_Basal Metabolic Rate and Intrinsic Rate of Natural Increase_
Basal metabolic rate represents the minimum energy required by an
animal to maintain basic homeostasis (Lusk, 1917:141; Kleiber, 1932,
1961:251; Benedict, 1938; Brody, 1945:59; Robbins, 1983:105-111). For
mammals, [.H]_b appears to be determined by complex interactions
between their body size (Kleiber, 1932, 1961:206; Benedict, 1938;
Brody, 1945:368-374; Hemmingsen, 1960:15-36; McNab, 1983b; Calder,
1987), the climate in which they live (Scholander et al., 1950c; McNab
and Morrison, 1963; Hulbert and Dawson, 1974; Shkolnik and
Schmidt-Nielsen, 1976; McNab, 1979a; Vogel, 1980), their food habits
(McNab, 1978a, 1978b, 1980a, 1983a, 1984a, 1986a, 1986b, 1988a, 1989),
and their circadian period (Aschoff and Pohl, 1970; Prothero, 1984).
Some species have higher mass-specific [.H]_{b} than others, and this
variation appears to be tied to ecological circumstances rather than
taxonomic affinities (McNab, 1988a, 1989). Basal metabolic rate is
important ecologically because it serves as a measure of a species'
minimum "obligatory" energy requirement, and under many circumstances,
it represents the largest energy demand associated with a daily energy
budget (King, 1974:38-55; McNab, 1980a; Mugaas and King, 1981:37-40).
Recently it also has been implicated as a permissive factor with
respect to r_{max} of mammals (Hennemann, 1983; Lillegraven et al.,
1987; Nicoll and Thompson, 1987; Thompson, 1987) via its direct effect
on their rates of development and fecundity (McNab, 1980a, 1983a,
1986b; Hennemann, 1983; Schmitz and Lavigne, 1984; Glazier, 1985a,
1985b). The implication of this latter point is that those species
with higher [.H]_{b}'s also have faster rates of development and
greater fecundity and hence enjoy the competitive advantage of a
higher r_{max}. Basal metabolism is, therefore, "a highly plastic
character in the course of evolution" (McNab, 1988a:25) that has a
profound influence on each species' life history.
_Minimum Thermal Conductance_
Whole-body resistance to passive heat transfer is equal to tissue
resistance plus coat resistance. Within limits, these resistances can
be altered; tissue resistance can be varied by changes in blood flow,
whereas coat resistance can be changed by piloerection, molt, and
behavior. When whole-body resistance is maximized (maximum tissue and
coat resistances), passive heat transfer is minimized. The inverse of
resistance is conductance; therefore, maximum whole-body resistance is
the inverse of minimum thermal conductance (C_{m}). Minimum thermal
conductance is readily derived from metabolic chamber data, and it is
commonly used to describe an animal's capacity to minimize passive
heat transfer. Minimum thermal conductance interacts with [.H]_{b} and
body mass to set the maximum temperature differential a mammal can
maintain without increasing its basal level of heat production. The
low temperature in this differential is the lower critical temperature
(T_{lc}).
Mass-specific C_{m} for mammals is negatively correlated with body
mass (McNab and Morrison, 1963; Herreid and Kessel, 1967; McNab, 1970,
1979b; Bradley and Deavers, 1980; Aschoff, 1981), and for any given
mass its magnitude is 52% higher during the active, rather than the
inactive, phase of the daily cycle (Aschoff, 1981). However, some
mammals have C_{m}'s that are higher or lower than would be predicted
for them on the basis of body mass and circadian phase. Seasonal
variation in C_{m} (higher values during summer than winter) has been
reported for many northern mammals that experience large annual
variations in air temperature (Scholander et al., 1950a; Irving et
al., 1955; Hart, 1956, 1957; Irving, 1972:165). Some tropical mammals
with very thin fur coats, and others with nearly hairless bodies, have
high C_{m}'s (McNab, 1984a), as do burrowing mammals (McNab, 1966,
1979b, 1984a) and the kit fox, _Vulpes macrotis_ (Golightly and
Ohmart, 1983). Some small mammals with low basal metabolic rates tend
to have lower than predicted C_{m}'s: small marsupials (McNab, 1978a),
heteromyid rodents (McNab, 1979a), several ant eaters (McNab, 1984a),
the arctic hare, _Lepus arcticus_ (Wang et al., 1973), the ringtail,
_Bassariscus astutus_ (Chevalier, 1985), and the fennec, _Fennecus
zerda_ (Noll-Banholzer, 1979). Thus, in spite of its mass dependence,
C_{m} also has been modified during the course of evolution by
selective factors in the environment and by the animal's own metabolic
characteristics.
_Capacity for Evaporative Cooling_
Latent heat loss occurs as a result of evaporation from the
respiratory tract and through the skin, and except under conditions of
heat stress, it "is a liability in thermal and osmotic homeostasis"
(Calder and King, 1974:302). E_{c}, defined as the ratio of
evaporative heat lost to metabolic heat produced, can be used to
quantify thermoregulatory effectiveness of evaporative cooling and to
make comparisons of heat tolerance between species. Thermoregulatory
effectiveness of latent heat loss is not just a function of the rate
of evaporative water loss but also of the rate of metabolic heat
production (Lasiewski and Seymour, 1972). For example, a low metabolic
rate minimizes endogenous heat load and thus conserves water, whereas
the opposite is true of high metabolic rates (Lasiewski and Seymour,
1972). Some mammals that live in arid regions have evolved low
metabolic rates and thus capitalize on this relationship to reduce
their thermoregulatory water requirement (McNab and Morrison, 1963;
McNab, 1966; MacMillen and Lee, 1970; Noll-Banholzer, 1979). What is
evident, therefore, is that an animal's capacity for increasing latent
heat loss must evolve together with its [.H]_{b} and C_{m} in response
to specific environmental demands.
_Diet_
McNab (1986a, 1988a, 1989) demonstrated that, for mammals, departures
of [.H]_{b} from the Kleiber (1961:206) "norm" are highly correlated
with diet and independent of phylogenetic relationships. McNab's
analysis indicates that for mammals that feed on invertebrates, those
species with body mass less than 100 g have [.H]_{b}'s that are equal
to or greater than values predicted by the Kleiber equation, whereas
those with body mass greater than 100 g have metabolic rates that are
lower than predicted. Grazers, vertebrate eaters, nut eaters, and
terrestrial frugivores also have [.H]_{b}'s that are equal to or
greater than predicted, whereas insectivorous bats, arboreal
folivores, arboreal frugivores, and terrestrial folivores all have
rates that are lower than predicted. McNab (1986a) found animals with
mixed diets harder to categorize, but in general he predicted that
their [.H]_{b}'s would be related to (1) a food item that is
constantly available throughout the year, (2) a food item that is most
available during the worst conditions of the year, or (3) a mix of
foods available during the worst time of the year. Although these
correlations do not establish cause and effect between food habits and
[.H]_{b}, McNab's analysis does make it clear that the relationship
between these variables has very real consequences for an animal's
physiology, ecology, and evolution.