80 HOMOIOTHERMISM 



seems that while a diurnal variation occurs in hirds, it 

 can readily be changed by changing the time of activity. 

 Owls and other nocturnal birds show their maximum 

 temperatures in the night. 



It is an interesting speculative consideration that 

 maximum and minimum temperature oscillations in the 

 bodies of animals coincide with those of the sea. The 

 maximum temperature of the sea is late in the afternoon 

 while the minimum is late in the night, near dawn. The 

 normal variation in shallow waters is similarly about 1 

 deg. C. It appears possible that there is in the body tem- 

 perature of the homoiotherms an indication of the tem- 

 perature of the sea at the time when their ancestors mi- 

 grated from the water to the land. If this is true, a 

 homoiotherm, although it at present maintains a constant 

 temperature much above that of its surroundings, has 

 mechanisms for keeping pace with temperatures to which 

 the ancestors were accustomed or perhaps to those which 

 they found advantageous in their original marine envi- 

 ronment. 



Age. — ^At birth the temperature of the homoiotherms 

 varies with the species. Some are more affected by en- 

 vironmental temperature than others. Young marsu- 

 pials, such as opossums, with their undeveloped nervous 

 system, have little resistance to temperature changes in 

 the surroundings. On the other hand the young of the 

 guinea pig will soon after birth maintain its body within 

 a fraction of a degree of that of the mother. A human 

 infant has a slightly higher temperature at birth than 

 its mother. During childhood the body temperature 

 gradually falls to that of the adult. In old age the tem- 

 perature rises as a rule and attains a maximum at about 

 eighty years. The young of certain birds are practically 

 poikilothermic at hatching (Kendeigh and Baldwin, 



