88 



THE RISE OF ANIMAL LIFE 



PHYSICAL FACTORS IN THE 

 ENVIRONMENT 



Temperature 



Everyone is fully aware of his own sensi- 

 tivity to change in temperature. We usually 

 want our houses at a relatively constant 

 temperature of about 25° C. and experience 

 discomfort if it deviates a few degrees one 

 way or the other. Our internal environment 

 is even more critical — there a rise of a few 

 degrees indicates sickness of a serious 

 sort. What is true of man in this respect is 

 equally true of all animals. When we con- 

 sider that the temperatures known to us 

 range from 273° C. below zero to several 

 thousand degrees above zero, it is rather 

 remarkable that life exists in that extremely 

 narrow range of a few degrees above freez- 

 ing to about 45° C. Even within these nar- 

 row limits the physiological processes do 

 their best work at an optimal point around 

 the middle, on either side of which the rate 

 of physiological reaction falls off. Animals 

 tend to seek out a temperature that, at least 

 most of the time, will permit their bodily 

 activities to proceed at an optimal rate. 



Since animals are found in all parts of 

 the earth except the polar regions, they 

 must find ways of surviving extremes of 

 temperature with the least amount of dis- 

 comfort to themselves. Those living in 

 colder regions either have a constant body 

 temperature ( monothermal ) or else have 

 developed a hardiness to cold that permits 

 them to survive. The internal environment 

 of the warm-blooded animals — birds and 

 mammals — is constant and always main- 

 tains the temperature at which physiologi- 

 cal activities can proceed at an optimal 

 rate. Cold-blooded animals ( poikilother- 

 mal), on the other hand, vary their internal 

 temperature and rate of reaction in accord- 

 ance with the external environment. When 

 the temperature drops, the animal becomes 

 sluggish, even to the point of complete in- 

 activity. Some can stand freezing for short 

 periods of time. On a chilly morning in 



the fall of the year it is simple to capture 

 a cold-blooded animal, from a common 

 housefly to a rattlesnake, but the task be- 

 comes more difficult on a hot summer day 

 when the temperature approaches 100° F. 

 Only at the higher temperature are all 

 activities at their maximum. 



During cold seasons some mammals 

 undergo a period of inactivity called hiber- 

 nation, when their temperature drops and 

 metabolic processes are reduced to a mini- 

 mum. Hibernating rodents, such as the 

 ground squirrel, pass into almost complete 

 inactivity, their heart and breathing rates 

 slowing down markedly. Indeed metabo- 

 lism is just enough to keep the animal alive. 

 The energy to maintain life is derived from 

 stored fat, hence the fat bear in the fall and 

 the lean bear in the spring of the year. 



Other animals survive periods of intense 

 heat by going into an inactive state called 

 aestivation. This is strikingly demonstrated 

 by the African lungfish which lives in re- 

 gions that are apt to dry up during the 

 summer months (Fig. 5-1). With the ap- 

 proach of hot weather and desiccation, the 

 fish burrows in the mud and secretes a 

 capsule in which it passes the warm dry 

 months. When the temperature drops and 

 moisture returns, it resumes its active life 

 once more. 



Some cold-blooded animals put forth 

 communal effort to prevent too great a drop 

 in temperature. Bees, for example, become 

 very active on cold winter days, beating 

 their wings almost continuously. This keeps 

 the temperature in the hive above freezing 

 even though the outside temperature may 

 be several degrees below zero. Snakes fre- 

 quently aggregate in dens in the fall of 

 the year for the apparent purpose of keep- 

 ing warm. Even though they are cold- 

 blooded, their temperature stays slightly 

 above that of the external environment. 

 Coiling about one another in large masses, 

 the whole group stays a little warmer be- 

 cause the individual heat loss is reduced. 



Keeping in mind that all living things are 



