PARENTAL BEHAVIOR 



1355 



developed for cancer research, found strain 

 differences with respect to the frequency of 

 "cannibahsm" — eating of the young. 



King (1958) found that females of two 

 closely related subspecies of deer mouse 

 differed with respect to the intensity with 

 which they would protect the young against 

 an intruder. The difference between these 

 two subspecies may be a difference in ag- 

 gressiveness, which is known to be capable 

 of alteration by selective breeding (Scott, 

 1958), and which might here be seen to be 

 influencing the expression of parental be- 

 havior. A number of other behavioral and 

 physiologic characters which may be re- 

 lated to the expression of parental behavior 

 have been shown to be susceptible of al- 

 teration by selective breeding: prolactin 

 content of the pituitary gland (Grosvenor 

 and Turner, 1957) ; temperature prefer- 

 ences (Wolburg, 1952) ; exploratory be- 

 havior (Carr and Williams, 1957; Mc- 

 Clearn, 1959) ; efficiency of lactation 

 (Falconer, 1953) ; hoarding (Stamm, 1954, 

 1956) ; various aspects of "emotionality" 

 (Broadhurst, 1958), etc. (Fuller, 1960). It 

 may be suggested that systematic study of 

 the genetics of parental behavior may soon 

 be as rewarding as has been the study of 

 various genetic aspects of sexual behavior 

 (Young, 1957). 



E. THE EOLE OF PARENTAL BEHAVIOR IN 

 THE DEVELOPMENT OF THE YOUNG 



The biologic function of parental be- 

 havior is, of course, to provide conditions 

 which foster the development of the eggs 

 and young. The requirements of the eggs 

 and young thus form part of the complex of 

 environmental conditions, in adaptation to 

 which parental behavior has evolved. The 

 characteristics of the parent are, of course, 

 also a source of selection pressure guiding 

 the evolution of the characteristics of the 

 young. This is not the place for a full dis- 

 cussion of this problem, but we may briefly 

 indicate some of the ways in which parental 

 behavior is adapted to the requirements of 

 the development of the young. It is apparent 

 that experience gained from contact with 

 other animals including the parents is pre- 

 requisite for the normal display of much 

 of hormonally induced behavior. 



1. In Birds 



Incubation behavior is clearly related 

 to the fact that most bird embryos require 

 for their development temperatures higher 

 than the usual environmental temperatures. 

 Egg temperatures are consistently main- 

 tained at a high level by the transfer of 

 heat between the ventral surface of the 

 incubating parent's body and the surface 

 of the egg (Kossack, 1947). We have al- 

 ready pointed out that the average tempera- 

 ture of eggs during incubation is approxi- 

 mately the same in birds of the same species 

 breeding in different climates, probably in- 

 dicating that the varying incubation be- 

 havior of the parents produces an approxi- 

 mately constant temperature of the eggs 

 (Irving and Krog, 1956). Farner (1958) 

 measured the temperature at the egg-body 

 surface in the nests of incubating yellow- 

 eyed penguins, and compared them with 

 cloacal temperatures. He found that, al- 

 though the body temperature of the bird did 

 not change during the incubation period, 

 the incubation temperature (temperature at 

 the egg-body interface) gradually increased 

 from about 20 to 25°C. to about 38°C. 

 during the incubation period. This gradual 

 increase in temperature is presumably due 

 to some combination of increasing efficiency 

 of incubation behavior and increasing vas- 

 cularity of the incubation patch. Metabolic 

 activity of the developing embryo does not 

 seem to be involved, because the curve of 

 temperature change was the same for nests 

 with eggs which failed to develop as for 

 nests with live eggs. 



There is great interspecific variation in 

 the speed of development of avian embryos, 

 and in the incubation behavior of the par- 

 ents (Stresemann, 1934). Some indication 

 of the significance of the details of pa- 

 rental incubation behavior in fostering the 

 development of the eggs may be seen in 

 the fact that great difficulties are en- 

 countered in attempting to incubate eggs 

 of wild birds in artificial incubators. Daniel 

 (1957) found that the eggs of red-winged 

 blackbirds developed well in artificial incu- 

 bators up to the 7th day, and that eggs 

 collected after the 7th day could be brought 

 to hatching in such an incubator, but that it 



