364 



Comparative Animal Physiology 



hypothalamus.28 i^ man, development of temperature regulation may take 

 several years.-" Temperature regulation is perfected in chickens 4 to 5 days 

 after hatching.^-*" In the house wren temperature regulation appears gradually 

 during the two weeks after hatching (Fig. 95). Regulation is accompanied by 

 increasing sensitivity of the heat center and increasing basal metabolism. In 

 the wren, for example, metabolism as measured by COo output increases with 

 age up to 12-15 days; as regulation appears the CO2 production is highest at 

 low air temperature and is minimal at 37.8°. The air temperature at which 

 metabolism is minimal is well below body temperature. Similarly, water loss 

 was increased by 4.6 times from 21.7° to 37.8° in wrens 0, 3, and 6 days old, 

 but hardly at all in adults; however, at air temperature of 40° the water loss 

 was twice the loss at 37.8°, regardless of age.'-*** Young swifts are able to survive 



60° F 



4o°C - 



as'c 



ad-'c 



34°C 



32 °C 



oNlGHT 

 • DAY 



ICC 



2o<'C 



30°C 



105° F 



lOO°F 



95°F 



-90°F 

 40°C 



EXTERNAL TEMPERATURE 



Fig. 94. Body temperature as a function of air temperature in a nocturnally active 

 Central American opossum, Metachirtis. From Morrison.^"' 



fasting longer than adults, because their metabolism and temperature fall and 

 they enter a hunger coma.^*''^ 



Thermal Stimulation. The mechanisms of heat regulation are activated in 

 two ways: by thermal receptors in the skin, and by direct stimulation of the 

 thermoregulator in the brain by changes in blood temperature. The thermal 

 receptors in the skin may elicit vasomotor reflexes even though the individual 

 is not conscious of heat or cold sensation. Observations on man show that 

 the receptors for cold are probably the bulbs of Krause, located at the outer 

 border of the skin layer containing blood vessels, that is, at a depth of about 

 0.1 mm., whereas the warmth receptors are the Ruflini organs, which lie deeper 

 in the skin (0.3 mm.), where blood vessels are abundant.'-- '•* Much sensory 



