EVOLUTIONAFY ASPECTS IN MAMMALS 



This low level of metabolism in these primitive forms is, of course, 

 of the greatest interest. Martin found about 2.5 times increase in 

 metabolism of Echidna at environmental temperatures of 5 C. He 

 points out that none of the experimental animals he used maintained 

 a constant body temperature throughout the range of air tempera- 

 tures. 



Of great significance to our understanding about how homeo- 

 thermy has developed phylogenetically is the finding that monotremes 

 depend only on variation in heat production and not on physical mech- 

 anisms for their maintenance of body temperature. At high air tem- 

 peratures, that is about 30° C,the respiratory rates of the marsup- 

 ials are affected very little by the high temperatures; whereas in 

 the monotremes, the breathing frequencies are decidedly lower in 

 30° G air than in cooler air. Martin was unable to find any sweat 

 glands in Echidna , and he also demonstrated that this species is de- 

 void of vasomotor adjustments importantforheatdissipation or con- 

 servation. Without any means for adjusting the core temperature by 

 physical means, Echidna is vulnerable at high air temperatures and 

 dies easily of heat apoplexia at a body temperature as low as 38 C. 

 Under natural circumstances, I would assume that behavior mecha- 

 nisms are indispensible to Echidna. The animal is known to bury it- 

 self several feet in the ground and only emerges after sundown on hot 

 days. This, in turn, willdrastically curtail the animal's activity time 

 and range. 



Kathleen Robinson, in 1954, studied heat tolerance in Australian 

 monotremes and marsupials. She confirmed Martin's earlier find- 

 ings about the lackofpanting and vasomotor adjustment in the mono- 

 tremes. She measured the evaporative heat loss and found that it is 

 higher in platypus than Echidna, indicating some activity of the sweat 

 glands in the former. These are distributed mainly on the snout, but 

 there is also one apocrine gland opening into the follicle of each of 

 the large hairs. The platypus shows also some adjustment in posture 

 to facilitate heat loss. As its body temperature rises, it rolls over to 

 its back with the under surface exposed and the legs outstretched. 

 With higher body temperatures, the platypus becomes restless, and 

 indicates some impairment in neuro- muscular coordination. The 

 animals do not salivate or lick their coats during heat exposure. On 

 the basis of the scattered data presented so far, we may be justified 



