HEAT REGULATION IN SMALL AND LARGE DESERT MAMMALS 



Professor Knut Schmidt -Nielsen 

 (Cincinnati, U.S.A.) 



Desert rodents usually lead a nocturnal life and spend the day in underground 

 burrows. In this way they escape the excessive heat load that would be imposed by 

 high solar radiation, high air temperature and high ground surface temperatures. 



The burrow temperature normally does not exceed 31*^, even on the hottest 

 day, as shown for example by Vorhies' investigations of the microclimate of kan- 

 garoo rat burrows. 



Kangaroo rats, like other rodents have no regular sweat glands and do not 

 sweat. However, if they are exposed to high temperatures, it will be found that to 

 some extent these animals are able to keep the body temperature below that of the 

 environment by the evaporation of water. 



In laboratory experiments it was found that the body temperature of the kan- 

 garoo rat will increase beyond the usual of 36- 37°C if the surrounding temperature 

 rose above about 35°C. A further increase in ambient temperature would lead to a 

 corresponding increase in body temperature, apparently without causing any physio- 

 logical reaction that would keep the body temperature from rising. However, if the 

 body temperature approached the lethal limit (around 42°C) a copious secretion of 

 saliva would occur, wetting the fur under the chin and throat, and evaporation would 

 keep the body temperature from rising further. This 'emergency heat regulation', 

 which is used only when conditions are critical to survival, will enable the animal 

 to keep its body temperature even slightly below that of the surroundings for a 

 short time. 



There were differences in the reaction of different individuals, and it was found 

 that some kangaroo rats could survive at 43°C for at least 20 minutes in experiments 

 where white rats died at 39°C. The amount of water used for this evaporation is so 

 great that to continue for a long time would be impossible. The animals under the 

 conditions mentioned above had lost about 15% of their total body water, which we 

 know is not far from the 20% which is considered the limit for desiccation that can 

 be tolerated by mammals. 



(Similar reactions of excessive salivation under heat stress have been found in 

 other animals, such as mouse, white rat, guinea pig, cat, swine, etc. For compari- 

 son, white rats were tested in the same experiments as kangaroo rats. It was found 

 that there was no apparent ability to lower the body temperature in the rats, and they 

 died at much lower air temperatures (39°C) than the kangaroo rats although the lethal 

 body temperature is nearly the same in the two species.) 



The very high rates of evaporation in a small animal which uses water for heat 

 regulation is due to the fact that the relative surface area is greater in a small 

 body than in a larger one. 



The amount of heat that should be dissipated in order to keep the body tempera- 

 ture constant in a hot environment equals the sum of the heat of metabolism and the 



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