90 John B. Calhoun 



other vocalizations also occiii- when rats are on the surface near their 

 burrows and also while at the source of food. I can only suspect that among 

 these there is specific communication among alpha members, the dominant 

 males, neighboring colonies, and that rats at the food source emit a signal 

 which might be termed a "here is food" signal. Unless there are such signals, 

 it is difficult for me to understand many of the observed behaviors of rats 

 which clearly indicated that they were aware when other rats were or were 

 not at the food source, even though they could not see it. 



B. Howler Monkeys, a Compact Colony Living Species 



Carpenter (1962) summarized the results of field research on this species 

 {Alouatta palliata) during the past thirty years. Tabular data on 136 dis- 

 tinct groups show the number of adult males and females and the number 

 of immature individuals in each group. I derived Fig. 32 from these data. 

 Two large groups containing 27 and 31 adults, respectively, were omitted 

 from the analysis. 



Regardless of group size all groups contained more females than males. 

 Excluded males live in a state of near isolation and have very little associa- 

 tion either with each other or the organized groups. As with the Norway 

 rat, reproduction within compact colonies apparently requires a reduction 

 in the number of males in groups for effective reproduction to take place. 



One of the central hypotheses in my formulation of the social use of 

 space is that constellation formation must have served as a mold which 

 so guided evolution that beha\'ior and physiology would become fixed so 

 that they would have optimum expression in a group of 12 adults. For 

 howler monkeys, groups of 9 to 11 are encountered more frequently than 

 smaller and larger ones (Curve A, Fig. 32). And although the decline in 

 frequency of groups containing more than 1 1 adults is not so rapid as ex- 

 pected (Table \'III), the observed data do reveal a marked decrease in 

 frequency of larger groups. Furthermore, only 3% of the 136 groups ex- 

 ceeded the maximum of 19 anticipated by the theory. The theory indicates 

 no expectation of groups containing less than 7 adults, yet 26.5% of the 

 136 groups of howler monkeys did contain less than 7 adults. However, it 

 must be pointed out that such theory presupposes completion of all social 

 processes culminating in a group having considerable stability of member- 

 ship. Incipient groups formed from the fragmentation of larger ones and 

 larger ones approaching the point where fragmentation is imminent should 

 logically both be excluded in comparing observed and theoretical fre- 

 quencies of group sizes. However, present knowledge prevents such a 

 comparison. 



