Behavioral Ecology VoL 8 No. 1: 28-36
The energetic costs of fighting in the house
cricket, Acheta domesticus L.
Mace A. Hack
Department of Biology, University of California San Diego, La Jolla, CA, USA
The cost of performing an agonistic behavior, or tactic, will have consequences for an individual's rate of cost accrual, the
tactic's evolutionary stability if used as an assessment signal, and its pattern of use in the behavioral choreography of a contest.
Few studies have attempted to quantify the costs of fighting, particularly with regard to energy expenditure. Flow-through
respirometry revealed that house cricket males can expend energy at relatively high rates when fighting (more than eight times
resting levels) depending on the particular tactic performed. Acoustic signalling (striduladon) constituted the least costly of
seven measured agonistic tactics, while wrestling with an opponent, the most energetically costly tactic, consumed oxygen at a
net rate more than 40 times that of striduladon. Low-cost tactics are used by opponents more frequently than more costly
tactics, providing evidence for an escalating tactical convention based on energetic costs. The moderate energetic cost of some
tactics suggests they may function as reliable signals in the assessment of wrestling ability. Net energy costs per contest increased
linearly with contest duration for both contest winners and losers, but winners tended to expend more energy per contest than
losers. The likely fitness effects of energy expended while fighting are discussed. The results of this study indicate that energy
expenditure is an important cost shaping contest strategies in Achtta dtmtstxau. Key words: aggression, assessment, energy,
Gryilidae, respirometry, signal honesty. [Bthav Eeol 8:28-36 (1997)]
G
ame theoretical approaches toward understanding the
evolution of behavior rely fundamentally on describing
the fitness costs and benefits of alternative behavioral strategies. In the context of animal conflict, numerous empirical
studies have demonstrated changes in fighting behavior with
the manipulation of a resource's fitness value (reviewed by
Enquist and Leimar, 1987), confirming the predicted influence of benefits on contest strategy (Enquist and Leimar,
1987; Hammerstein and Parker, 1982; Maynard Smith and
Parker, 1976). Contest models also predict that the costs of
fighting should determine an individual's strategy (Enquist
and Leimar, 1983; Hammerstein and Parker, 1982; Maynard
Smith and Parker, 1976), yet very few studies have measured
the costs of fighting beyond quantifying contest duration (but
see Austad, 1983; Hammerstein and Riechert, 1988; Riechert,
1988). Time spent fighting is measured easily and may represent a true cost to fitness (Parker and Thompson, 1980;
Riechert, 1988), but its insensitivity to variance in cost among
alternative behaviors reduces its accuracy as a cost measure
for species that perform multiple agonistic tactics. For these
species, determining relative cost accrual rates to test contest
models requires the measurement of fighting costs in currencies likely to vary with tactic choice.
Quantifying the specific costs associated with alternative agonistic tactics is also germane to the description of tactic function. Both theory (Enquist and Leimar, 1983; Enquist et al.,
1990; Parker and Rubenstein, 1981) and empirical evidence
(Enquist and Jakobsson, 1986; Keeley and Grant, 1993; Leimar et al., 1991) suggest that many contests are resolved
through the repeated, mutual assessment of relative "resource-holding power" (RHP) (sensu: Parker, 1974), rather
than as a result of "all-out combat" to inflict costs (sensu:
Maynard Smith and Harper, 1988). Thus, some agonistic tac-
M. A. Hack is now at the Department of Environmental and Evolutionary Biology, Nicholson Bldg., University of Liverpool, P.O. Box
147, Liverpool L69 SBX, UK.
Received 10 April 1995; revised 15 August 1995; accepted 29 November 1995.
1045-2249/97/J5.00 O 1997 International Society for Behavioral Ecology
tics function as signals of RHP (Qutton-Brock and Albon,
1979; Riechert, 1978; Rubenstein and Hack, 1992), and possibly motivation (Poole, 1989; Turner and Huntingford,
1986). However, conflicts of interest between the sender and
receiver of a signal, inherent to contests, will favor manipulative or unreliable signaling (Krebs and Dawkins, 1984). Unless an aspect of the signal and RHP are obligately correlated
(e.g., call fundamental frequency and body size: Davies and
Halliday, 1978), reliable signals of RHP will remain evoludonarily stable only if the signaler incurs a cost (Enquist, 1985;
Grafen, 1990; Maynard Smith, 1991; Zahavi, 1977a; Zahavi,
1977b). Description of the costs associated with a signal can
indicate both a tactic's relative reliability as a signal and the
type of information it communicates (Grafen, 1990).
Finally, a tactic's relative cost is predicted to affect its pattern of use in the choreography of acts during a single contest
(Enquist et al., 1990; Enquist et aL, 1985; Parker and Rubenstein, 1981). Observations of fighting behavior from a variety of species concur that there are general rules determining the temporal sequence and relative frequencies of tactics
during contests (Qutton-Brock and Albon, 1979; Enquist et
al., 1990; Hack, 1994; Keeley and Grant, 1993; Riechert, 1978;
Rubenstein and Hack, 1992), often suggesting an escalation
sequence from low cost to progressively more costly tactics.
However, a significant impediment to testing the effect of economics on these observed patterns of tactic use is the lack of
quantitative data on tactic costs.
Two cost currencies are likely to vary among alternative agonistic tactics: injury risk and energy expense. Tactics involving physical contact presumably pose a greater risk of injury
to opponents than displays, or those without contact (Berzins
and Caldwell, 1983; Clutton-Brock et aL, 1979; Riechert,
1978). Alternative tactics may be further differentiated by
their energetic costs, although direct measurements of energy
expenditure during contests are rarely made (but see Bennett
and Houck, 1983; Smith and Taylor, 1993). Variance in tactic
"intensity," used to estimate fighting costs and test contest.
models (Catterall, 1989; Riechert, 1988), implies variance in
energetic cost among tactics, yet the subjectively determined
intensity ranks of tactics may have little basis in reality. Nevertheless, some indirect evidence indicates the importance of
Hack • Energetics of fighting
energy costs to contest behavior, induding increased feeding
rates as a result of more intense resource defense (Ewald and
Rohwer, 1980), lower fat reserves among die losers of contests
(Marden and Waage, 1990), and die apparent physical exhaustion of contest losers (Gutton-Brock et aL, 1979; Rand
and Rand, 1976; Wells, 1988). dearly, more direct measurements of energy expenditure while fighting are needed.
In diii study, I quantify die energetic costs of fighting for
male house crickets {Achtta donusticus) by continuously monitoring their instantaneous rates of oxygen consumption. Field
crickets, or members of die family Gryilidae, are particularly
well-suited to die study of contest energetics. Male crickets do
not have specialized weapons, nor is body size always a strong
determinant of fighting success (Burk, 1983; Hack, 1994). Indeed, dominance relationships are temporally unstable in
most spedes diat have been studied (Alexander, 1961; Burk,
1983; Dixon and Cade, 1986; Hack, 1994; Sandford, 1987;
Simmons, 1986), implying die importance of transient characters, such as physical condition, in die determination of
fighting success. In addition, an individual's behavior varies
considerably during a single contest (Adamo and Hoy, 1995;
Alexander, 1961; Hack, 1994; Simmons, 1986), from low-intensity display to highly escalated, intense physical contact, so
diat fighting costs in addition to time are likely to influence
contest behavior. Injuries to die antennae, cerri, and legs resulting from fighting have been reported anecdotalh/ for some
spedes (Alexander, 1961; Burk, 1979; Sandford, 1987; Simmons, 1986), but their rarity suggests diat energy costs may
have a more general and important influence on contest strategy in gryUids.
29
tion were all performed with custom software for die Macintosh. AD reported rates of oxygen consumption have been
standardized to STPD (0°C, 760 torr, dry) conditions.
More detailed descriptions of die experimental apparatus
and die methodology of data collection are given in Hack
(1994).
Sturco contests
To elicit die full repertoire of agonistic behaviors observed in
natural interactions, I simultaneously placed two males into
die same metabolic chamber. Some tactics can only be observed and measured under these conditions since they require die coordinated actions of both opponents. To control
the timing of male interaction, I installed a screen baffle midway down die metabolic chamber's length. When dosed, the
baffle prevented a cricket from moving between die halves of
die chamber without impeding air flow. Separating males
within die chamber allowed diem to setde to a stable resting
rate of O, consumption before die initiation of fighting.
I conducted a total of nine two-male trials, ranging in duration from 20 to 50 min. No male was used in more than
one trial. Male body mass ranged from 0.293 to 0.481 g (i ±
SD = 0.372 ± 0.057) and adult age (i.e., days since edosion)
varied from 14 to SO days (x ± SD •= 19.3 ± 6.2). Opponents
were matched for .mass to within ±6% and were identical in
adult age (except for one pair differing by three days). Mass
and age matching minimized variance in resting VO, between
opponents and simplified the data analysis.
Behavioral analysis
METHODS
FTow-tfazougb respiroosefery
The male crickets used in diis study came from a laboratory
population maintained at 23-26°C on a 12.5:11.5 light:dark
schedule. Temperature and light conditions during die collection of respirometry data were identical to diose under
which die crickets were raised.
I measured die oxygen consumption rates of crickets in an
open, flow-dirough system using a dual-sensor oxygen analyzer (S-SA Applied Electrochemistry Inc.). Differences in die
fractional concentration of oxygen between an empty control
chamber and an identical chamber containing die test animals were sampled three times per second, at a resolution of
0.001% oxygen, and averaged to produce a single datum per
second. Control and metabolic chambers consisted of hard
plastic tubes, each with a jotal volume of 35 mL Chamber
temperature varied less than 0.1 °C during any single trial (Bailey Instruments BAT-12 digital thermocouple thermometer),
indicating diat die test animals' own activity did not significandy raise chamber air temperature. The range in chamber
temperature across die entire study did not exceed 0.6°C
(24.5-25.rC). An R-2 pump and flow-control unit (Applied
Electrochemistry Inc.) maintained a fixed flow rate of 40
ml/min through the experimental chambers. I independently
verified die flow rate with a Gilmont flow meter before and
after each experimental triaL
I corrected all samples of excurrent O, differences to reflect instantaneous rates of oxygen consumption (VCv Bartholomew et al., 1981). The use of tubular chambers and die
relatively small air volume between chamber and sensor
(<10%.of die chamber's volume) ensured appropriate mixing characteristics for applying die instantaneous correction
(Frappell et aL, 1989). Baseline correction for sensor drift,
curve smoothing to remove random noise in sensor readings,
calculation of effective volume, and die instantaneous correc-
I filmed male behavior during each trial using a Canon VC-30
color video recorder. A Hoya +1 diopter lens provided doserange magnification and a small condenser microphone (Realistic Back Electret), placed inside die chamber, recorded
male sounds. Slow-motion analysis of die videotapes allowed
me to quantify tile frequencies of 13 agonistic tactics used by
males (Appendix). Two additional behaviors, courtship display (Alexander, 1962) and walking, were also quantified. I
denned a "step" as die basic unit of walking and measured
its frequency by arbitrarily choosing a hind leg and counting
only its cydes of movement
Step, kick, head butt, head charge, fore leg punch, and
mandible lunge occurred in discrete acts; one act of these
behaviors counted as a single repetition in die analysis. Stridulation, shake, antennae lash, and stridulation lash each consisted of a small number of acts too rapid to distinguish individually, yet all three tactics generally occurred in discrete
bouts of S 1 s; one bout of these behaviors was treated as a
single repetition. Cerd raise, courtship display, mandible
flare, mandible spar, and wresde constitute behavioral states;
one second of these behaviors counted as a single repetition.
Video and VOj records were temporally aligned by correcting for die time an air sample required to travel from the
metabolic chamber to die sensor. I determined this time lag
using diree independent methods: (1) die time elapsed between injection at die influx port of an Cydefirient sample
until peak deflection at die sensor; (2) division of die total
volume of die metabolic chamber and its downstream plumbing to die sensor by die known flow rate; (3) inspection of
die video record to find short (<3 s), isolated bursts of activity
and matching of these actions to die oxygen consumption
record. Estimates of die lag time from all three methods
agreed to within ± 5 s and was determined to be 30 s.
Cricket fights tended to occur in short bursts of combat
followed by longer periods of relative inactivity. To account
for any oxygen debt incurred by opponents, and because of
Behavioral Ecology Vol. 8 No. 1
so
the several-second error in temporally matching the activity
and VO, records, I divided complete trials into contiguous
time intervals of 60 s for the multiple regression analysis.
Regression smlysis
I used multiple regression to estimate the independent contribution of each tactic's performance to the total oxygen consumed by fighting males. This involved simultaneously regressing the oxygen consumed by both males combined, per sample interval, against die cumulative frequency of each tactic
performed during the same interval. (I calculated combined
oxygen consumption per sample interval by integrating the
observed VO, versus time plot) Since data were pooled across
pairs (see below), I first removed variance among pairs in
resting oxygen consumption rates by subtracting the oxygen
consumed at rest by both males from their total combined
oxygen consumption. This resulted in a measure of net combined oxygen consumption, or that attributable solely to the
activity of one or both males. Multiple regression through the
origin of net consumption values resulted in a set of slopes
(partial regression coefficients), each representing the incremental or net oxygen consumed per single repetition of a
particular tactic By assuming males of equal mass and age
consumed equivalent amounts of oxygen when performing
the same behavior, I was able to sum the frequency of each
behavior across opponents, and thereby reduce by a factor of
two the number of independent variables entered into the
model. This essentially attributed all net oxygen consumption
to a single hypothetical male and was valid provided intermale
variance in VO, per tactic did not exceed variance in mean
VO, among alternative tactics.
I calculated mass-specific estimates of tactic cost by dividing
net combined oxygen consumption by the mean mass of opponents. (Mean mass closely approximated the actual mass of
each male since opponents were weight-matched to within
±6%.) I used mean mass, rather than the sum of opponent
masses, because males did not perform the same behaviors at
equal frequencies per sample interval; their contributions to
net combined oxygen consumption were not equivalent. If
one male rested during the sample interval while the other
performed several agonistic acts, the division of net combined
oxygen consumption by their summed mass would underestimate the oxygen consumed per act repetition, per male. In
this case, the net oxygen consumed would be attributed to a
male twice the actual size of the actor, and the energy cost
per unit mass would be half the actual cost to the actor.
Ideally, intrapair and interpair variance in the energy cost
per tactic could be distinguished with a separate multiple regression analysis for each pair. However, the division of trials
into 60-s sampling intervals meant that 60 measurements of
VO, were summed in this interval to produce a single datum
for the multiple regression. This substantially reduced intrapair variance in tactic frequency for some tactics. For example,
wresding typically occurred in several short bouts (3-7 s) during a contest and did not recur in successive fights between
the same males; all measurements of VO, while wrestling often occurred within 1—2 sample intervals. To estimate the energetic cost of tactics like wrestling, I pooled multiple samples
from each pair across pairs.
Pooling data, or treating samples from the same pair similarly to samples from separate pairs, introduces concerns regarding psoudsMplieaiMa (Hurlbort, 1984). In this specific
analysis, pseudoreplication can lead to the inaccurate estimation of partial regression coefficients and an increased
probability of falsely rejecting the null hypothesis (b, ™ 0).
However, several factors reduce the likelihood of both problems (Leger and Didrichsons, 1994): (1) most tactics (exclud-
ing cerci raise and courtship) were performed by at least six
of nine pairs; (2) pairs contributed similar numbers of samples (only one escalated fight); (3) intrapair variance in energy expenditure per tactic probably equaled that between
individuals of different pairs; and (4) a conservative a level
in most cases offset the increased degrees of freedom resulting from pooling.
Duly oxygen consumption
Measurements of oxygen consumption per day by isolated
males under semi-natural conditions can be used to gauge the
relative energetic cost of fighting. Single males (n • 14)
placed in sealed containers (950 ml or 1760 ml), containing
sand, food, water, and a burrow, performed many natural behaviors, including digging, moving around the container to
forage, and advertising acoustically for mates. After 24 hours
at 24-25°C on a 13:11 lighcdark cycle, I drew a 60 ml sample
from each chamber and pushed it through a single channel
of the oxygen sensor (flow rate of 35 ml/min via a syringe
pump from IXT.C. life Science). I then used the change in
oxygen concentration to calculate a male's total daily oxygen
consumption (Bartholomew et al., 1981). Male body mass and
adult age in this experiment averaged 0.272 g (SD «• 0.058,
range =• 0.169-0.400) and 18.2 days (SD - 8.8, range •= 2 30), respectively. I conducted a second set of trials with tile
same males, each 48 hours in duration, to estimate daily variability in total oxygen consumption.
RESULTS
of oxygen consumption during staged fights
All trials contained one or more peaks of oxygen consumption
coinciding with periods of intense combat (Figure 1). Less
intense periods of agonistic interaction between and following
activity peaks consisted of stridulation, shaking, and chasing
around the chamber. The mean and maximum combined
VO, for bodi males while fighting averaged 2.280 ml-g-'-h"1
(SD - 0.847, range - 1.224-3.786) and 4.081 ml-g-'-h"1 (SD
- 1.793, range = 1.438-7.134), respectively. Combined VO,
at rest, measured for each pair during tile period of complete
inactivity preceding the initiation of fighting, averaged 0.841
m l g - ' h - ' (SD - 0.080; range •» 0.722-0.950). Mean combined VO, during fights ranged from 1.3 to 4.8 times (mean
« 2.8) combined VO, at rest, while maximum combined VO,
ranged as high as 8.1 times (mean = 4.9) resting combined
o
Oxygen consumed per tactic
A total of 173 60-« sample intervals from nine male pairs were
analyzed. Sample intervals containing qualitatively variable
and difficult to quantify behaviors, such as antenna! grooming
and digging, were excluded from the analysis. Of tile 15 behaviors recorded, several appeared similar enough in form for
their frequencies to be combined. This reduced the independent variable set and further simplified the regression analysis.
The rapid steps involved in the tactics head charge, mandible
hinge, and mandible spar were added to the frequency of
steps in other contexts to create a single measure for locomotion during fights termed "charge." Each second of mandible sparring added to the duration of mandible flaring accounted for the mandible and head movements involved in
this tactic Stridulation lash was decomposed into stridulate
and antennae lash.
The full regression model of 11 behaviors explained 90.4%
of the variance in the net oxygen consumed by fighting crick-
31
Hack • Energetics of fighting
Jl
/T
K I
^tM
gM^
0
100
too
S00
400
100
ISO
700
100
900
WB
Figure I
Net oxygen consumed per repetition (± SE) for seven tactics in the
agonistic lepenoue of A. dowmticus. Energy costs per tactic are die
partial regression coefficient! resulting from die multiple regression
of net oxygen consumption against the cumulative frequencies of
each tactic SD(stridulate), ( - 2.16, p < .05; MF(mandible flare), t
- %&1, p < .01; SKXshake), t - 6.15, p < .0001; CH(charge), t 13.76, p < .0001; AL(antennae lash), t - 5.60, p < .0001; CR(cerd
raise), t - 2.05. p < .05; WR(wresue), t - 5.60, p < .0001. Factorial
scopes were calculated by assuming one tactic repetition per s and a
restingVo, of 0.117 til O,r'»"'•
/
aoo
400
IOO
too
Trial Duration (•)
700
100
Figure 1
Continuous plots with time of instantaneous oxygen consumption
rates (VO,), combined for both opponents, during staged agonistic
interactions. The complete trials for two male pain are shown as
examples. Each datum represents the mean of three instantaneous
measurements sampled over a l-« interval. The bar underscoring
the trace of data points depicts male activity: open — inactive,
hatched - pre- or peat-fight stridulation, shaking and/or chasing,
jolid - fighting. Maximum combined VO, dearly peaks during
periods of intense combat, then quickly diminishes to
approximately resting levels.
ets (F l l i a , =» 148.34, p < .0001). I reduced this model through
a forward stepwise procedure to afinalmodel (adj. R* = 0.90,
Fg.i<B " 203.23, p < .0001) composed of seven agonistic tactics
with estimated slopes, or partial regression coefficients, significantly different from zero (Figure 2). CoUinearity among
tactic frequencies was generally low (r < .5 in all but three
comparisons). One nonagonistic behavior, courtship display,
also remained in 1the final model ( b , ± S E ° 0.12 ± 0.04 uJ
O^g^-repetition" , t •• 3.09, p < .01); the implications of this
result will be addressed in a separate study (Hack, in preparation). The net oxygen consumed per repetition for three
tactics—head butt, fore leg punch, and kick—could not be
accurately resolved with this dataset since the frequencies of
these tactics did not explain statistically significant proportions of variance in net oxygen consumption.
The net oxygen consumption of fighting male crickets differed more than 40-fold between a single repetition of stridulation, the least energetically costly behavior, and one of
wresding, the most costly. In general, the tactical repertoire
divided into two groups with regard to energetic costs (Figure
2). One chirp of stridulation consumed less oxygen than one
second, of mandible flare (r = 2.01. d/= 165, p < .05), although both displays only increased VO, by 17% and 70%
above resting levels, respectively. In contrast, tactics involving
rapid locomotion and physical contact, such as charge, antennae lash, and wrestle, raised active VO, 6-8timesresting levels
(increases of 523-705%). Energy expenditures of this magnitude correspond well with the ma-trimnm combined VO, observed directly during fights, as expected if the multiple regression technique indeed provides accurate estimates of tactic cost. Within the group of more costly tactics, oxygen consumption per. repetition did not substantially differ, but all
had greater energetic costs than stridulate and mandible flare
(all p < .001). Two additional displays, shake and cerd raise,
fell within the group of higher cost tactics. Shake consumed
significantly more oxygen than stridulate (t •• 5.81, df= 165,
p < .001) and mandible flare (t « 4.68, df= 165, p < .001),
presumably because it involved the rapid movement of the
whole body rather than just the tegmina or mandibles and
head. Cerci raise also involved lifting die entire body, but its
estimated energetic cost did not statistically differ from those
of the other displays.
Examination of each tank's pattern of use suggests an important influence of energetic cost onfightingstrategy. Across
all 12 observed fights, opponents used tactics of high energetic cost significantly less often than tactics of lower energetic
cost (Figure 3). In addition, die proportion offightsin which
house cricket males performed a particular tactic decreased
with increasing tactic energy cost (r, = —0.811, p < 0.05, n
- 7).
Total energy expended per opponent
Measures of tactic energetic cost provide a means of calculating die cumulative net energy expended per opponent. I calculated a male's net oxygen consumption per fight by tallying
die frequencies with which it performed each tactic, multiplying by each tank's cost per repetition, and summing across
tactics. The accuracies of calculated totals are likely to be high
since die seven tactics measured account for 90% of die variance in male oxygen consumption.
Cumulative net oxygen consumption per opponent averaged 41.81 uJ-g-'-fight"1 (SD •=• 38.66, range = 1.22-160.38).
In nine of 12fightsdie winner's cumulative net consumption
exceeded die loser's, although die average magnitude of this
difference was not large (x ± SE = 11.92 ± 537, paired t =
32
Behavioral Ecology Vol. 8 No. 1
10000
100
200
300
400
S00
eoo
700
Fight Duration (•)
p
((iloxyg«n/g)
Figure 3
The relationship between the oxygen consumed per repetition of
each tactic and its cumulative frequency (log-transformed) over 12
fights (nine male pairs). More energetically costly tactics are used
lets frequently by opponents: r* - 0.667, Fl£ - 10.00, p < .05, Y «
8.06 - 4.46-X. Tactic key: SD (stridulate), MF (mandible flare), SK
(shake), CH (charge), AL (antennae lash), CR (cerd raise), WR
(wrestle).
Figure 4
Net oxygen consumption per fight as a function of fight duration,
plotted separately for winners and losers (n - 12 fights from nine
opponent pain). A linear function with time fits the energy
expenditure! per fight of winners better than those of losers:
winner's net O, - 0.193-duradon + 3.068, r* « 0.876, Fm, 70.89. p < .0001; loser's net O, - 0.126-durauon + 6.818, r* 0.665, F u o - 19.89, p < .01. Winners and losers tended to diverge
in net energy expenditure per fight as fight duration increased
(ANCOVA: status • deration fM - 3.48, p - .08).
2.14, df™ \\,p<
.06). Cumulative net consumption for both
winners and losers increased linearly with right duration (Figure 4). The net consumption of winners also tended to increase at a greater rate with fight duration [i.e., the difference
between the net consumptions of winners and losers grew
larger as fight duration increased (r* •• 0.566, F u o » 13.04,
p < .01)]. Differences in the frequencies of two tactics, stridulate and shake, largely accounted for the disparity between
the energy expenditures of winners and losers.
direct comparison. However, the accuracy of the metabolic
measurements can be evaluated by comparing oxygen consumptions measured for mechanically similar behaviors in
other orthopterans: (1) the cost per charge exceeds by only
60 to 90% estimates of the cost per stride during steady-state
walking (Full et aL, 1990; Stevens and Josephson, 1977), and
(2) the cost per pulse of agonistic stridulation falls within die
range of costs per pulse for mare-attraction stridulation (reviewed by Bailey et aL, 1993).
Daily energy expenditure
Relation of energetic cost to fitness
The total oxygen consumed per male at 24-26°C averaged
4.334 ml/day (SD •» 1.055, n - 14, range - 2J52-5.916), or,
in mass-specific units, 16.240 ml-g-'-day-' (SD = 3.808, range
- 9.350-22.326). Daily oxygen consumptions during the subsequent 48 hour period were very similar (mean difference
= 0.481 ml-g-'-day-1. paired t «=• 0.395, DF = 13, NS), suggesting the lack of variance in daily energy expenditures for
the three-day period. Using the mean daily energy expenditure of isolated males to gauge the relative energetic cost of
a single fight reveals a small impact on total daily energy budget The total energy expended by a male per fight ranged
from < 0.1 to 1.4% (x ± SD - 0.4 ± 0.37%) of mean daily
energy expenditure in the absence of fighting. In comparison,
fight duration ranged from < 0.1 to 0.8% (x ± SD = 0.3 ±
0.24%) of a day.
The influence of energy expenditure on the evolution of
fighting strategy will ultimately depend upon its consequences
for an individual's survival and reproduction. It is difficult to
determine these consequences under laboratory conditions,
but some proximate measures are available. The proportion
of an individual's daily energy budget spent fighting represents energy not spent more directly on survival or reproduction. The daily energy expenditures measured under seminatural conditions in this study cannot substitute for actual
field measurements, but if they are accurate, they are likely to
be most representative of males occupying burrows or other
calling sites. Male gryUids adopting this strategy are relatively
sedentary and primarily call to attract mates (Cade and Cade,
1992; Evans, 1983; Hissmann, 1990; Zuk et aL, 1993)—a daily
pattern of behavior similar to that observed among the experimental males.
A single escalated fight constitutes, on average, less than 1%
of the average daily energy budget measured in the absence
of fighting. However, paired male A. domesticus may fight several times before dearly determining control of a resource
(e.g., burrow, female) or establishing a consistent dominance
relationship (Hack, 1994). High interaction rates have been
reported under laboratory and field conditions for other grylbds (52 figha/h: Alexander, 1961; six escalated fighu/h: Sim-.
moss, 1986) and the generally short (few days) tenure of
males occupying burrows in field populations further suggests
frequent, intense competition (Campbell and Shipp, 1979;
Evans, 1983; Hissmann, 1990; Rost and Honegger, 1987).
DISCUSSION
Fighting constitutes an energetically costly activity for male
house crickets. Mean rates of oxygen consumption during escalated combat range as high as five times resting oxygen consumption. Several agonistic tactics raise rates of oxygen consumption SBC to eight times above resting levels, making diem
more energetically costly than odier common cricket behaviors, including walking, grooming, calling for mates, and
courtship (Hack, 1994). Since this is one of the first studies
to quantify the energetic costs of alternative agonistic tactics
for any species, few data from other studies are available for
Hack • Energetics of fighting
Thus, frequent fighting may result in a cumulative energetic
cost that constitutes a significant proportion of an individual's
daily energy budget. Daily energy expenditures on fighting
may be even greater in other gryOids, since A. domtsticus appears to be less aggressive than other field cricket species.
Energy expended while fighting must either be replenished
through foraging or cause a permanent reduction in reserves.
Foraging presumably involves increased predation risk, and
the time it requires may represent a significant loss of mating
opportunities in species with short adult life-tpans, such as
crickets and msects in generaL Depleted energy reserves reduce an individual's ability to secure a mating territory in the
damselfty Calopltryx maculata (Marden and Waage, 1990) and
to attract mates through signaling in the bushcricket Rtquma
vftHcahs (Simmons et aL, 1992). The physiological cost to a
female GryUus rubens of simply maintaining operative flight
muscles is substantial enough to reduce her fecundity by more
than 20% (Mole and Zera, 199S). A similar trade-off may apply to male gamete production in crickets, since mating success often depends on a male's ability to mate multiply and
rapidly with the same female (Sakaluk and Cade, 1983; Simmons, 1988b; Simmons, 1988c; Zuk, 1987). Assuming an average spermatophore mass in A. domtsticus of 0.9 mg of protein and a net cost of an escalated fight equal to 0.05 ml O,
for a 0.5 g cricket, the energy required to produce a single
spermatophore is equivalent to approximately 19 escalated
fights (efficiency equations from Woodring et al., 1979). For
A. domtsticus, a moderate number of fights per day potentially
incurs enough of an energetic cost to reduce a male's mating
success by limiting spermatophore production. Indeed, energetic costs imposed by gregarine parasites have similarly been
shown to reduce spermatophore production in two gryllid
species (Zuk, 1987).
Tactic coat and function
Wrestling incurs the greatest energetic cost among A. domesticus tactics and presumably also entails the greatest risk of
injury. Contests that escalate to wrestling typically end with
the loser getting thrown or flipped by its opponent and rapidly retreating. Thus, wrestling represents all-out combat and
preceding tactics potentially allow the assessment of likely success in a wresding bout. For a tactic to signal RHP or motivation reliably, it must either be obligately correlated in some
aspect of performance with a quality contributing to fighting
success (Maynard Smith, 1982; Maynard Smith and Harper,
1988) or incur a cost to produce (Enquist, 1985; Grafen, 1990;
Maynard Smith, 1991; Zahavi, 1977a; Zahavi, 1977b). Signaling costs include prior investment in costly structures (Alvarez, 1990; Petrie, 1988), a commitment to performing costly
behavior (Enquist, 1985; Enquist et aL, 1985), retaliation by
an opponent that inflicts costs if the signal is false (Meller,
1987), and the immediate performance costs focused on in
this study.
Measurements of energetic performance cost provide a basis for evaluating the communicative function of alternative
agonistic tactics in die repertoire of A domtsticus. Among tactics with negligible risks of injury, shake and antennae lash
incur relatively large energetic performance costs per repetition and may reliably signal a male's ability to incur die greater energetic cost of all-out combat (Grafen, 1990; Zahavi,
1977a; Zahavi, 1977b). Bodi tactics are frequendy performed
before escalation to wresding and in die intervals between
bouts of wresding. In contrast, stridulate and mandible flare
expend relatively litde energy and consequently should be evolutionarily unstable as signals of fighting ability. Nevertheless,
house cricket contests, and those of other gryUids, often do
not escalate beyond stridulation and mandible flaring (Ada-
S3
mo and Hoy, 1995; Alexander, 1961; Burk, 1983; Hack, 1994;
Simmons, 1986), suggesting that both tactics still function as
effective signals in die resolution of contests. The performance of mandible flare may commit an individual to more
cosdy subsequent behavior and thereby reliably signal motivation (Enquist, 1985); it often direcdy precedes die more
cosdy tactics of mandible spar and wrestle.
Stridulation does not consistently appear to precede costly
behavior by either opponent, although it is more often performed by die winner direcdy after its opponent's retreat in
A, domtsticus (Hack, 1994) and odier gryilida (Adamo and
Hoy, 1995; Alexander, 1961; Burk, 1983; Simmons, 1986). This
observation suggests that stridulation constitutes a conventional signal (sensu: Maynard Smith and Harper, 1988) of
fighting ability or prior fighting success. Its relatively low cost
implies a low reliability as well. However, it may be informative
in some contexts since a receiver pays litde cost to perceive it
accurately (Dawkins and Guilford, 1991); a signaler will stridulate widiout requiring reciprocally cosdy action from die receiver and, as an acoustic signal, it can be detected from a
distance widiout exposing die receiver to attack. The costs of
probing die signal's reliability dirough physical contact or
more energetically cosdy behavior may not always be justified
if resource value is low or die likelihood of success is small.
Aggressive stridulation may alternatively function as an obligate indicator of body size since acoustic parameters of die
similar advertisement call correlate with body size in GryUus
bimaculatus (Simmons, 1988a). However, such correlations
may not be general to gryllid signals (Souroukis et al., 1992).
Furthermore, it is unclear why die signaling of relative body
size should occur as frequendy after a contest has been resolved as during it, when assessment of RHP would have die
most utility.
Charge (including head charge and mandible lunge) and
cerd raise have relatively high energetic costs, but probably
do not function as signals in die process of assessment Charges often displace an opponent and resemble all-out combat.
Cerd raise is typically given by a retreating individual and
appears to protect these sensory organs from charges and potential damage. It may also signal retreat, or submission, and
discourage further attack from die contest winner.
Do energy costs tnflWmT tactic use?
The lower frequency of more energetically cosdy tactics implies an ^-glaring tactical convention (Enquist et aL, 1990;
Parker and Rubenstein, 1981) whereby opponents begin contests with relatively low-cost tactics and escalate to progressively more cosdy tactics as die contest proceeds. If low-cost contests are more common than high-cost contests, as predicted
if opponents are usually asymmetric in fighting ability or motivation (Enquist and Leimar, 1983; Hammerstein and Parker,
1982; Maynard Smith and Parker, 1976), tactics used to begin
contests will be much more frequendy observed than more
cosdy tactics employed only during die final stages of highly
escalated contests. Indeed, tactics such as stridulate and mandible flare generally preceded more energetically cosdy tactics
during fights, although small sample sizes preclude statistical
verification of these patterns. [The influence of energy cost
on contest choreography is currently being analyzed widi a
larger dataset (Hack, in preparation)] Although escalating
tactical conventions appear general to gryflids (Adamo and
Hoy, 1995; Alexander, 1961; Simmons, 1986), and may be a
common aspect of animal contest behavior (Qutton-Brock.
and Albon, 1979; Keeley and Grant, 1993; Riechert, 1978;
Rubenstein and Hack, 1992), diis is one of die few demonstrations of an escalating convention based on actual measurements of tactic cost.
34
The argument that energetic costs influence the use of particular tactics by A. dorrusticus males is subject to an important
caveat. Collinearity among the possible types of cost incurred
during a contest can confound the dear identification of one
cost type as the primary determinant of contest strategy. In
perhaps the best study to date of contest strategy economics,
estimated energy costs of fighting for the spider Agdenopsis
aptrta closely followed theoretical predictions of total contest
fitness costs, yet die injury risks incurred during contests reduced the evolutionary importance of energy expenditures to
negligible levels (Hammentein and Riechert, 1988; Riechert,
1988). However, this conclusion is unlikely to apply to house
cricket contests since they never result in lethal or debilitating
injury (n *> >3000), whereas 36% of A. aptrta contests end
in severe injury in some contexts. House cricket contests occasionally result in minor injuries, suggesting some influence
of injury'riskon fight strategy, though not one strong enough
to account for either differences in die use of tactics unlikely
to cause injury or the observed patterns of total cost in contest! without tactics of high injury risk.
Implications of cost measures for contest theory
Contest models (Enquist and Leimar, 1983; Enquist et aL,
1990; Hammerstein and Parker, 1982; Parker and Rubenstein,
1981) make several assumptions about opponents' relative
rates of cost accrual: (1) costs accrue as linear functions of
time, or as nonlinear functions provided those for two opponents do not intersect; (2) cost accrual rate is a primary asymmetry distinguishing opponents; and (3) the poorer fighter,
as defined, accrues costs more quickly than its opponent In
diis study, the cumulative net energy expenditures of bodi
contest winners and losers fit linear functions with time, supporting the first assumption. It is more difficult to verify the
second and third assumptions. Opponents have different rates
of injury cost accrual if one is more likely to be damaged in
combat (Austad, 1983). In contrast, asymmetry in accrual
rates for energy or time costs are likely to result from opponent differences in their abilities to recover from the same
absolute energy expenditure or duration of lost opportunity
(Grafen, 1990; Parker and Thompson, 1980). Similar net energy expenditures per contest by A. domtsticus opponents suggest that good fighters start with greater energy reserves (Marden and Rollins, 1994) or perhaps forage more efficiently
(vehrencamp et al., 1989). If poor fighters accrued energy
costs more quickly, because of for example wasteful activity,
the net energy expended per fight for losers should have consistendy exceeded diat of winners, yet die opposite result was
obtained.
Activity differences presumably account for the greater net
energy expended per fight by winners relative to losers. The
artificially confined space of the metabolic chamber partially
explains this observation since it prevented die full retreat of
die loser in some cases and prolonged die winner's aggressive
behavior. However, greater activity (stridulation, shaking) by
a contest winner immediately after die retreat of its opponent
typifies die fighting behavior of gryllids (Adamo and Hoy,
1995; Alexander, 1961; Bailey and Stoddart, 1982; Burk, 1983;
Simmons, 1986) and implies a functional importance to die
greater energy expenditures of winners. Perhaps by advertising dieir recent victory, winners reinforce a dominance relationship and strengthen the ofEect of prior experience in subsequent contests widi die same opponent (Adamo and Hoy,
1995; Alexander, 1961; Burk, 1979; Hack, 1994; Simmons,
1986). As diis study demonstrates, significant energetic costs
incurred by both contest winners and losers can have important effects on die choreography of tactics by each opponent
Behavioral Ecology VoL 8 No. 1
and diereby shape their overall strategies in die resolution of
conflict
APPENDIX
Description of tactics observed in die contests of male house
crickets, categorized by die degree of physical contact each
entails (modified from Alexander, 1961).
Display (no physical contact):
Cerd raise: a high lifting of die abdomen's tip and cerci
Mandible flare: a presentation of open mandibles to an opponent
Stridulation: a chirp of sound produced by rubbing die
wings together
Shake: a quick rocking of die whole body forward and back
several times
light physical contact:
Head butt: a slow push of die head into an opponent
Fore leg punch: a batting of an opponent widi one of the
fore legs
Antennae lash: a whipping of an opponent's dorsum and
head widi both antennae, often accompanied by a rapid
fanning of die fore legs without physical contact
Stridulation lash: an antennae lash and stridulation performed simultaneously
Mandible span both males rapidly maneuver around each
other, barely apart or briefly touching, widi flared mandibles
Hard physical contact:
Head charge: a rapid ram of die head into an opponent
Kick: an outward thrust of die hind leg, capable of throwing
an opponent
Mandible lunge: a head charge widi flared mandibles, resulting in a bite
Wrestle: a grasping and locking together of mandibles with
an opponent, followed by a twisting of die head in an
attempt to overturn or throw it
I would like to thank S. Vehrencamp for the equipment and advice
the provided in collecting the retpirometrv data. M. ChappeU'i custom software and respirometry expertise were also critical to the success of this study. J. Bradbury, J. Kohn, K. Williams, S. Vehrencamp,
M. Chappell, L. Simmonj, W. Cade, and an anonymous reviewer provided many helpful comments in the preparation of diis manuscript.
This research was partially funded by a Predoctoral Fellowship from
the National Science Foundation.
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