Geoscience Reference
In-Depth Information
Costs and benefits must ultimately be calculated in terms of an animal's fit-
ness, 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
C
D
plus
C
R
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
C
D
plus
C
R
exceeds
B
in the long term, then the animal must reduce
C
D
, or
C
R
, both of which have lower limits.
C
D
generally cannot be reduced
below basic maintenance costs, or basal metabolism; however, hibernation and
estivation are methods some animals can use to reduce
C
D
below basal metab-
olism. Reducing
C
R
might reduce
B
because benefits can be experienced only
through attending to critical, local resources, which is
C
R
. If
C
R
can be reduced
through increased efficiency,
B
need not be reduced when
C
R
is reduced or
need not be reduced as much as
C
R
is reduced. Ultimately, in the long term, if
C
D
+
C
R
>
B
then the animal cannot survive using local resources. If the ani-
mal 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. Oper-
ational 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 ap-
proaches 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 loca-
tions that the animal visits over time. An animal's home range usually cannot
