EVOLUTIONARY ASPECTS IN MAMMALS 



processes. All these features will result in an intensification 

 of the life processes, making possible a greater exploitation of 

 the environment. 



To keep a system at a constant temperature in a steady state 

 situation, a change in heat loss must be balanced by a correspond- 

 ing change in heat production, or vice versa. This requires a sen- 

 sitive mechanism set to a particular temperature which exerts 

 control over heat production or heat dissipation, or both. The sim- 

 plest regulator we can think of in this respect would be what we 

 are all familiar with in houses, refrigerators, etc., a regulator 

 that turns on the heat if the temperature falls and shuts it off if 

 the temperature rises above the set level. We can certainly appre- 

 ciate the limitations of such a simple system. In order for it to 

 work efficiently, the temperature of the object ought to be appre- 

 ciably higher than that of the environment. Moreover, as the 

 environmental temperature rises and exceeds that of the object, 

 the whole system of regulation would fail. We shall soon see 

 that this simplest possible system of regulation is exactly what 

 we find in the lower mammals like the Echidna. We may ask why 

 life did not choose the other possible way to achieve simple regula- 

 tion, by regulating heat loss rather than heat production. An obvi- 

 ous consequence of regulation of heat loss only would be a far 

 higher fuel cost. Securing the necessary fuel for such a regulation 

 would require both time and range of activity that were not pos- 

 sible for the earliest mammals. The first records of body tem- 

 peratures in monotremes were made about 75 years ago. In 1883 

 Maclay published records of cloacal and intra-abdominal tempera- 

 tures in two specimens of Echidn a aculeata. He found an average 

 temperature of 28 C. Lendenfeld (1886) reported a marked increase 

 (2 G) in the female Echidna after egg laying. Richard Semon (1894) 

 seems to have made the first systematic study of body tempera- 

 tures in monotremes on a fairly large number of specimens. The 

 cloacal temperature he measured ranged between 26.5 C and 



34.0 C and intraperitoneally from 29.0 C to 36 .9 G. This repre- 



o o 



sents a fluctuation of 7 G to 7.5 G at air temperatures ranging 



only from 18 G to 24 G. Semon points to a clear intermediate pos- 

 ition of the monotremes between the reptiles and the higher mam- 

 mals, but he does not classify themaspoikilothermic, as was usual 



77 



