foals, can also shiver. Puppies and other less-developed neonates cannot shiver and rely on the warmth of mother and the nest to protect them from cooling. Brown fat is present in some of the latter neonates and in other small mammalsand provides a source of nonshivering thermogenesis.Chronic exposure of animals to cold results in increased secretion of thyroxine and an increase in basal metabolism, which increases basal heat production. When animals are housed where they receive natural light, the thickness of the hair coat increases at cold times of the year. Hair growth is the result of decreasing daylight as cold weather approaches.
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