674 PHOTOPERIODISM IN VERTEBRATES 



They live in the humid Tropics, but are far enough from the equator 

 to be exposed to considerable seasonal changes in day length. 



It has recently been found that day length affects not only reproduc- 

 tion but also food consumption, appetite, and growth. Dessauer 

 ( 1955a,b) suggested that the marked seasonal fluctuations in body fat, 

 liver storage, and appetite found in Anolis carolinensis were influenced 

 by day length. This suggestion was confirmed by Fox and Dessauer 

 (1957), who found that 18 hours of light per day in fall and winter 

 increased the appetite of both young and adult Anolis carolinensis and 

 caused a conspicuous increase in growth of immature animals. 



So far, little has been said about the physiological mechanisms of 

 reptilian photoperiodism for the simple reason that little information 

 is available. There can be no doubt, however, that the pituitary is an 

 intermediary between light and the gonads. In lizards and snakes, as 

 in birds (both the Squamata and birds are thought to be derived from 

 some common diapsid ancestor), the skull is notably open and 

 fenestrated. There is little doubt that in these reptiles light could reach 

 the hypothalamus or pituitary directly through the orbit (as in some 

 birds), or even through the otic region. Consequently, there is no a 

 priori reason why the eye need be implicated as the initial receptor of 

 the photoperiodic stimulus. It is, however, quite probable that in turtles 

 the eye may be the receptor because turtles are anapsids and have an 

 unfenestrated and relatively heavy skull through which light is not 

 likely to penetrate to the pituitary region. It is, of course, safest merely 

 to say that the mechanism of the reception of the light stimulus as 

 well as its mode of endocrine mediation remains not only unknown, 

 but almost unstudied. Here lies an intriguing field of research but one 

 in which, judging by the history of research on photoperiodic mecha- 

 nisms in other organisms, answers wifl prove elusive. 



SUMMARY 



Although photoperiodic responses have been demonstrated in the 

 reproductive physiology of turtles and lizards, and in the feeding, and 

 growth of at least one lizard, the biology of most terrestrial reptiles 

 obscures the role of day length as a clearly delimited environmental 

 stimulus. Under natural conditions the effects of day length on reptiles 



