Definition of Home Range

Most animals are not nomadic but live in fairly confined areas where they enact their day-to-day activities. Such areas are called home ranges.

Burt (1943:351) provided the verbal definition of a mammal's home range that is the foundation of the general concept used today: "that area traversed by the individual in its normal activities of food gathering, mating, and caring for young. Occasional sallies outside the area, perhaps exploratory in nature, should not be considered part of the home range." This definition is clear conceptually, but it is vague on points that are important to quantifying animals' home ranges. Burt gave no guidance concerning how to quantify occasional sallies or how to define the area from which the sallies are made. The vague wording implicitly and correctly allows a home range to include areas used in diverse ways for diverse behaviors. Members of two different species may use their home ranges very differently with very different behaviors, but for both the home ranges are recognizable as home ranges, not something different for each species.

How does an animal view its home range? Obviously, with our present knowledge we cannot know, but to be able to know would provide tremendous insight into animals' lives. Aldo Leopold (1949:78) wrote, "The wild things that live on my farm are reluctant to tell me, in so many words, how much of my township is included within their daily or nightly beats. I am curious about this, for it gives me the ratio between the size of their universe and mine, and it conveniently begs the much more important question, who is the more thoroughly acquainted with the world in which he lives?" Leopold con tinued, "Like people, my animals frequently disclose by their actions what they decline to divulge in words."

We do know that members of some species, probably many species, have cognitive maps of where they live (Peters 1978) or concepts of where different resources and features are located within their home ranges and of how to travel between them. Such cognitive maps may be sensitive to where an animal finds itselfwithin its home range or to its nutritional state; for example, resources that the animal perceives to be close at hand or resources far away that balance the diet may be more valuable than others. From extensive research on optimal foraging (Ellner and Real 1989; Pyke 1984; Pyke et al. 1977), we know that animals often rank resources in some manner. Consequently, we might envision an animal's cognitive map of its home range as an integration of contour maps, one (or more) for food resources, one for escape cover, one for travel routes, one for known home ranges of members of the other sex, and so forth.

Why do animals have home ranges? Stamps (1995:41) argued that animals have home ranges because individuals learn "site-specific serial motor programs," which might be envisioned as near reflex movements that take an animal along well traveled routes to safety. These movements should enhance the animal's ability to maneuver through its environment and thereby to avoid or escape predators. Stamps argued that the willingness of an animal to incur costs to remain in a familiar area implies that being familiar with that area provides a fitness benefit greater than the costs. For animals with small home ranges that live their lives as potential prey, Stamps's hypothesis makes sense. However, many animals, especially predatory mammals and birds, have home ranges too large and use specific places too seldom for site-specific serial motor programs to have an important benefit. Site-specific serial motor patterns of greatest use to a predator would have to match the escape routes of each prey individual, but each of these might be used only once after it is learned. The reason that animals maintain home ranges must be broader than Stamps's hypothesis.

Nonetheless, Stamps has undoubtedly identified the key reason that animals establish and maintain home ranges: The benefits of maintaining a home range exceed the costs. Let CD be the daily costs for an animal, excluding the costs, Cr , of monitoring, maintaining, defending, developing, and remembering the critical resources on which it based its decision to establish a home range. In the long term, CD plus CR must be equal to or less than the benefits, B, gained from the home range, or

Costs and benefits must ultimately be calculated in terms of an animal's fitness, but if the critical resources are food, then costs and benefits might be indexed by energy. If the benefits are nest sites or escape routes, energy is not an adequate index. If CD plus CR exceeds B for an animal in the short term, then the animal might be able to live on a negative balance until conditions change. If CD plus CR exceeds B in the long term, then the animal must reduce CD, or Cr, both of which have lower limits. CD generally cannot be reduced below basic maintenance costs, or basal metabolism; however, hibernation and estivation are methods some animals can use to reduce CD below basal metabolism. Reducing CR might reduce B because benefits can be experienced only through attending to critical, local resources, which is CR. If CR can be reduced through increased efficiency, B need not be reduced when CR is reduced or need not be reduced as much as CR is reduced. Ultimately, in the long term, if CD + Cr > B then the animal cannot survive using local resources. If the animal cannot survive using local resources, it must go to another locale where benefits exceed costs, or it must be nomadic and not exhibit site fidelity.

Because maintaining a home range requires site fidelity, site fidelity can be used as an indicator of whether an animal has established a home range. Operational definitions of home ranges exist using statistical definitions of site fidelity (Spencer et al. 1990). The goals of such definitions are good but the methods sometimes fail to define home ranges for animals that exhibit true and localized site fidelity. For example, Swihart and Slade (1985a, 1985b) used data for a female black bear (Ursus americanus) that I studied in 1983—1985 and determined that she did not have a home range because the sequence of her locations did not show site fidelity as defined by their statistical model. However, the bear's locations were strictly confined for 3 years to a distinct, well-defined area (figure 3.1). Consequently, researchers must sometimes use subjective measures of site fidelity, such as figure 3.1, to augment objective measures that sometimes fail, probably because statistical models have assumptions that are not appropriate for animal movements. Nonetheless, tests of site fidelity should be disregarded only when other objective approaches to site fidelity exist.

An animal's cognitive map must change as the animal learns new things about its environment and, hence, the map changes with time. As new resources develop or are discovered and as old ones disappear, appropriate changes must be made on the map. Such changes may occur quickly because an animal has an instantaneous concept of its cognitive map. A researcher, in contrast, can learn of the changed cognitive map only by studying the changes in the locations that the animal visits over time. An animal's home range usually cannot

Adult Female Black Bear 61 in 1983, 1984 and 1985

Figure 3.1 Location estimates for adult female bear 61 in studied in 1983, 1984, and 1985 in the Pisgah Bear Sanctuary, North Carolina, U.S.A. Note that in each year, bear 61's locations were confined to a distinct area and that the area did not change much over the course of 3 years. This bear showed site fidelity, even though her location data did not conform to the rules of site fidelity for Swihart and Slade's (1985a, 1985b) model. The lightly dotted black line marks the study area border.

Figure 3.1 Location estimates for adult female bear 61 in studied in 1983, 1984, and 1985 in the Pisgah Bear Sanctuary, North Carolina, U.S.A. Note that in each year, bear 61's locations were confined to a distinct area and that the area did not change much over the course of 3 years. This bear showed site fidelity, even though her location data did not conform to the rules of site fidelity for Swihart and Slade's (1985a, 1985b) model. The lightly dotted black line marks the study area border.

be quantified, practically, as an instantaneous concept because the home range can only be deduced from locations of an animal within its home range and the locations occur sequentially (but see Doncaster and Macdonald 1991). Thus, for most approaches, a home range must be defined for a specific time interval (e.g., a season, a year, or possibly a lifetime). The longer the interval, the more data can be used to quantify the home range, but the more likely that the animal has changed its cognitive map since the first data were collected.

In addition, no standard exists as to whether one should include in an animal's home range areas that the animal seldom visits or never visits after initial exploration. Many researchers define home ranges operationally to include only areas of use. Nonetheless, animals may be familiar with areas that they do not use. An arctic fox (Alopex lagopus) may be familiar with areas larger than 100 km2, yet use only a small portion (ca. 25 km2) where food is concentrated (Frafjord and Prestrud 1992). Areas with no food are not visited often, if ever, despite and because of the animal's familiarity with them. Should such areas be included in the fox's home range? Other areas with food might not have been visited in a given year simply by chance. Should those areas be included in the fox's home range? Pulliainen (1984) asserted that any area larger than 4 ha (an arbitrary size) not traversed by the Eurasian martens (Martes martes) he and his coworkers followed should not be included in the martens' home ranges. Through a winter, a marten crosses and recrosses old travel routes, leaving progressively smaller and smaller areas of irregular shape surrounded by tracks. Pulliainen presumed that a marten's radius of familiarity, or radius of perception, would cover an area of 4 ha or less. But how wide might an animal's radius of perception be? Some mammals can smell over a kilometer, see a few hundred meters, but feel only what touches them. Which radius should be used, or should a multiscale radius be used? In addition, areas not traversed may have been avoided by choice. Hence, should no radius of familiarity be considered? If we do not allow some radius of familiarity, or perception, around an animal, we are reduced, reductio absurdum, to counting as an animal's home range only the places where it actually placed its feet. Clearly, this is not satisfactory.

Related to this final problem is how to define the edges of an animal's home range. For many animals, the edges are areas an animal uses little but knows; the animal may actually care little about the precision of the boundaries of its home range because it spends the vast majority of its time elsewhere. Except for some territorial animals, the interior of an animal's home range is often more important both to the animal and to understanding how the animal lives and why the animal lives in that place. Gautestad and Mysterud (1993, 1995) and others have noted that the boundaries of home ranges are diffuse and general, making the area of a home range difficult to measure. That the boundary and area of a home range are difficult to measure does not reduce in any way the importance of the home range to the animal and to our understanding of the animal, however. Even crudely estimated areas for home ranges have led to insights into animal behavior and ecology (see the review by Powell 1994 of home ranges of Martes species), suggesting that home range areas should be quantified. However, we must keep in mind that home range boundaries and areas are imprecise, at least in part, because the boundaries are probably imprecise to the animals themselves.

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