670 PHOTOPERIODISM IN VERTEBRATES 



for approximately two months during the winter caused only slightly 

 greater testicular development in the horned lizard, Phyrnosoma 

 cornutum, than that which occurred during hibernation in the same 

 season. Since the mean hibernating temperature of this species is 

 more than 15°C below the experimental temperature used, it is hard 

 to tell whether or not the slight gonadal response was photoperi- 

 odically induced. Burger (1937) reported that in the turtle Pseudemys 

 elegans an artificial increase of day length started in the middle of 

 November inhibited the spermatogenic cycle already in progress and 

 induced the initiation of a new cycle; this suggests that day length 

 can contribute to the rhythm of the reproductive cycle. Clausen and 

 Poris (1937) found that the daily addition of 6 hr of light starting in 

 late November had a marked stimulatory effect on spermatogenesis 

 in Anolis carolinensis. Using circumstantial natural history evidence, 

 Baker (1947) suggested that lizards of the genus Emoia living in the 

 New Hebrides are probably paced at least in part by day length in 

 their reproductive periodicity. Miller (1948) reported that testicular 

 development of Xantusia vigilis could be affected by light. Bartholo- 

 mew (1950, 1953) found (1) that increased length of day was more 

 important than increased temperature in accelerating gonadal growth 

 in Xantusia vigilis, ( 2 ) that males were more responsive than females 

 to photoperiodic changes, and (3) that rate of photoperiodic response 

 of males increased directly with increasing temperature. Galgono 

 (1951) examined the responses of Lacerta sicula to various combina- 

 tions of temperature and photoperiod and suggested the interesting 

 possibility that temperature might control spermatogenesis while 

 photoperiod controlled testicular secretion. 



If any one conclusion can be drawn from the welter of papers that 

 have appeared during the 35-year history of research on animal photo- 

 periodism, it is that the evaluation of the role of day length in the 

 determination of breeding season cannot be made on the basis of 

 laboratory data alone. A detailed familiarity with the natural history 

 and behavior of an animal is as important as an analysis of its 

 endocrine physiology and its responses to light. Although knowledge 

 of reptile behavior and ecology lags behind that of birds and mam- 

 mals, information is available which is pertinent to the understanding 

 of the relation of day length to breeding season. 



