1342 



HORMONAL REGULATION OF BEHAVIOR 



to the secretory cells of the pituitary gland. 

 There is now abundant additional evidence 

 of the correctness of Harris' concept of the 

 neurohumoral relationship between the hy- 

 pothalamus and the anterior pituitary gland 

 (Harris, 1955). 



In summary, there is ample anatomic 

 and physiologic basis for concluding that 

 the nervous system exercises detailed con- 

 trol over the activity of both the anterior 

 and the posterior lobe of the pituitary gland. 



2. Hormone Secretion as a Reflex 



The preceding remarks imply that it 

 should be possible for endocrine secretion 

 to occur as a reflex response to stimulation 

 of afferent neural structures and, there- 

 fore, hormone secretion may, in some situa- 

 tions, occur as a reflex response to external 

 stimuli of the kind which we ordinarily 

 know to give rise to behavioral responses. 

 There is considerable direct evidence that 

 this is indeed so. 



In his classic papers on sexual cycles, 

 F. H. A. Marshall (1936, 1942, 1956) 

 pointed out the role which external stimuli 

 might play in the regulation of breeding 

 periods. The importance of such factors is 

 becoming increasingly clear, not only in 

 determining the timing of breeding periods 

 during the year, but also in organizing the 

 succession of changes within the breeding 

 season itself (Maschkowzew, 1940; Aschoff, 

 1955). 



Most animals breed only during a par- 

 ticular season of the year, and it has long 

 been known that, in many species of birds 

 and mammals, changes in light stimula- 

 tion due to the changing length of the day 

 constitute one of the principal regulators of 

 the timing of the breeding season (Rowan, 

 1926, 1931; Bissonnette, 1937; Farner, 

 1955). Other factors, such as temperature, 

 also play a role (Pitt, 1929; Marshall and 

 Coombs. 1952; Engels and Jenner, 1956; 

 Marshall and Disney, 1956). 



Some tropical and Australian birds breed 

 irregularly, whenever heavy rains occur 

 (Baker, 1938; Marshall, 1951). In such 

 species, the gonads of birds collected in 

 the same locality just before a rainy period 

 are inactive, whereas one or two months 

 after a rainy period, birds of the same spe- 

 cies have active gonads, but only in the 



locality in which the rain has occurred 

 (Keast and Marshall, 1954; Benoit, 1956). 

 We have already discussed (see above, p. 

 1280) the experimental demonstration by 

 Marshall and Disney (1957) of the nature 

 of this effect. 



Stimuli provided by the mate are of im- 

 portance in the development of gonadal 

 activity in many animals. Although the in- 

 creasing length of the day in spring initi- 

 ates gonad development in many birds, it 

 is commonly found that they do not come 

 into full breeding condition unless further 

 stimulated by the presence of the mate 

 (Riley and Witschi, 1938; Bissonnette, 

 1939; Burger, 1953; for review see Lehr- 

 man, 1959a). In colonial birds, mutual 

 stimulation among the members of the 

 colony appears to have the effect of syn- 

 chronizing their reproductive cycles (Darl- 

 ing, 1938), so that larger colonies have 

 shorter over-all breeding seasons than 

 smaller ones; in very large colonies, groups 

 of birds in any one part of the colony may 

 have their breeding times more closely syn- 

 chronized than those of the colony as a 

 whole (Neff, 1937; Lack and Emlen, 1939; 

 Disney and Marshall, 1956). 



Female rabbits and domestic cats ovu- 

 late as a result of the stimulus provided 

 by participation in copulation (Heape, 

 1905), which causes the release of gon- 

 adotrophic hormone from the animal's hy- 

 pophysis. This effect can be duplicated by 

 artificial mechanical stimulation of the va- 

 gina in cats, although less readily in rab- 

 bits (Greulich, 1934; Sawyer, 1949; Saw- 

 yer and Markee, 1959). Whitten (1956a) 

 has shown that the timing of the estrous 

 cycle of the female mouse can be modified 

 by stimuli provided by male mice. This 

 stimulation is probably olfactory, inasmuch 

 as the length of the estrous period can be 

 changed by placing a male in a small basket 

 within the female's cage for several days, 

 or by placing the females in cages recently 

 vacated by males. Further, removal of the 

 olfactory bulbs causes regression of the 

 ovaries although it seems to have no such 

 effect on the male gonads (Whitten, 1956b; 

 Lamond, 1958, 1959). Conversely, forcing 

 the association of female mice in large 

 groups appears to suppress gonadotrophic 

 activity (Whitten, 1959) . 



