PHOTOPERIODISM 481 



determined by temperature. In 1932 Baker and Ranson, however, 

 reported that reproduction in the field mouse, Microtus agrestis, almost 

 ceased when the day length was reduced from 15 to 9 hr, the nights 

 changing from short to long. At that time Bissonnette (1932a) reported 

 work on reproduction of the ferret, Putorius vulgaris (Mustela fure) . He 

 found that the female could be brought into full oestrus by lengthening 

 the period of light during winter. In both cases light of low intensity 

 was adequate for induction of oestrus when used to extend the normal 

 day. These are "long-day" animals, whereas others such as man appear 

 to be indeterminate (Whitaker, 1938); however, note the comments of 

 Tennyson (1842) and Cook (1894). 



PHOTOPERIODIC RESPONSES OF ANIMALS 



Descriptive surveys of photoperiodic responses were made on many 

 plant varieties in the two decades following the period of discovery. In 

 the latter part of this period and more recently, attention turned to 

 physiological aspects of the subject. Results of the physiological work 

 on plants are discussed in detail and serve to illustrate the occurrence 

 of the phenomenon. Photoperiodic responses of animals have not been 

 studied as extensively as those of plants, and less is known about the 

 physiological aspects. For this reason a brief presentation of descriptive 

 aspects of the work with animals is given as a background. 



Sexual reproduction in many types of animals is definitely regulated 

 by photoperiodism. Just as there are long-day, short-day, and day- 

 neutral plants, there are similar groups of animals. Some aphids are 

 examples of short-day types. Marcovitch (1924) found that the pro- 

 duction of the sexual forms, which occurs in late summer or fall, is con- 

 ditioned by photoperiodism rather than other environmental factors such 

 as temperature. Deer, sheep, and goats are other examples of short-day 

 animals. In these the normal breeding season occurs during the short 

 days of autumn, and the young are born in the spring (Bissonnette, 1936b, 

 1941 ; Simms, 1950; Yeates, 1949). Hafez (1950) found that the duration 

 of the breeding season of sheep is related to their origin, breeds from the 

 Scottish Highlands having a considerably shorter season than those from 

 southern England, and the latter, a shorter season than breeds of Spanish 

 origin. 



Long-day animals include a great number of birds. Bissonnette (1936b) 

 listed junco, crow, canary, starling, mejiro, turkey, chicken, sparrow, 

 mourning dove, duck, pheasant, ciuail, and grouse. Long-day mammals 

 include ferrets (Bissonnette, 1932a), field mice (Baker and Ranson, 1932), 

 raccoons (Bissonnette and Csech, 1937), and many others. Ground 

 scjuirrels, guinea pigs, and a fish, the stickleback, are reported to be 

 uninfluenced by photoperiodism (Bissonnette, 1936b). Rowan (1938) 



