George B. Wislocki Qao 



and the antlers are cast earlier in mature bucks than in younger animals. It 

 seems reasonable to interpret these observations in the sense that the more 

 mature the animal the earlier in the season the hyjjophysis is activated and 

 hence the sooner the hypophysis in turn activates the antler pedicles and the 

 testes. In senescent animals (Seton/' Brehm/' Anthony''), conversely, the 

 antlers diminish in size, appear later in the season, and the velvet is retained 

 longer. In addition, in the senile roebuck the antlers are reported as never 

 being shed (Schumacher"). These age changes indicate a lessening of the 

 potency of the hypophysial-testicular mechanism. Finally, it is known that 

 deer of a given species give birth to their young earlier in the southern than 

 in the northern parts of the United States (Audubon and Bachman^'), and 

 similar findings have been recorded for Europe (Brehm,^" Jacobi"), observa- 

 tions which again point to seasonal influences in the control of reproduction. 

 Moreover, many species of tropical and subtropical deer tend to have ex- 

 tended sexual seasons and may breed at any time of the year (Marshall"'^, 

 besides having no definite period for the appearance of the antlers (Brehm'"), 

 observations which indicate that, in latitudes in which climatic changes are 

 relatively slight, sexual periodicity and antler growth are not sharply regu- 

 lated. Many of these various observations indicate the probable role of ex- 

 teroceptive stimuli in regulating the periodicity of antlers and gonads and 

 these leads should be more carefully followed up. 



We return at this point to the thesis of Marshall, to the effect that, whereas 

 many mammals have their sexual seasons in spring or summer imder the 

 exteroceptive influence of light, the ungulates have their period of rut or 

 estrus in the fall, when, if exteroceptive stimuli are involved, the animals 

 must be reacting to a diminution of light or possibly to temperature. From the 

 observations reported in the present study, it appears very likely that in deer, 

 as in many other seasonal animals, the hypophysial-gonadal cycle is also in- 

 itiated by the exteroceptive stimulus of light. In ordinary seasonal mammals 

 we are accumstomed to a short duration of these cycles, consisting of stimula- 

 tion of the pittiitary gland followed immediately, in days or weeks, by the 

 sexual season. In the white-tailed deer it is evident that the annual hypo- 

 physial-gonadal cycle is initiated in early summer, the major difference be- 

 tween deer and other mammals beino that in the former the entire cvcle 

 requires five or six months for its completion, whereas in the latter the cycle 

 is usually very much shorter. Thus it is apparent that in deer the pituitary- 

 gonadal cycle is initiated, as in other seasonal mammals, in the spring or 

 simimer, under conditions of maximal light, whereas only the terminal phe- 

 nomena of rut occur in the fall at a time of diminishing light. That withdrawal 

 of light may be an active factor in precipitating the terminal phase of the 

 ungulate cycle is suggested by recent observations by Bissonnette'" to the effect 

 that the onset of the sexual season in goats, which normally occurs in the fall, 

 may be hastened by subjecting them to diminished light. If this be the case in 



