JOHANSEN 



this connection, as reported by Scho lander et al. (1950) that some 

 of the smaller arctic mammals with furry insulation like that of 

 tropical mammals are dependent upon burrows and nest-building 

 for their survival. Notably, measurements made by Scho lander et al. 

 showed that the insulative value of a lemming nest is roughly 1.5 

 times that of the lemming fur. If the nest is covered with snow, its 

 insulative value would presumably be even greater. In considering 

 the evolution of homeothermism, then, one must include behavioral 

 patterns as essential and indispensable parts of the whole system. 



The rather loosely connected information I have given you may 

 qualify for a tentative outline of one probable way the evolutionary 

 sequence of homeothermy has taken place. Figure 20 is an attempt 

 to put the factors together, but only in a qualitative way, since 

 measurements are lacking in most parts. The first successful 

 efforts to maintain a fairly uniform level of body temperature began 

 on the psycho-physiologicalorbehavioral level. Such an achievement 

 would necessitate a well-developed sensory system for temperature, 

 as well as a nervous coordination of the effector mechanism, 



MONOTREMATA 



EUTHERIA 

 MARSUPIALIA ' INSECTIVORA CHIROPTERA XENARTHRA CARNIVORA 



^^ ^yC^^x^^^y^y^-^-^^y'^y^y^^^^-'^y^^ 



-.*#5%;<%%%^S»;*%%;?5S%««S%??%iS^ 



fgm/x stnsotion and CMS inttgrvtiofi of responses to the environment 



CNS th«rmostotlc control of chemieol and physical tamp regulation 



Figure 20. Simplified schematical drawing of a possible route for the evolu- 

 tionary sequence of homeothermy in mammals. (Johansen, 1961). 



124 



