The Utility of Break-Back Traps in Population Studies of Small Mammals

Ordinary snap or break-back traps are so extensively used in small mammal population studies that their suitability is seldom questioned. Indeed, their usefulness and efficiency have been regarded so highly that the problem of developing better mouse-trap has been looked upon as more or less insoluble. Students of small mammal populations are constantly faced with the problem of dealing with populations composed of several species with varying habitat requirements and habits. These populations, although heterogeneous both in species and behaviour patterns, are almost always sampled with a single type of trap. The suitability of the traps for taking any or all species has not been evaluated, although, as Edwards (1952) has pointed out, most trappers of small mammals feel that some species are easier to take than others. Thus, a bias in sampling for population composition is immediately apparent. Apart from these difficulties involving selectivity, there is a further consideration in the efficiency of the trap in taking any animals at all. When a trap is set and is later found to contain an animal, there is positive proof that the animal was present, but when the trap is found to be empty one is left to speculate as to whether this means a real absence of animals or simply that the animals have in some way avoided the trap. It can reasonably be supposed that often an animal may pass a trap without being attracted to it by reason of failure to sense the bait or because