90 VERTEBRATE RESPIRATION 



to substantial losses of heat from the respiratory tract. Although 

 birds pant to some extent, they have no sweat glands and cannot 

 lose water by that route. Mourning doves pant when their body 

 temperature rises and at air temperatures of 39 ° C. they consume 

 four times more water than at 23° C. In small birds the water 

 loss may be enormous (a wren loses almost 40 % of its body 

 weight per day as water) and in the smallest forms with the 

 highest metabolic rates the loss of water may be a factor limiting 

 their survival. In some humming birds the loss through the 

 respiratory tract must approach 75 % of the body weight per 

 day. Small mammals also have similar problems because of their 

 high metabolic rate and relatively large surface areas. Many 

 small rodents have no sweat glands and do not pant, but avoid 

 the heat by living in burrows beneath the desert. Many are 

 nocturnal in their habits but a notable exception is the antelope 

 ground squirrel which inhabits the Mohave Desert of California. 

 This squirrel has a broad thermal neutral zone, for between 

 temperatures of 90° and 107° F. its metabolic rate remains 

 virtually constant. No other non-sweating mammal has a zone 

 which extends so high in temperature. In the absence of sweat- 

 glands it goes in for drooling, i.e., it spreads saliva over its 

 cheeks and head with its paws in order to reduce the body 

 temperature. Fortunately this mammal has an extremely efficient 

 kidney and can withstand considerable dehydration. 



(iii) Nervous mechanisms 



The nervous control of temperature regulation is based upon 

 information derived from receptors which detect a steady level 

 as well as changes in body temperature. Many are found in the 

 skin but it has recently been shown that receptors in the hypo- 

 thalamus of the fore brain are of great importance in triggering 

 the effector mechanisms which regulate the temperature. In 

 addition to its receptive function, the hypothalamic centre is 

 also the prime thermostat of the mammalian body. 



Using special methods, the total energetic output from the 

 skin has been determined regardless of the position of the sub- 

 ject. Heat carried from the body by evaporation was also 



