26 John B. Calhoun 



cursions from home. During these outward excursions the animal remains 

 in a refractory state during which it is unable to respond to stimuli en- 

 countered. Only at the termination of trips, including the period of wander- 

 ing at end of trips, will the animal be able to respond to objects or situa- 

 tions such as it had already passed on the outward trip. However, those 

 objects which were bypassed on the outward trip do alter the probability 

 of terminating trips. The greater the number of such objects, or the greater 

 the intensity of stimuli emanating from them, the greater will be the 

 probability of a trip stopping after reaching every distance from home. 

 Should novel stimuli be encountered during a trip, the shorter will be the 

 interval after reaching home before initiating another trip. This holds in 

 so long as the novel stimuli are not excessive. In this case, the animal will 

 remain at home for unusually long periods before again venturing forth. 

 The probability of wandering increases with distance from home. These 

 several aspects of locomotion and responsiveness to stimuli lead to a 

 probabiUty of responding within the region about an animal's home, 

 which so closely resembles that of the bivariate normal distribution func- 

 tion that the latter may be used to represent home range. 



VI. Continuous Removal Trapping of Small Mammals 



When a large number of snap-traps are set within a habitat where mice 

 and shrews live, it is usually observed that more individuals are captured 

 on the first than on the second or third day, and that fewer animals enter 

 traps on the third than the second day. It has generally been assumed 

 that this decline in catch through time would continue were the traps left 

 set for a longer period. Based upon this assumption, various equations 

 (e.g., Zippin, 1956; Calhoun and Casby, 1958) have been developed for 

 estimating the number of individuals which would eventually enter the 

 traps. 



Furthermore, it has also generally been assumed that the catch for a 3- 

 or 4-day period would reflect the relative species composition of the com- 

 munity being sampled. This assumption served as the basis for establishing 

 the North American Census of Small Mammals (Calhoun, 1949-1957). 

 During the nine years, cooperators provided results (see NACSM Release 

 No. 9) for 1615 standard trap-lines consisting of 20 stations, usually 50 

 feet between stations, 3 traps per station, run for three consecutive nights. 

 Despite the laudable objectives of this cooperative endeavor, this tre- 

 mendous effort has, for reasons we shall shortly see, provided inadequate 

 data for insight into either relative density or species composition. 



Questioning the usefulness of short-term removal trapping developed 



