PARENTAL BEHAVIOR 



1297 



tei-A'al before laying of the new clutch bears 

 no relationship to the age of the old clutch 

 (Amantea, 1928; Nice, 1937, 1949; Koba- 

 yashi, 1953a; Giirr, 1954). For example, 

 Paludan (1951) notes that, when 35 herring 

 gull nests were destroyed by a storm, the 

 laying of the first eggs of the new clutches 

 was closely spaced between 11 and 14 days 

 later, although the original clutches had 

 varied in age from 1 to 21 days. However, 

 quantitative data from studies of waterfowl 

 (Sowls, 1949) and pheasants (Seubert, 

 1952) in wildlife preserves indicate that the 

 interval before relaying is somew^hat longer, 

 the older the original clutch at the time of 

 its destruction. 



4- Some Remarks on the Onset of Incuba- 

 tion 



In the light of all the facts just presented, 

 what may we say about the nature of the 

 endocrine changes underlying the onset of 

 incubation behavior during the normal 

 breeding cycle? 



The first point to be dealt with is the 

 problem of the exact time when prolactin 

 appears during the cycle. Saeki and Tanabe 

 (1954, 1955) found that the prolactin con- 

 tent of the pituitary gland rises immediately 

 after domestic hens begin to show incuba- 

 tion behavior. However, the spacing of their 

 tests was such that it is not clear whether 

 the prolactin content rises just before or 

 just after the beginning of incubation. Simi- 

 larly, Bailey's (1952) description of the 

 development of the incubation patch in song 

 birds permits the conclusion that prolactin 

 is secreted during early incubation, but does 

 not reveal whether the secretion of this hor- 

 mone first reaches significant levels just 

 before or just after the beginning of incuba- 

 tion behavior. Lahr and Riddle (1938) esti- 

 mated the presence of prolactin in the blood 

 of pigeons by arresting mitoses in the crop- 

 sac epithelium by means of colchicine in- 

 jection, a method which gives a very sensi- 

 tive indication of the effect of prolactin on 

 the rate of growth in the epithelium (Le- 

 blond and Allen, 1937) . They found that the 

 average number of mitoses ]ier high power 

 microscopic field rose from about 4 just be- 

 fore the 1st egg to about 32 after the laying 

 of the 2nd egg. They also found that the 



injection of 60 l.V. of prolactin would in- 

 crease the average number of mitoses per 

 field from about 7 to about 25 within a half 

 hour after the injection. Since pigeons (at 

 least in captivity) begin to incubate some- 

 time between the laying of the 1st and of the 

 2nd egg, these data again are not sufficiently 

 exact to indicate whether the prolactin se- 

 cretion began just before or just after the 

 beginning of incubation behavior. 



Inasmuch as the presence of eggs stimu- 

 lates prolactin secretion, and placing eggs 

 in the newly built nests of black-headed 

 gulls suppresses the laying of subsequent 

 eggs, provided the birds incubate (Weid- 

 mann, 1956), Eisner (1958) suggested that 

 ])rolactin secretion starts as a result of the 

 onset of incubation, which in turn is caused 

 by other hormones such as progesterone 

 (Cole, 1930; Riddle and Lahr, 1944; Lehr- 

 man, 1958b). This is a plausible suggestion, 

 and should be considered in future research 

 on this problem, despite some difficulties 

 such as the fact that progesterone does not 

 induce incubation behavior in domestic hens 

 (see above, page 1293). 



A second problem is the manner of hor- 

 mone action in inducing incubation behav- 

 ior. (This is, of course, a problem with re- 

 spect to all hormone-induced behavior.) 

 Some species of birds incubate eggs in spite 

 of the absence of incubation patches (Bai- 

 ley, 1952). Nevertheless, a number of coin- 

 cidences suggest that the incubation patch 

 should be investigated as a possible source 

 of stimulation for incubation, or as a source 

 of tension which is reduced by incubation 

 behavior. First is the fact that the occur- 

 rence of incubation patches in male and in 

 female birds generally corresponds to that 

 of incubation behavior in the two sexes. 

 Second, the incubation patch normally de- 

 veloi)s just at the time when incubation be- 

 havior is beginning. Third, the characteris- 

 tics of the incubation patch suggest that it 

 is sensitive to temperature changes and 

 adapted for heat exchange with the egg, and 

 the effects of temperature changes on the 

 incubation behavior of the bird suggest that 

 the temperature-sensitivity of the incuba- 

 tion patch may be an important factor. 

 Finally, prolactin induces incubation be- 

 havior in hens only if they are laying at 



