EVOLUTIONABY ASPECTS IN MAMMALS 



primitive characteristic. It seems obvious to me that an animal 

 this size together with a number of other smaller ones, like for 

 instance the birch mouse (weighing 7 to 12 grams) I have studied 

 (Johansen and Krog, 19 59), and the little pocket mouse ( Perognathus 

 longimembris ) studied by Bartholomew and Cade (1957), have to 

 allow for a greater variability in the body temperatures to regulate 

 at all. We can appreciate the enormous activity needed by these 

 animals to secure enough fuel for their high- paced metabolic 

 machinery. The labile body temperature is thus in a number of 

 species expressive of a specialization rather than a primitive 

 character. It should prove most interesting to study temperature 

 regulation in shrews of the northern-most habitats. 



The exceedingly specialized bats, the Chiroptera , offer a num- 

 ber of interesting features in mammalian phylogeny. They are 

 mostly confined to the tropics, although some small Micro- 

 Ghiroptera, Vespertilionidae, approach the arctic regions on the 

 Scandinavian Peninsula and in Alaska. It seems to the benefit of 

 all of us, however, that I leave the discussion of this important 

 order to a far greater specialist than myself. Dr. Morrison, who 

 has done extensive research on a number of species both among 

 the Micro- and Mega- Chiroptera (Morrison, 1959). Before I 

 leave the important Insectivores, let me remind you that they are 

 generally small in size, mostly confined to the tropical and tem- 

 perate regions. Theydisplay a rather great lability in body tempera- 

 ture, and many of them are hibemators. A substantial number of 

 species are typical substratum dwellers, with burrowing habits. 

 Some are excellent nest builders. Their body temperatures tend to 

 show a large range and lability, but are regulated at a higher level 

 than those of most marsupials. 



Let us next turn to the phylogenetically very interesting group, 

 Xenarthra. They consist of the armadillos, the Dasypodidae, the 

 anteaters, the Myrmecophagidae, the sloths, the Bradypodidae, and 

 the Old World scaly anteaters, the Manidae. The extremely inter- 

 esting and phylogenetically important Orycteropus called the 

 aardvark, or Cape Anteater, is a zoologically very isolated form. 

 This animal retains a number of characteristics present in the 

 earliest eutherians. Some authors, like H. Winge (1941) place it 



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