AMER. ZOOL., 28:877-884 (1988)
The Energetic Costs of Fighting1
SUSAN E. RIECHERT
Department of Zoology, University of Tennessee,
Knoxville, Tennessee 37996-0819
SYNOPSIS. Current evolutionary theory predicts that energy expenditure will be adjusted
in contest situations to the value of the disputed resource and the relative probability of
winning it. Estimates of energy expended in contest situations support this prediction. I
report on the energetic costs of display relative to other contest costs to individual fitness
(e.g., risk of predation, losses in feeding time, injury and mortality) in territorial disputes
of the spider Agelenopsis aperta. Cost estimates obtained in terms of decrements to milligrams wet-weight of future egg production resulting from single contests indicate that
actual energy expenditure in these territorial disputes represent insignificant costs. These
costs are, in fact, 5—6 orders of magnitude smaller than the costs associated with injury,
potential predation and even loss in food as a result of time spent in the interactions.
Review of the literature indicates that in most instances, energy expenditure may be
correlated with some other factor upon which selection is acting (e.g., short contests where
predation risk is high, variation in levels of escalation exhibited). Two exceptions include
the tremendous losses of workers to reproductives in ant, termite, and bee colony territorial
disputes and the production of specialized agonistic organs exhibited by some corals, sea
anemones and corallimopharians when encountering neighbors within "territorial"
boundaries.
Rubenstein, 1981) and to a given individual's probability of winning the dispute
(Maynard Smith and Price, 1973; Maynard
Smith, 1974). Thus, individuals would
expend more energy when fighting for a
mating opportunity in a population where
females were in short supply than they
would for mating opportunities in a situation where access to receptive females was
readily available. Individuals also are predicted to expend more energy in disputes
where size, residence time and/or age and
experience do not favor one opponent over
another.
My discussion concerns the extent to
which the predicted relationship holds
between energy expenditure in contests
and the resulting fitnesses individuals
achieve. In particular I consider the fitness
cost of energy expended during contests
when compared to other potential fitness
costs associated with "fighting." These latter costs include debilitating injury and
mortality (Geist, 1974; Wilkinson and
Shank, 1977; Hamilton, 1979; Silverman
and Dunbar, 1980; Bernstein, 1981; Clutton-Brock et al., 1982), exposure to predators (Hammerstein and Riechert, 1988),
' From the Symposium on Energetics and Animal lost feeding opportunities (Chelazzi et al.,
Behavior presented at the Annual Meeting of the 1983) and physiological stress (reviewed in
American Society of Zoologists, 27-30 December
King, 1973).
1986, at Nashville, Tennessee.
INTRODUCTION
Under current evolutionary dogma, we
assume that the genes of individuals showing quantitative advantages in fitness of
even the smallest magnitudes will come to
fixation over time. The link between energy
expenditure and subsequent fitness is also
assumed, since energy expended in activities is energy that is not available for
reproduction. It is not surprising then that
agonistic behavior might be considered to
be energetically costly (Hamilton, 1964;
Rand and Rand, 1976; Gorton and Gerhardt, 1979; Riechert, 1979; Parker and
Thompson, 1980; Caldwell, 1982; Otronen, 1984). (By agonistic behavior I am
referring to King's [1973] definition: "all
behaviors associated with the contest or
struggle between individuals.")
The recent development of evolutionary
game theory has led to the formalization
of these predictions. Time and energy
expenditures in contests are assumed to be
adjusted both to the value of winning the
contest (Maynard Smith and Parker, 1976;
Parker and Thompson, 1980; Parker and
877
878
SUSAN E. RIECHERT
The assumption that energy expenditures in contests represent important fitness costs has not been tested to any extent
because of the difficulties associated with
measuring energy expenditures and
expressing these and other contest costs in
the same fitness currency for comparison.
It is particularly difficult to obtain data on
the energetic costs of the activities exhibited in disputes, because the cost of most
act types which are short in duration cannot be measured. Even total energy expenditure in contests is difficult to measure,
since placing contestants in respirometer
chambers and obtaining contest behavior
and associated estimates of the respiration
exhibited are problematic. Doubly-labeled
water techniques have been used to estimate the costs of competitive interactions
that take place over an extended period
{e.g., frog chorusing [Ryan, 1985; Taigen
and Wells, 1985; Wells and Taigen, 1986]).
Others have used indirect estimates such
as energy expenditure reflected in
increased feeding rates (Ewald and Rohwer, 1980; Berger, 1986) and the ranking
of behavior patterns in terms of increasing
intensity (Rand and Rand, 1976; Riechert,
1978*, 1979). The only study to date that
places both energy expenditure in contests
and other contest costs in a common currency for examination of their relative contributions to individual fitness is my work
on the within-species competitive behavior
of the spider, Agelenopsis aperta. I present
the A. aperta territorial dispute system in
the next section as an example of how such
studies might be completed and the game
theoretic predictions concerning the significance of energy expenditure tested.
is ultimately determined by local insect
levels (Riechert, 1978a, 1981),butisproximally under genetic determination (Riechert, 1987).
The agonistic behavior A. aperta exhibits
in disputes over webs and associated territories has been the subject of extensive
study (Riechert, 19786, 1979, 1981, 1982,
1984, 1986, 1987; Maynard Smith and
Riechert, 1983). Energy expenditure estimates used in the study of contest costs for
this spider were calculated from the following formula modified from that presented in Riechert (1979):
C = 4.7c|W n y RTv
+ WK
where: c = resting metabolism for the spider body temperature experienced in the
given contest in J/mg/sec, 4.7 is a constant
which corrects for maximum aerobic and
anaerobic expenditures over resting for an
ectotherm with the physical properties
exhibited by this spider (calculation by R.
Full based on relationships described in:
Palidino and King [1979], Taylor et al.
[1980, 1982], Garland [1982]), Wo and Wv
are the body masses of the territorial owner
and intruder or visitor, respectively, R, and
Rj are the ranks of given behavior patterns
exhibited by the owner and intruder,
respectively, in the contest, and D; and Dj
are the durations of the given ranked act
exhibited by the owner and intruder in the
contest. To summarize, the estimate is
based on the idea that acts performed with
greater intensity or speed cost more energetically to complete per unit time than
AN EXAMPLE: AGELENOPSIS APERTA
acts in which the movements are perTERRITORIAL DISPUTES
Agelenopsis aperta (Gertsch) is a promi- formed more slowly. The 35 act types
nent spider resident of arid grasslands in exhibited in the territorial disputes of this
the southwestern United States. The spi- spider have been assigned rankings in order
der builds a sheet-web with an attached of increasing intensity from 1 to 35 (Riefunnel from which it can escape daily tem- chert, 19786). The rank score of each
perature extremes and predation while behavior pattern exhibited by a given
waiting for encounters with potential prey. opponent in a contest is multiplied by its
This species is territorial in that individuals duration in the contest. The cost for a given
throughout their lives defend areas in individual then represents these values
excess of their webs against intrusion by summed over all behavior patterns and
conspecifics. Territory size for this species multiplied by estimated resting metabo-
879
COSTS OF FIGHTING
lism for a spider of that body temperature
and weight and by the 4.7 x correction.
Estimates of energy expended in A. aperta
territorial disputes suggest that the game
theoretic predictions concerning the significance of this cost are indeed correct.
The energy expended in territorial disputes is correlated with the foraging value
of the disputed territory (Riechert, 1979).
In one population, for instance, energy
expenditure is logarithmically correlated
with the rates of encounter with prey and
thermal environments associated with different microhabitats. In this particular case,
the availability of sites providing both high
levels of prey and favorable thermal environments for spider foraging is very low
(<3% of the available cover: Riechert,
1981). Here, energy expenditure in contests over sites providing adequate food is
significantly higher than that exhibited in
contests by individuals of a population
occupying more favorable local conditions
where adequate sites are present in abundant supply (Riechert, 1979, 1986). In
other work, Riechert (19786) suggests that
ownership-intruder status and weight biases
show significant correlations with estimated energy expenditure in disputes.
Equally weighted opponents and smaller
owners of high quality territories tend to
expend more energy in territorial disputes
than smaller intruders and larger owners
for which contest outcome is clearly defined
in favor of the larger resident spider.
Energy expenditures in different body
weight and site value contexts for the territorial disputes of a desert grassland population of this spider are shown in Figure
1. Note that the major difference is with
the value of the disputed resource and that
relative weight is only important when the
disputes involve territories of high quality,
i.e., those which ensure enough food per
day to permit survival to reproduction (22
mg dry weight/day of insect prey [Riechert, 1978a]).
In modeling the territorial disputes of
this spider as an evolutionary game, however, Maynard Smith (1973) and Hammerstein and Riechert (1988) find that
energy expenditure in the contests contributes little to the determination of payoffs in fitness associated with winning or
.0015
in
o
u
o
o
^
.0010
.0005
WEIGHT
BIAS;
SITE
QUALITY:
OWNER
EQUAL INTRUDER
LOW
OWNER
EQUAL INTRUDER
HIGH
FIG. 1. Whisker plot of estimated contest cost for
different territorial dispute contexts of the spider
Agelenopsis aperta in a desert grassland study area in
south-central New Mexico. Horizontal bars equal
medians. Surrounding boxes represent upper and
lower quartiles, respectively and vertical lines represent ranges. See text for methodology underlying
energy estimates.
losing particular disputes. Because A. aperta
territorial disputes often lead to escalated
behavior in which the probability of injury
is as high as 36%, any energetic costs
accrued in the course of the contests are
dwarfed when compared to the consequences to fitness of being injured or suffering mortality. In Table 1,1 provide estimates of the payoff decrements to milligram
egg production received by spiders engaged
in single territorial disputes in either a desert grassland habitat where adequate territories are present in limited supply or a
desert riparian habitat along a stream
where suitable sites are abundant. These
results are derived from the evolutionary
game theoretic analyses of Hammerstein
and Riechert (1988). The energetic cost of
disputes are included here under the general category of display. Display costs are
low relative to those associated with injury.
Of course, if an individual suffers mortality
during the course of a dispute, it loses its
entire expected input to reproduction for
the remainder of its life. To a spider suffering the loss of a single leg, there is a
880
SUSAN E. RIECHERT
TABLE 1. Payoff components: Expected egg-mass increments and decrements respectively in mg wet weight ofevents
associated with territorial disputes of two populations of the
funnel spider, Agelenopsis aperta.*
Population
Desert
grassland
(New Mexico)
Desert
riparian
(Arizona)
Payoff component
Value of winning average
1.6
quality site
16.7
6.2
Cost of leg loss
14.2
Cost of lethal injury
93.7
84.1
0.1
3.0
Cost of display
* Display costs consist of all costs not associated with
injury (e.g., risk of predation, energy expenditure, loss
in food intake).
subsequent 10% decrease in its prey capture success rate and a 25% decrease in its
probability of winning territorial disputes.
These factors represent major subsequent
energy losses to the spiders which far outweigh the immediate energetic costs of
fighting.
When display costs are subdivided into
contributing factors (energy expenditure,
loss in feeding and possibility of predation),
it is apparent that the actual energy
expended in a dispute represents but a small
fraction of total display costs (Table 2).
Several orders of magnitude higher is the
expected loss in food intake resulting from
the time spiders are engaged in fighting
during periods when they would otherwise
be attacking prey. The prominant contributing factor to display costs in the riparian
population is potential predation during the
course of the disputes in which the opponents are exposed on the web to visual
predators. Predation by birds is an important limiting factor to the riparian population (Riechert, 1986) and because vigilance by the spiders towards potential
predators is only !40th of that exhibited
during foraging activity itself, potential
predation during contests represents a
major cost to the projected fitness of individuals of this population.
I conclude from this analysis of the A.
aperta territorial system that actual energy
expenditure during contests does not represent a significant cost and can, in fact, be
eliminated from payoff calculations with
negligible loss of resolution. The question
remains as to whether the noted variation
in energy expenditure between contest
contexts reflects selection on energy
expenditure itself, or whether energy
expenditure is merely correlated with some
other phenomenon upon which selection
is operating. Likely possibilities include the
length of time individuals are willing to
spend exposed to unfavorable thermal conditions (e.g., higher temperatures which
may lead to water loss) or to potential predation. Energy expenditure also is directly
correlated with intensity of fighting which
is predicted to be context dependent in this
species.
CONTEST COSTS IN THE VERTEBRATES
Ectotherms
Spiders are small ectothermic animals
with recognized low basal metabolic rates
(Miyashita, 1969; Anderson, 1974; Tanaka
and Ito, 1982). It is possible that energy
expenditure may be more important in the
homeotherms. Taigen and Wells (1985)
found chorusing by the anuran amphibian,
Hyla versicolor, to be at the high end of the
energetic cost range for ectothermic vertebrate activities (a 22-fold difference
between resting metabolism and energy
expenditure during calling) (see also Ryan,
1985). In a subsequent paper (Wells and
Taigen, 1986) they identified a significant
effect of male competitive context on the
duration and rate of calling. These context
effects, however, did not extend to measurable differences in energy expenditure.
The primary function of chorusing in the
amphibia is in the attraction of mates. The
actual contest aspect of such calls comes
with increased male competition for
females at higher chorusing densities. Malemale competition in the form of chorusing
does not add significant cost to that already
expended in attracting females.
In a classic paper on the adjustment of
fighting behavior to resource value, Rand
and Rand (1976) suggest that energetic
costs of disputes between female iguanas
over nesting sites are adjusted to the value
of the disputed site—how close the process
of digging a nesting cavity is to being completed. Their conclusion is based on observations of contests in which the behavior
patterns are ordered on the basis of intensity. The authors assume that behaviors
COSTS OF FIGHTING
TABLE 2.
881
Breakdown of costs of display in territorial disputes q^Agelenopsis aperta.*
Decrement in wet weight milligrams egg production
Desert riparian
Desert grassland
Mean
Energy expended
Loss in food intake
Loss to predation
Mean
SE
5
4 .8 x 10
"
0.10
0
2.6 x 10"
0.001
7
2..0 x
10-6
0.06
2.4
SE
2
.5 x lO"8
0.001
0.72
Desert grassland, n = 267 contests; desert riparian, n = 153 contests.
completed with greater speed are more
costly energetically than slowly performed
behaviors. While this may be true on a per
second basis, actual energy expended in
the different behavior patterns may be
equivalent, since the slow behaviors take
longer to perform. Further, there is the
problem of risk of predation which was not
included in the study. Their conclusion
does not appear to be justified for the same
reason I was in error in my conclusions
presented in 1979.
Endotherms
Perhaps the best example of the energetic costs of competitive interactions in
the higher vertebrates comes from study
of the nectar feeding birds. The hummingbirds and sunbirds are known to vary
their territorial defense of a nectar source,
exhibiting vocal and visual displays from
perch sites rather than chases when nectar
supply is low (Ewald and Carpenter, 1978;
Frost and Frost, 1980). A similar shift in
the character of territorial defense is
observed when the time since last feeding
is considered: the longer the period the
greater the frequency of chases (Ewald and
Orians, 1983). Chases are approximately
100 times more costly energetically than
displays to the hummingbirds measured,
though these differences may be minimal
relative to the effects of ambient temperature on perching costs (Ewald and Carpenter, 1978; Ewald and Rohwer, 1980).
Ewald and Rohwer (1980), however, do
report that chasers fed more at nectories
to make up for depleted resources. Hummingbird chases then may represent an
example of energetically costly behavior.
The results presented here for the nectar feeders are confounded by the fact that
along with increases in nectar supply, there
is an increase in intruder pressure. Chases
in these cases might be necessitated by the
greater frequency of interaction and need
to quickly settle one dispute so that other
encroachers on the territory can be
addressed. Adjustment of energy expenditure to the value of the resource, then,
may be only correlated with the factor
selection is actually operating on. No mention was made of the potential for injury
in the chases and fights that occur in the
escalated territorial fighting of these
species, so comparison of the two costs can
not be made at this time. The system is an
important one to the topic under consideration here and hopefully further work
will be completed that addresses these
questions.
Harem defense in some herd animals is
another potential example of significant
energetic costs to endotherms. The two
best examples are from Berger's (1986)
treatise on wild horses and the review of
Clutton-Brock et al. (1982) on their studies
of red deer. Berger feels that energy
expenditure in harem maintenance is
costly, because it increases daily energy
needs by as much as 26%. Clutton-Brock
et al. (1979, 1982) report that fights
between male deer over harems are
"exhausting," something that leads to quick
challenges by additional rivals. However,
frequent injury and even mortality are
associated with the fights in both systems.
Exposure to predation, disease and subsequent losses of mating opportunities are
suggested outcomes of injury. The relationship between energy expenditure and
these potential losses in fitness has yet to
be quantified for either system.
Exceptional Energetic Costs
Examples of high energy expenditures
in contest behavior do exist. Competitive
interactions among some corals (Richard-
882
SUSAN E. RIECHERT
son etai, 1979; Chornesky, 1983; Hidaka
and Yamazato, 1984) and sea anemones
(Francis, 1976; Watson and Mariscal, 1983)
often involve induced morphogenesis of
specialized structures that possess numerous nematocysts or stinging cells. Specialized tentacles and marginal structures
called acrorhagi are directed toward
neighboring individuals, causing the rapid
necrosis of these "opponents' " tissues.
Although no estimates are available for the
energetic costs of producing these structures, they can be significantly longer or
have a much higher density of nematocysts, than do feeding tentacles.
Another system in which considerable
energy may be invested in contests is intercolony disputes of the social insects. In the
ants for instance, territorial disputes
between ant colonies can lead to the loss
of thousands of individuals. Driesen et al.
(1984), report per diem losses of as many
as 8,500 red wood ants per day in territorial disputes between neighboring colonies. Larger colonies of ants are known to
raid and kill off the workers of smaller colonies of the same species (Wilson, 1975)
and similar tatics are used in between
species agonistic interactions in ants (Way,
1953; Brown, 1959). Major worker caste
kills in between-colony territorial interactions have also been described for termites
(Darlington, 1982; Levings and Adams,
1984) and stingless bees (Johnson and
Hubbell, 1975). Again, no actual estimates
have been made for the energetic costs to
a reproductive of losing this many sterile
workers, but these losses represent major
proportions of the worker force. I expect
the cost is considerable.
CONCLUDING REMARKS
to substantiate or repudiate this conclusion.
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