EVOLUTIONARY ASPECTS IN MAMMALS 



behavioral response, in contrast to the pure physiological mechan- 

 isms of sweating, panting, and vasodilation. This specialization 

 limits the usefulness of the method since it can only operate effect- 

 ively in an animal resting, and would be less useful to a rapidly 

 moving animal. To briefly summarize the temperature regulating 

 ability of the marsupials living today is very difficult, if not impos- 

 sible. The group is exceedingly diversified, and having been prac- 

 tically indigenous to Australia with little competition for a very 

 long time, marsupial Life has adjusted to most habitats available. 

 With the very interesting exception of the Virginia opossum of 

 North America, the marsupials are confined to the tropical or 

 neotropical regions. This, however, may not be related so much 

 to inferiority to placental mammals in temperature regulation 

 as to their very specialized mode of reproduction. In this regard, 

 Bartholomew presents very interesting and important data on the 

 ontogenetic development of temperature regulation in the marsu- 

 pial, Setonix sp. I consider this a topic in itself, however, and can 

 find no time to discuss it now. The marsupials so far studied in 

 regard to temperature regulation indicate clearly a lower level 

 of resting body temperature in the more primitive forms like the 

 Dasyuridae which show values down to 33 G to 34 C at resting 

 conditions, at 20 G air temperature; whereas the specialized, 

 phylogenetically more advanced species show a higher resting 

 level and a smaller range of variation. As more information be- 

 comes available, like the important works on Australian marsu- 

 pials by Robinson and Morrison (1957) and Bartholomew (1956) , this 

 unique indigenous fauna may enable us to talk with more confidence 

 about the role of the phylogenetic position vis a vis the influence 

 of environmental factors for the establishment of homeothermy. 

 Common to temperature regulation in all marsupials is the presence 

 of physiological effector mechanisms of both chemical and physi- 

 cal temperature regulation. Some of them, like sweating and pant- 

 ing, seem generally to be of rather limited importance, being sub- 

 stituted by specialized behavioral responses like the coat licking. 

 In my own interpretation of Bartholomew's important work on 

 Setonix, it seems of the greatest significance that the integrative 

 control of the otherwise well-developed effector mechanisms 

 show some lag in precision, compared to higher placental mam- 

 mals. It is conceivable that even more striking differences in this 



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