CxONADAL HORMONES AND SOCIAL BEHAVIOR 



1245 



known, history of selection for fighting abil- 

 ity. Scott (1942) found that inbred strains 

 of C57/10 black, C3H agouti, and C (Bagg) 

 mice differed in aggressiveness, activity, 

 and other traits. When C57/10 and C prog- 

 eny were cross-fostered the behavior char- 

 acteristic of the strains remained true to 

 heredity (Fredericson, 1952) . 



A selective breeding program for levels 

 of aggressiveness in Leghorn chickens was 

 conducted by Guhl, Craig and Mueller 

 ( 1960) . The males and females of the parent 

 generation were of different strains merely 

 because these were available, and there was 

 no information on the relative aggressive- 

 ness of either strain. Selection in each gen- 

 eration was based on the percentage of ini- 

 tial encounters won, as supported by 

 rankings in the peck-order. Individuals 

 ranking highest and lowest were used for 

 breeding. This technicjue limited the num- 

 ber of individuals wdiich could be tested. 

 Some of the results are indicated in Figure 

 20.1 for four generations of selection. Re- 

 ciprocal crosses were made with the 



breeders of the F3 generation, and their 

 offspring tended to be intermediate in ag- 

 gressiveness. These results are indicated by 

 four })oints in the F4 . However, due to 

 uncontrollable circumstances, only a few 

 of these chicks lived to sexual maturity 

 and testing. The upper points are means of 

 7 males and 8 females from "high" dams, 

 and the lower two points are the means of 

 3 males and 1 female from "low" dams. 

 The number of individuals selected for 

 breeding and their tested progeny is given 

 beneath the figure for all generations ex- 

 cept the crosses. A x" analysis based on the 

 mean percentage of encounters won by in- 

 dividuals from the "high" and "low" lines 

 showed statistically significant differences 

 between the two lines of selection from the 

 F2 through the F4 in both sexes. 



Calhoun (1956) made an interesting at- 

 tempt to determine the extent to which 

 heredity might modify social behavior. He 

 used physiologically unstable DBA/2 and 

 physiologically stable C57BL/10 inbred 

 mice. Among other differing traits, the 



Fig. 20.1. Result.? of selective breeding for high and low levels of aggressiveness in White 

 Leghorn chickens. Figures show the percentage of initial encounters won by sires, dams, and 

 their progeny. All encounters were between individuals of high and low hnes. 



