EVOLUTIONARY ASPECTS IN MAMMALS 



shows also a higher average body temperature. At air temperatures 

 of 28° C to 23 C, Bradypus showed an average of 33 C, whereas 

 Choloepus showed 34.4 C . The difference in intramuscular tempera- 

 tures was even greater with values of 34.6 C for Choloepus and 

 32.4° C for Bradypus. Bradypus occurs only in the lower neo-tropi- 

 cal altitudes and is particularly prevalent in regions with small 

 fluctuations in air temperature and dense vegetation providing ample 

 shade. Choloepus has a similar habitat but is also able to withstand 

 colder "areas with occasional freezing in altitudes up to 7,000 or 

 8,000 feet (Britton, 1941). Choloepus has not yet been studied in 

 regard to temperature regulationinthesecolder areas; this project, 

 however, seems to promise a great deal. The body temperatures 

 of Bradypus drop precipitously when exposed to 10 G air tempera- 

 ture and reached 20° C after about 5 hours. Below this temperature, 

 a lethargic condition seemed to ensue. Gold was apparently a strong 

 stimulus to muscular activity. Marked hypertonus was noticeable, 

 but no shivering was visible at any temperature. This is of particu- 

 lar interest in light of the extremely low mass of skeletal muscle 

 in the sloth. Britton and Atkinson (1938) report that the skeletal 

 muscle mass in Bradypus is only 25% of the body weight. The cor- 

 responding figure for the higher mammals ranges between 45% and 

 55%. Upon exposure to sun, the rectal temperature rose 2 C to 

 4 C within 30 minutes, and the animals struggled vigorously to 

 get free. Getting freedom, they promptly sought shade under the 

 nearest tree. Irving et al. (1942), in their interesting study on res- 

 piration in the sloth, mentioned an oxygen consumption about half 

 of what is generally found in higher mammals , with values approach- 

 ing what Martin found for the monotremes and some marsupials. 

 Comparing the two sloths, thethree-toed is far inferior in its ability 

 to maintain a fairly uniform level of body temperature and is help- 

 lessly unable to venture into an environment outside the tropical 

 stability. The unsurpassed slowness is interesting in the light of 

 the small muscle mass. The low resting metabolism is shared with 

 the other members of the Xenarthra and the monotremes and many 

 of the marsupials. We have throughout this discussion seen that 

 very early mammals became able to increase their metabolism by 

 shivering, thus compensating for an increased heat loss. This me- 

 chanism appeared before the ability to regulate heat loss intrinsi- 

 cally. In the sloth things are seemingly different. The animal almost 



101 



