The Study of Mammal Movements: A Review

Abstract

Movements of mammals have been widely studied, although some studies appear to have lacked specific objectives. Movement data furnish basic information regarding a species, are useful in control programs, to the taxonomist, and to the wildlife manager. Beginning with Seton (1909), biologists have used the home range concept in their studies of mammals but have differed widely in their approaches to the determination of home range. This report suggests that the sizes and shapes of species home ranges have little significance in themselves and that researchers should concentrate on ecological studies. In general, herbivores seem to have smaller home ranges than carnivores; habitat affects the size of home range; and the average size of home range appears to decrease as the population density increases. Advantages of direct observations for determining mammal movements are: handling is minimized, movements of individuals being studied are not hampered, and direct information is obtained on where an animal is and what it is doing. Principal disadvantages of this technique are the time required, the limited number of animals that can be studied at one time, and the influence of the observer on the individual being studied. Many techniques for capturing, marking, and directly observing mammals have been developed and reported. Indirect observations include use of natural signs; capture, marking, and release methods; radioactive materials; dyes for urine and feces; photographic devices; and radiotelemetry. Although a few workers disagree, the techniques for locating and observing animals appear to be far ahead of the techniques for interpreting the data. Beginning with Hayne's (1949) report, several workers have suggested various statistical approaches to the study of home ranges and movements of mammals. These include methods for developing composite home ranges based on two or more observations for each individual; for determining when an animal has been captured near the limits of its range; and for estimating the probability that a capture site or point where an animal was observed was within some specific distance of a calculated center of activity. No one technique for determining location and no one technique for analyzing data gives the best answer for all species and all situations. The length of the observation period, conceded to be important, has rarely been considered sufficiently in calculating home range areas or distances of movement. Techniques or methods used for analyzing data include the minimum area method, the boundary strip method, the inclusive boundary strip method, the exclusive boundary strip method, the observed range length, the adjusted range length, the composite home range, the center of activity, activity radius, recapture radius, recapture center, standard diameter, standard range, and others. Recently various investigators have shown that home ranges of various species of mammals are rectangular, ellipsoidal, or linear, rather than circular. Probably no one shape will satisfactorily describe the home ranges of all species. It is suggested that studies should emphasize a mammal's specific needs under all circumstances all year rather than the distance it moves, the shape of its home range, or the area covered. The latter questions will be answered in finding out why an animal is at a particular place at a particular time. This paper reviews some of the techniques used for studying mammal movements, discusses the more common methods used to evaluate and interpret movements of mammals, and briefly looks at some of the needs in this area. The term mammal movements is used in a broad sense, including activity, home range, migration, immigration, emigration, and movements associated with behavior and territory. The difference between individual movements and movements of species or populations should be clearly understood. Individual movements are movements of individual mammals. A study of individual movements leads to conclusions about daily and seasonal movements, species movements (except for range extension), migratory movements, movements in defense of territory, and movements in any other category the researcher chooses to study.