54 John B. Calhoun 



they expand immediately as Cleihrionumys are removed. The vahdity of 

 this interpretation is revealed in Fig. 14B, which shows that during the 

 first 3 days of trapping as the Clethrionomys population is reduced, as in- 

 dicated by fewer numbers taken on successive days, the numbers of Pero- 

 myscus taken increases. This increasing catch can only result from sufficient 

 expansion of home range by survivors to bring about an increased prob- 

 ability of encountering traps. 



One might argue that these latter data for Pewmyscus merely indicate 

 that the snap-trap is initially a sufficiently strange object to elicit avoidance. 

 As time elapses these mice become accustomed to the presence of the trap 

 and thus later in time more individuals will enter traps. Two lines of evi- 

 dence of reasoning suggest the fallacy of this interpretation. If it were cor- 

 rect, we must conclude that Pewmyscus in Maine lack this strange object 

 response but those in New York have it highly developed (see Fig. 14). 

 There exists no logical basis for believing that such a difference character- 

 izes the populations of these two areas. Furthermore, in the trapping of 

 both Pewmyscus and Clethrionomys it is not an uncommon experience to 

 find either of these mice dead in a trap with a bloody stump of one hind 

 leg, while another trap, 2-5 feet away, is covered with fresh blood and fur. 

 The conclusion as to what happened is clear. The mouse happened to get 

 caught in one trap by one leg, it chewed or pulled itseh loose, then went 

 fairly directly to another trap, bit at the bait on the treadle and was thus 

 killed. If these mice have a strange-object reaction it must be of a suffi- 

 ciently low order of magnitude that even the recent loss of a leg in one trap 

 fails to increase it to the point of avoiding the next trap encountered. 



In the third area. Alberta, from which adequate data are available for 

 these two genera, both are relatively abundant but Pewmyscus exceeds 

 Clethrionomys (Table VII). Dming the fall season, nearly twice as many 

 of each genus are trapped as during the spring. During the period of low 

 spring densities, these genera exhibit nearly identical rates of decline 

 (Fig. 15 A) accompanying removal trapping. As with the Maine data, such 

 trends may be interpreted as indicating that at such densities neither 

 species is capable of markedly inhibiting the home range of the other. 

 However, by fall many Clethrionomys have contracted their home ranges 

 as a response to their exposure not only to more of their own kind but also 

 to more Peromyscus. Home ranges of Peromyscus remained unaltered, as 

 indicated by the similarity of rate of decline during both spring and fall 

 (Fig. 15). However, the daily catch for Clethrionomys remained nearly 

 identical through three successive days of removal trapping. Sufficient en- 

 largement of home ranges by surviving Clethrionomys must have taken 

 place each day to lead to an equivalent frequency of traps being encountered 

 on the following day bj^ red-backed mice, despite their fewer numbers 



