672 PHOTOPERIODISM IN VERTEBRATES 



If there were no other compHcating factors one could then claim 

 that outside the humid Tropics day length could not be the primary 

 factor timing reproduction in reptiles because the daily period of 

 activity would be determined strictly by environmental temperature 

 and the effective photoperiod would not be related to day length but 

 to the diurnal temperature cycle of the environment. Lizards are, 

 however, often active in environmental temperatures far below the 

 range at which their performance is adequate for survival. They 

 achieve this by raising their body temperature above ambient tempera- 

 ture by means of behavior patterns that allow the accumulation of 

 heat from solar radiation or from the substratum which in turn has 

 been heated by the sun. Pearson (1954) has shown with particular 

 clarity the behavioral responses that allow a lizard, Liolaemus multi- 

 formis, which lives at an altitude of over 15,000 feet in Peru, to 

 maintain a body temperature as much as 30°C above air temperature. 

 At the other extreme some lizards may be active at ambient tempera- 

 tures which exceed the range of body temperatures that they can 

 tolerate. Norris (1953) gives a graphic picture of the manner in which 

 the desert iguana by intermittent activity and frequent resorting to 

 shade and burrows manages to occupy a habitat in which soil and 

 temperatures are often above 50°C. 



Although darkness or light will determine the period of potential 

 activity depending on whether or not a reptile is nocturnal or diurnal, 

 the actual daily period of activity will depend on temperature. Because 

 environmental temperature in general increases with increasing day 

 length, length of day can still be a primary factor, but the effective 

 day length is determined by thermoregulatory behavior, not by sunrise 

 or sunset. Consequently, in reptiles the relation between long days and 

 short days may bear only a remote relationship to the concept as it 

 was originally formulated for plants, and outside the Tropics, it may 

 be meaningless. 



These ecological relationships make the context of photoperiodism 

 in reptiles (except aquatic forms) different from that of plants which 

 cannot avoid or seek out the light except by tropisms, from that of 

 fish and many amphibians for which water minimizes temperature 

 changes, and from that of birds and mammals which are essentially 

 independent of temperature in their activity. It may be suggested that 



