88 John B. Calhoun 



gradually ensued a change toward more males per female, and these females 

 were less successful in reproduction. At the lowest level a colony consisted 

 of only males, or if females occurred they were essentially asexual with 

 regard to reproductive effectiveness. The lower the colony's rank, the lower 

 the mean weight. 



Members of each colony represented more than one place of birth in the 

 pen. Members of the highest-ranked colony, a, mostly still lived at the 

 place of their birth or had come there from adjacent colonies. As social 

 rank of the colony decreased, its members represented ever more different 

 places of birth. Despite this disparity in places of birth characterizing each 

 colony, the total number of adults forming each colony varied little from 

 the mean number of 12 characterizing the loosely knit constellation of such 

 simple social types as Peromyscus and Clethrionomys. 



At this point we may consider possible events which fostered the evolu- 

 tion of the compact colony from the loosely knit constellation. The major 

 impact of the constellation way of living is that the most frequent group 

 size would be 12 individuals. It might, therefore, be anticipated that 

 evolutionary processes would adjust the physiology and behavior of such 

 species to be most effective and appropriate to interactions transpiring in 

 such a sized group. In accordance with the conservatism of evolution we 

 might anticipate restriction of group size about this optimum of 12 as 

 other factors caused the constellation to contract into a compact colony. 

 Restriction of location of food stands is the most likely candidate for an 

 appropriate environmental change. Provided the location of food became 

 restricted but abundant and relatively permanent at these locations, we 

 could expect types like the Norway rat to develop. They build burrows 

 not too far away from such spatially restricted sources of food and all 

 members of the colony participate in transporting this food into the burrow 

 where large caches are formed. 



Compact colony evolution produced a situation which necessitated 

 further evolution of the nervous system. If groups of 12 adults assembled, 

 either as a consequence of being born at the same place or from random 

 mixing, a more nearly equal sex ratio would characterize most colonies 

 than was the case in my study within the experimental enclosure. This 

 presents the opportunity for development of marked aggression among 

 males and consequent stress experienced by associated females. Actually 

 this was the initial situation always characterizing an incipient colony of 

 Norway rats. In the process one or more males were driven off and suffi- 

 cient females remained, although some always left to keep the colony at 

 near 12 individuals. The rejected males either joined another developing 

 colony lacking a male as dominant as the one at the colony they left or 

 they joined to form an all-male colony. Where there was a single very 



