PHOTOPERIODISM IN REPTILES 671 



It is quite clear tliat, with the possible exception of forms living in 

 the humid Tropics, the times of activity of terrestrial reptiles and 

 hence the periods during which light can effect their endocrine system 

 is dependent not only on day length but is profoundly affected by 

 local conditions of temperature as well as by the fundamental be- 

 havioral dichotomy of nocturnality versus diurnality. Unlike fish and 

 aquatic amphibians, which occupy an environment in which the rate 

 and extent of temperature change are minimized by water, most 

 reptiles live in an environment in which not only seasonal, but also 

 daily and hourly changes in temperature can shape the pattern of their 

 existence. To a person'who has not himself studied reptiles, particularly 

 lizards, in the field or who is not familiar with the literature on the 

 ecology of reptiles, the dependence of these animals on temporary 

 changes or local differences in temperature can hardly be anticipated. 

 Certainly the performance of reptiles in the laboratory does not pre- 

 pare one for it. In captivity at temperatures below their normal range 

 of activity, nothing dramatic happens. The animals just slow down, 

 and if the temperature is reduced sufficiently, they become torporous. 

 At temperatures above their normal range, they merely seem unusu- 

 ally alert and restless. Field observations, however, have made it 

 apparent that active reptiles maintain body temperature at a charac- 

 teristic level (Cowles and Bogert, 1944). This level may be similar in 

 species of the same genus occurring in markedly different climates, 

 but differ by several degrees in sympatric species of different genera 

 (Bogert, 1949). The extremes to which this adaptation to and 

 maintenance of a characteristic range of body temperature may be 

 pushed is shown with particular clarity by the desert iguana, Dipso- 

 saurus dorsalis, which is frequently active at body temperatures in 

 excess of 43°C (Cowles and Bogert, 1944), a level higher than that 

 characteristic of most birds and mammals and above the lethal level 

 for some species of Sceloporus (Cole, 1943). 



Since reptiles are incapable of effective physiological thermoregula- 

 tion, and the extent of their temperature acclimation (Dawson and 

 Bartholomew, 1956) is quite restricted as compared with most 

 poikilothermic animals (Bullock, 1955), the Hmited range of body 

 temperatures characteristic of normal activity for any given species 

 greatly restricts the daily periods of activity. 



