1. The Social Use of Space 53 



Peromyscus, there exist conditions limiting the extent to which this abihty 

 may develop. 



The typically low relative density of both species (Table VI) on Alt. 

 Desert Island, Maine, represents such a condition. As revealed in Fig. 14, 

 both species decline at about the same rate from day 1 through day 3. 

 This can happen only when no alteration in home range size transpu-es 

 over time or when the survivors of each species make equivalent but slight 

 increases in extent of home range. Appreciation of why inhibition of home 

 range fails to develop at low densities demands knowledge of variables we 

 lack. 



It demands that we know actual densities. The NACSAI census provide 

 only relative densities. However, we can make approximations. Run long 

 enough (30 days), the 950-foot-long B-type NACSM census procedure 

 should take all residents wdthin 3 home range sigma on either side of the 

 trapline and for a radius of this distance about the end of the line. If we 

 take 50 feet as approximating the average home range sigma of small 

 mammals, uninhibited by dominants, then approximately 8 acres are ex- 

 posed to such a trapline. Furthermore, examination of 30-day censuses re- 

 veals that for species with uninhibited home ranges, 25-50% of the 

 residents are taken during the first 3 days of trapping. On this basis, there 

 was on the average less than one Clethrionomys and less than two Peromys- 

 cus per acre in this Maine study. Since juveniles, with as yet probably little 

 influence on the spatial distribution of associates, comprised a portion of 

 the catch, it is quite likely that the average distance between home range 

 centers for each species exceeded 3 sigma. As shown in the prior theoretical 

 sections, learning of signals would most likely be fairly ineffective here 

 because contact between neighbors w^ould be infrequent. 



Under these circumstances the signals emitted by each species should 

 have acquired little in the way of negatively stimulating characteristics 

 for its own members. It follows that inhibition of home range size will 

 have been negligible and thus the two species, which probably have nearly 

 the same size home range, should encounter traps with nearly equal fre- 

 quency, and thus the rate of decline in catch from days 1 through 3 should 

 be nearly equal. However, in the one Mt. Desert Island, Maine, study where 

 eight NACSM lines were run for 15 days (Fig. 13A), it is apparent that 

 home ranges of Peromyscus were slightly contracted. Fifty per cent of the 

 15-day total for Clethrionomys was attained by day 6, but not until day 10 

 for Peromyscus. Thus, where both species regularly occur at low densities, 

 Peromyscus is only moderately subordinate to Clethrionomys. 



In the Adirondacks, where Peromyscus most frequently has a low density 

 and Clethrionomys a much higher one (Table VI), Peromyscus is markedly 

 subordinate. Its home ranges not only are markedly contracted, but also 



