502 



PHYSIOLOriY OF GONADS 



NDJFMAMJJASON 



INSECTIVORA 



MOLE SHREW 



COMMON SHREW 

 CARNIVORA 



BROWN BEAR (EUR.) 



FERRET 



COYOTE 



WILD CAT (EUR.) 



BAD3ER (AMER.) 

 LAGOMORPHA 



HARE (ENG.) 



COTTONTAIL (N . ENG.) 

 RODENTIA 



13-L. GROUND SQUIRREL 



WOODCHUCK 



GRAY SQUIRREL 



DORMOUSE 



FIELD MOUSE (EUR. ) 



MUSKRAT (MARYLAND) 

 (IOWA) 



PORCUPINE 

 ARTIODACTYLA 



LLAMA 



ROE DEER 



MULE DEER 



GIRAFFE 



SHEEP (HAMP ) 



BIGHORN 



GOAT 



YAK (RUSS. ) 



INDIAN ANTELOPE 

 PERISSODACT YLA 



HORSE 



Fig. 8.3. Some representative seasonal breeders. Solid bars indicate breeding seasons 

 (according to Asdell, 1946); blank intervals, periods of anestrum. Months of the year repre- 

 sented by letters at top of chart ; winter and summer solstaces marked by wavy lines. South- 

 ern hemisphere seasons converted to corresponding ones of the northern hemisphere. End 

 of season for the Bighorn is vmcertain. 



The complexity of the i)rol)lem is well 

 illustrated by the 13-lined ground squirrel 

 whose breeding season, like that of a multi- 

 tude of small rodents, comes in the spring. 

 Moore, Simmons, Wells, Zalesky and Nel- 

 son (1934) reported that increasing illumi- 

 nation, elevated temperature, and feeding 

 all failed to bring the females into estrus 

 out of season. If, however, hibernation was 

 first induced by low temperature and dark- 

 ness, premature estrus would follow. The 

 conclusion was reached that hibernation it- 

 self is a necessary prerequisite. Ovarian de- 

 velopment actually begins, under natural 

 conditions, in early January in the midst 

 of hibernation. Females exix'i-imentally 

 maintained "continually for several months 

 in cold and darkness, with more or less 

 normal hibernation, [exhibit] sexual de- 

 velopment at any time of the year, and 

 periods of estrum have thus been . . . main- 

 tained for many months. ..." The impres- 



sion is given tliat tlie conditions favoring 

 hibernation also favor sexual development 

 to such extent that breeding potentiality 

 continues for a few months after emergence, 

 in spite of elevated temperatures and long 

 periods of illumination. In another rodent, 

 Peromyscus leucopus, however, the length 

 of daily illumination is of paramount im- 

 portance. Temperature changes (4 to 25°C. ) 

 have no effect on rejiroduction when lighting 

 is adequate (Whitaker, 1940). Whereas a 

 similar primaiy dependence on lighting 

 can be shown in a number of other species 

 from several orders, it is unwise to general- 

 ize that this is usually true. 



IV. Attainment of Maturity. Emergence 

 of Full Ovarian Function 



Ahhough ('merg(>nce of the ovary from 

 the state of quiescence is gradual, there is 

 usually some outward sign that allows the 

 observer to say that puberty has ari'ived 

 or the breeding season has begun. In ])ri- 



