EXPERIMENTS ON LIGHT AND TEMPERATURE 



IN A WILD MAMMAL WITH AN ANNUAL 



BREEDING SEASON 



LEMEN J. WELLS 



Department of Anatomy, University of Minnesota, Minneapolis 



The breeding season of the ground squirrel {Citellus tridecemlineatus) 

 occurs in the spring, soon after the animals emerge from hibernation. 

 During the summer, in males and females, the reproductive tract 

 retrogresses to a juvenile-like condition, and the hypophysis cerebri 

 is no longer rich in gonadotropin. 



Using incandescent bulbs and a time switch, we began experiments 

 on the daily ration of light in the autumn. The light was increased 

 gradually, within a period of 2 months, from 12 hr per day to 22 hr 

 per day. Thereafter, such animals were given 22 hr of light per day 

 for 3 months. These treatments failed to hasten the functional de- 

 velopment of certain endocrine glands: gonads, hypophysis, adrenals, 

 and thyroid (Moore et al, 1934; Simmons, 1934; Wells, 1935; 

 Zalesky, 1934, 1935). 



Subjection of animals to a constant temperature of 4°C, begin- 

 ning in the spring, produced definite effects (Wells, 1938; Wells 

 and Zalesky, 1940; Zalesky and Wells, 1940). The gonads failed to 

 undergo retrogression, the hypophysis continued to show a high level 

 of gonadotropin, and the adrenal cortex resembled that found during 

 the breeding season. 



Experimental combinations of light and temperature were tried: 

 ultraviolet and 4°C; 12 hr of light per day (incandescent bulbs) and 

 18°C; darkness and summer room temperature. The effects upon 

 the endocrine glands were like those in experiments in which 

 temperature alone was modified (Wells, 1938; Wells and Zalesky, 

 1940; Zalesky and Wells, 1940). 



The several observations suggest that in this species low environ- 

 mental temperature stimulates the functioning of the hypophysis 



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