PARENTAL BEHAVIOR 



1809 



endogenous hormones and nest-building be- 

 havior, 



3. Induction of Nest-huilding Behavior by 

 External Stiinidi 



Temperature changes and nest-huilding 

 behavior. Kinder (1927) kept rats at vari- 

 ous temperatures, and found that the 

 amount of nest-building activity was in- 

 versely proportional to the environmental 

 temperature. For example, at room tempera- 

 tures around 90°F. rats used, on the av- 

 erage, 6 to 11 stri])s of paper per 6-hour 

 period; when the room temperature fell to 

 40 to 60°F. the consumption of nesting ma- 

 terial rose to 38 to 50 strips during a like 

 period. Nests built at temperatures of 90°F. 

 were small, scattered, and loose, in contrast 

 to the large compact nests built by rats 

 kept in the lower temperatures. Roller 

 (1956) observed similar effects in mice. He 

 found that nest-building did not occur at 

 temperatures above 85°F., and that the 

 amount of nest-building increased with de- 

 creasing temperature down to about 50°F. 



This effect of temperature on nest-build- 

 ing activity, plus the fact that thyroidec- 

 tomy increases, and exogenous thyroid ex- 

 tract inhibits, such activity, has suggested 

 to all workers on this problem that nest- 

 building activity serves in part a thermo- 

 regulatory function, and that the cyclic 

 variations in nest-building behavior in rats 

 and mice can be partly explained as tem- 

 perature-regulating devices (Kinder, 1927; 

 Richter, 1937; Koller, 1956). We shall re- 

 turn to this point later, in our discussion of 

 the mechanisms of hormonal effects on be- 

 havior (see below, p. 1346). 



The effect of the young. Koller (1952, 

 1956) reported that the introduction of 

 young mice (less than 15 days old) into the 

 cage with an adult female causes an abrupt 

 increase in the amount of nest-building be- 

 havior on the following night and on subse- 

 quent nights. Females that had been using 

 an average of about 7 gm. of nesting mate- 

 rial per night increased their consumption 

 to al)out 25 gm. This increase in nest-build- 

 ing activity appears to be a consequence of 

 the "adoption" of the young by the female. 

 If the young animals are placed in the cage 

 under a wire cover, so that the female can- 



not retrieve them or touch them, they do 

 not cause any increase in nest-building ac- 

 tivity. Further, about 25 per cent of Roller's 

 subjects killed or ate the young, and in those 

 cases there was no increase in nest-build- 

 ing activity on the night following the intro- 

 duction of the young. Nest-building activ- 

 ity is normally maintained at a high level 

 after parturition, during the period when 

 the young are being cared for by the mother. 

 Roller found that when the young were re- 

 moved immediately after parturition the 

 level of nest-building activity (which is 

 very high during the last half of pregnancy) 

 abruptly dropped back to the nonbreeding 

 level. 



Leblond (1940) similarly reported that 

 introduction of young mice into the cage in- 

 duced nest-building behavior in the adults, 

 but beyond this statement his data are 

 ambiguous on this point because his quan- 

 titative scores were arrived at by lumping 

 nest-building behavior together with other 

 aspects of ''maternal behavior." 



Roller reported that male mice kill and 

 eat young animals left in the cage with 

 them, and therefore that no increase in 

 nest-building behavior was induced by this 

 treatment. On the other hand, Leblond and 

 Nelson (1937a; Leblond, 1940) found that 

 "maternal behavior" (which included an 

 undefined amount of nest-building behav- 

 ior) was induced in male mice by keeping 

 young in their cages. It is not apparent 

 whether this difference between the results 

 of Leblond and Nelson and those of Roller 

 is due to a difference in the strain of mice 

 used or in the conditions of their exjieri- 

 ments. 



One is temi)ted to suggest that the ability 

 of young mice to stimulate increased nest- 

 building behavior in adults is due to a 

 hormonal change induced by stimuli coming 

 from the young, but this does not seem very 

 likely. To begin with, the presence of young 

 mice maintains a high level of nest-building 

 behavior in their mothers after parturition, 

 when the level of progesterone (which can 

 induce the nest-building behavior) has 

 fallen abruj^tly. The presence of the young 

 is known to stimulate and maintain the se- 

 cretion of prolactin (see below, p. 1325) , but 

 ]irolactin administered to adult females does 



