Physiology & Behavior 71 (2000) 335 ± 341
Post-fight levels of plasma lactate and corticosterone in male copperheads,
Agkistrodon contortrix (Serpentes, Viperidae): differences between
winners and losers
Gordon W. Schuett*, Matthew S. Grober
Department of Life Sciences, Arizona State University, P.O. Box 37100, Phoenix, AZ 85069-7100, USA
Received 2 February 2000; received in revised form 16 May 2000; accepted 7 July 2000
Abstract
During the mating seasons (late summer and spring), male copperheads (Agkistrodon contortrix; Serpentes, Viperidae) engage in
aggressive physical interactions for priority of access to females. These fights generally involve two individuals and are characterized by
prominent vertical displays, a high degree of physical contact, and the absence of biting. Ritualized aggression does not occur in females.
Although intrasexual aggression in conspecifics has obvious energetic costs (e.g., lactate accumulation) that can affect subsequent behavior,
few studies have addressed these costs in reptiles, and no studies have examined snakes. Moreover, recent studies suggest
psychoneuroendocrine (catecholamines, glucocorticoids) regulation of metabolism during and following aggressive episodes. There were
three main questions addressed in this study. Do winners and losers of staged, pair-wise encounters show differences in post-fight (60-min)
levels of plasma lactate and corticosterone (CORT)? Are levels of plasma lactate correlated with levels of plasma CORT? Is fight duration
correlated with levels of plasma lactate and CORT? Two different control groups (cage and arena) were used. Body length, body mass,
duration of fighting, and season of testing were not correlated with levels of plasma lactate and CORT. At 60-min post-fight, losers had
significantly higher levels of mean plasma lactate and CORT when compared to levels in winners and controls, and there were no significant
differences between winners and controls. From our results, we suggest the following conclusions. First, elevated levels of CORT in losers,
but not winners, result from psychoneuroendocrine factors rather than simple exercise. Second, elevated levels of CORT in losers retard
metabolic recovery resulting in higher lactate levels in losers, whereas winners return to pre-fight levels within 60-min post-fight. Last, the
CORT response has a net negative effect on metabolic recovery and may be implicated in the protracted suppression of aggressive behavior in
losers. D 2000 Elsevier Science Inc. All rights reserved.
Keywords: Reptilia; Pitviper; Agonistic behavior; Metabolism; Energetics; Anaerobiosis; Glucocorticoids
1. Introduction
Males that engage in conspecific fights for priority of
access to reproductively active females and/or critical resources incur a variety of proximate costs, e.g., physical
injury and high levels of energy expenditure [10,16,19,21].
Gaining a better understanding of proximate mechanisms of
the energetics of sexual aggression is needed to elucidate the
effects of energetics and behavior on individual fitness
[2,29]. Although there is a growing literature on the ener-
* Corresponding author. Tel.: +1-602-543-6021; fax: +1-602-5436073.
E-mail address: [email protected] (G.W. Schuett).
getics of male ± male agonism in vertebrates, there is little
information on the role of steroid hormones, such as
glucocorticoids, on specific aspects of metabolism, such as
glycolysis [8,10]. Indeed, as stated by Haller (Ref. [10], p.
599), ``One of the most neglected areas in the physiology of
aggression is the energetic background to this behavior.''
Furthermore, in comparison to studies on fishes, amphibians, and mammals, few studies on the energetics of male
aggression concern reptiles (e.g., Refs. [3,7,9,19,20,32,33]).
In snakes, unlike most other vertebrates, fighting behavior is limited to males, occurs exclusively (or nearly so)
for access to mates, and is taxon-specific (i.e., it is absent
in certain lineages; Schuett et al., unpublished data).
Territoriality, which is common in lizards [11], is not
known in snakes [5]. Fighting between males in free-
0031-9384/00/$ ± see front matter D 2000 Elsevier Science Inc. All rights reserved.
PII: S 0 0 3 1 - 9 3 8 4 ( 0 0 ) 0 0 3 4 8 - 6
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G.W. Schuett, M.S. Grober / Physiology & Behavior 71 (2000) 335±341
ranging snakes can be infrequent due to a number of
factors (e.g., low male density), but it is, nonetheless,
important to individual reproductive success and, hence,
individual fitness [12,23].
Previous laboratory work on male ± male fighting in adult
copperheads, Agkistrodon contortrix (Serpentes, Viperidae)
has demonstrated that winners and losers of staged fights
show large differences in post-fight behavior [23]. Losers
exhibit stress-induced inhibition of aggression for up to 7
days post-fight [23] and loss of courtship behavior for 24-h
or greater [24]. Moreover, losers show significantly higher
levels of plasma corticosterone (CORT) compared to winners and controls at 60-min post-fight [25]. Although it is
clear from previous studies that exercise is associated with
increases in both lactate [8] and CORT [8,11] in vertebrates,
it is also plausible that psychoneuroendocrine responses to
social interactions (e.g., male agonism) can drive similar
changes in lactate and CORT levels [33]. In step with these
ideas, we had three goals in this study. First, to determine
whether winners and losers of staged, pair-wise encounters
in male copperheads show differences in post-fight (60-min)
levels of plasma lactate. Second, to compare levels of
plasma lactate to levels of plasma CORT at 60-min postfight. And last, to determine the relationship of fight duration to levels of plasma lactate and CORT.
2. Materials and methods
2.1. Animals
Adult male and female southern copperheads (A.
contortrix) from areas near Tyler, TX (Smith County)
were used. All subjects were housed in plastic boxes
(36.5 23.0 13.5 cm) with heat tape (250 W) situated
beneath the front end. Pre-killed laboratory rodents were
offered for food about every 10 days during their active
season (mid-February to mid-November), and water was
available ad libitum year round. Further details of their
care in the laboratory are published [23]. The Animal
Care and Use Committee, University of Wyoming,
Laramie, WY, approved all protocols for this study.
2.2. Agonistic trials
Procedures for the agonistic trials have been described
[23]. Briefly, all trials were performed in a large, plyboard
arena (180 60 62 cm) with a glass front. Agonistic
trials were initiated by placing a female in the center of the
arena, immediately followed by placing two males at the
opposite ends. The male subjects were randomly chosen
from specific snout-vent length (SVL) classes. SVL of
males (N = 44) used in the trials ranged from 52.0 to 76.5
cm (control group: mean = 63.7, SEM = 1.34 cm; loser
group: mean = 64.8, SEM = 1.37 cm; winner group:
mean = 68.1, SEM = 2.2 cm; no significant difference in
mean SVL between the three groups, F2,41 = 1.82, P = 0.17).
Body mass (g) ranged from 101.5 to 343.0 g (control group:
mean = 180.3, SEM = 8.31 g; loser group: mean = 190.0,
SEM = 11.79 g; winner group: mean = 221.86, SEM =22.70
g; no significant difference in mean body mass between the
three groups, F2,41 = 2.53, P = 0.092). Trials were aborted
after 15 ± 20 min when fighting did not occur. When
fighting occurred, the animals were not disturbed until
winners and losers were established (usually within 5 ± 15
min; see Schuett [23] for details). At 15-min post-fight, all
subjects were returned to their respective cages for 45-min
prior to collection of blood. In the arena control (control
A), trials were run by placing a single adult male and
female (but not a second male) in the arena for 15 ± 20
min. Following this period, subjects were returned to their
respective cages for 45-min prior to collecting blood
(males only). Cage controls (control B) consisted of adult
males that were tested in their cages with no females
present. The control trials were run the same day and time
period as the experimental group. All trials were conducted
from 1100 to 1300 h to control for possible diel variation
in circulating levels of CORT [30].
2.3. Collection of blood
The method of blood collection to obtain plasma for
assaying lactate and CORT has been described [25].
Briefly, all male subjects were placed under light anesthesia (halothane) prior to collection of blood by cardiocentesis (heart puncture). Although measurement of plasma
lactate does not necessarily yield accurate values of the
intensity of glycolysis in other tissues [7], destruction of
the subjects to obtain whole-body homogenates was not
feasible in this study. Nonetheless, as noted by Seymour
(Ref. [28], p. 494), ``The persistence of lactate in the blood
[of snakes] makes this metabolite a useful indication of
anaerobiosis which may have occurred hours before sampling.'' A small volume of blood (0.5 ± 1.0 ml) was
collected, transferred to a sterile, 1.5-ml centrifuge tube,
and centrifuged at 5000 rpm at 20°C for 5-min. Plasma
was collected, placed in new centrifuge tubes, and stored in
an ultralow freezer (ÿ 80°C) until analyses were performed
(1 ±12 months).
2.4. Measurement of plasma lactate and CORT
For lactate measurements, plasma was thawed at room
temperature (25°C), vortexed, and analyzed using electrochemical detection (YSI Model 23 L-lactate analyzer,
Yellow Springs Instrument, Yellow Springs, OH). The
procedures outlined in the YSI 23 L-lactate analyzer
instruction manual were followed for all assays. The YSI
lactate analyzer detects lactate concentration from 25 ml of
plasma. A 5-mM L-lactate standard was used to calibrate
the analyzer prior to performing the assays and after every
two to four samples. All samples were analyzed in
G.W. Schuett, M.S. Grober / Physiology & Behavior 71 (2000) 335±341
duplicate unless repeat values varied by more than 0.2
mM, but this rarely occurred. In a few cases, the analysis
was repeated until two duplicate samples were within 0.2
mM. In samples where plasma lactate concentrations were
higher than 15 mM, samples were diluted with distilled,
deionized water appropriately.
Measurement of CORT was by radioimmunoassay
(RIA) and follows the previously published procedures
[25]. Briefly, RIA kits were used (ImmuChem double
antibody corticosterone, RIA I; ICN Biomedicals, Inc.)
with no modifications, and validation was accomplished
using both parallelism and quantitative recovery. Parallelism was demonstrated between inhibition curves of rat
standards provided in the kits and serial dilutions of
plasma samples from adult male copperheads. Quantitative recovery of CORT added to adult male copperhead
plasma samples was 100%. Assays were run in triplicate
and analyzed in one assay run. The intra-assay coefficient
of variation was 7.56%. Mean ( ‹ SEM) values for CORT
are expressed as nanograms per milliliter (ng/ml). A
portion of the CORT values we report here have been
previously published [25].
2.5. Statistical analyses
Statistical methods follow Zar [35], and tests were
performed using STAT 2000 [4] and StatView 5.02
(SAS). Diagnostic analyses of data distribution (normality
and equality of variance) were run prior to performing
statistical tests. Seasonal differences, differences between
control groups, and differences between winners and
losers were tested with ANOVA. Associations between
plasma lactate and body size (SVL, mass) and fight
duration were tested using regression analysis. ANOVA
was used to test for the effect of fight outcome. The
association between plasma lactate and plasma CORT was
tested using regression analysis. Data are presented as the
arithmetic mean ‹ SEM unless otherwise indicated. All
statistical tests were two-tailed, and the alpha level of
significance was set at P0.05.
3. Results
Results of the measurements of plasma lactate and
CORT from 44 adult male copperheads of four different
experimental groups (winners, losers, and two control
groups) are presented in Table 1. Diagnostic analyses
showed that the assumptions of normality (skewness and
kurtosis) and equality of variance were met in all cases.
Mean plasma lactate levels (mM) of males in the two
different mating seasons (spring and late summer) were
not significantly different (tarena = ÿ 0.26, df = 9, P = 0.80;
tcage = 1.33, df = 9, P = 0.22; tloser = ÿ 0.841, df = 9, P = 0.42;
twinner = 0.301, df = 9, P = 0.77). Because arena and cage
controls were not significantly different (P = 0.74), we
337
simplified comparisons between control and experimental
groups by collapsing the data from the two controls groups
into a single control to represent the variation in plasma
lactate in snakes with no fighting experience. As noted
above, non-fighting male snakes were either solitary (cage
control) or in the presence of a single female but not a male
(arena control). Moreover, because seasonal differences in
winners and losers were not detected, we collapsed them
into single groups.
Plasma lactate levels (mM) were not correlated with
2
SVL or body mass in the control group (r SVL
= 0.090,
2
F1,20 = 1.98, P = 0.170; r mass = 0.041, F1,20 = 0.85, P = 0.37),
2
2
winners (r SVL
= 0.221, F1,9 = 2.54, P = 0.145; r mass
= 0.189,
2
F1,9 = 2.10, P = 0.19), or losers (r SVL = 0.018, F1,9 = 0.17,
2
= 0.004, F1,9 = 0.038, P = 0.850). In regresP = 0.690; r mass
sion analyses, plasma lactate levels (mM) in winners and
losers were not correlated with duration of fighting when the
fight of longest duration (171 min) was removed from the
data set as an outlier (r 2 = 0.121, F1,8 = 1.10, P = 0.325,
N = 10 staged fights). We felt removing this single, unusual
case was justified because all other fights lasted circa 30 min
or less (mean = 8.46, SEM = 11.53 min), which is more
typical of fight duration observed in captivity [23,26].
The effect of winning and losing on plasma lactate levels
was directly compared to the control group using ANOVA
because the other factors investigated (SVL, body mass,
fight duration, and season) were not found to be correlated
to plasma lactate levels. Mean plasma lactate (mM) of
winners, losers, and the control were not equivalent (Fig.
1A, F2,41 = 6.407, P = 0.004). Pair-wise comparisons revealed that losers had greater mean plasma lactate
(mean = 10.37, SEM = 0.89 mM) than winners (mean = 6.47,
SEM = 0.89 mM; t = ÿ 3.11, P = 0.003) and the control
group (mean = 6.38, SEM = 0.63 mM; t = 3.67, P = 0.0007).
The winner and control groups were not significantly
different (t = 0.08, df = 31, P = 0.93).
Mean plasma CORT levels (ng/ml) of males in the two
different mating seasons were not significantly different
(tarena = ÿ 1.759, df = 9, P = 0.112; tcage = ÿ 1.242, df = 9,
P = 0.246; tloser = ÿ 0.497, df = 9, P = 0.631; twinner = ÿ 1.74,
df = 9, P = 0.115). The two control groups (arena and cage)
were not significantly different (P = 0.84); thus, as in the
above analysis, we collapsed the two CORT control groups
into a single control group. Moreover, because seasonal
differences in plasma CORT in winners and losers were not
detected, we collapsed them into single groups.
Plasma CORT levels (ng/ml) were not correlated with
2
= 0.014,
SVL or body mass in the control group (r SVL
2
F 1, 2 0 = 0.279, P = 0.603; r ma s s = 0.040, F 1 , 20 = 0.825,
2
= 0.192, F1,9 = 2.132, P = 0.178;
P = 0.374) or winners (r SVL
2
rmass = 0.301, F1,9 = 3.876, P = 0.081); however, in losers,
2
= 0.498,
CORT was positively correlated with SVL (r SVL
2
F 1,9 = 8.91, P = 0.015) but not body mass (r mass = 0.308,
F1,9 = 4.01, P = 0.080). Plasma CORT levels (ng/ml) in
winners and losers were not correlated with duration of
fighting when the fight of longest duration (171 min) was
338
G.W. Schuett, M.S. Grober / Physiology & Behavior 71 (2000) 335±341
Table 1
Post-fight (60-min) plasma lactate and plasma CORT values of winners, losers, and controls (C) in spring and late summer. Control A (CA) denotes that the
male was tested in the arena with a female only. Control B (CB) denotes that the male was tested directly from its permanent cage
Post-fight
Subject
SVL (cm)
Mass (g)
Date
Spring
Winner
Loser
CA
CB
Winner
Loser
CA
CB
Winner
Loser
CA
CB
Winner
Loser
CA
CB
Winner
Loser
CA
CB
Winner
Loser
CA
CB
60.0
56.5
71.0
61.0
57.0
60.0
62.5
74.5
62.0
69.0
60.0
59.0
76.5
69.0
58.5
61.0
70.0
60.5
72.0
52.0
71.0
70.0
68.5
62.0
122.0
110.5
198.5
177.0
129.0
149.0
171.5
256.0
183.5
222.5
158.5
184.0
256.0
196.0
132.0
185.5
205.5
146.0
202.5
101.5
246.0
243.0
190.5
155.5
03-24-92
1.00
03-25-92
26.40
03-26-92
1.00
04-02-92
17.81
04-07-92
3.50
04-14-92
8.00
Late summer
Winner
Loser
CA
CB
Winner
Loser
CA
CB
Winner
Loser
CA
CB
Winner
Loser
CA
CB
Winner
Loser
CA
CB
76.0
68.0
72.5
57.0
69.5
62.5
70.0
62.0
73.5
66.5
62.0
58.0
58.5
62.5
71.5
67.0
74.5
68.0
64.5
55.0
313.0
211.5
245.5
114.0
209.0
200.0
185.0
215.5
292.0
218.0
200.5
186.5
141.5
195.0
220.5
176.5
343.0
198.5
190.0
121.5
09-11-91
5.50
09-01-92
171.15
09-09-92
4.48
09-16-92
12.00
09-16-92
4.87
removed from the data set as an outlier (r 2 = 0.001,
F1,18 = 0.018, P = 0.896).
The effect of winning and losing on plasma CORT
levels was directly compared to the control group using
ANOVA because the other factors investigated (SVL, body
mass, fight duration, season) were not found to be correlated with plasma CORT levels (with the single exception
noted above). Mean plasma CORT (ng/ml) of winners,
losers, and the control were not equivalent (F2,41 = 39.16,
Contest duration (min)
Lactate (mM)
CORT (ng/ml)
3.78
10.94
10.52
7.08
4.13
6.41
3.26
2.82
6.42
7.34
6.18
8.28
7.42
8.43
6.20
5.38
7.60
8.47
6.28
8.12
3.44
7.78
6.29
7.46
17.92
37.73
21.62
10.03
23.03
58.26
1.96
32.84
23.80
83.39
16.89
21.92
20.78
89.80
13.85
27.57
16.01
52.07
20.06
25.74
35.28
53.93
29.36
5.77
4.38
7.40
11.54
4.22
10.24
19.20
6.29
2.16
10.22
13.52
6.42
6.34
4.95
10.33
11.69
5.82
8.57
14.29
5.32
2.67
49.65
67.72
38.81
24.17
49.13
57.70
28.61
40.74
14.94
63.92
20.11
14.88
19.07
71.41
14.05
22.72
49.33
75.31
37.84
43.60
P = 0.0001). Pair-wise comparison revealed that losers had
greater mean plasma CORT (mean = 64.66, SEM = 3.89 ng/
ml) than winners (mean = 28.99, SEM = 3.89 ng/ml,
P = 0.0001; Fig. 1B) and the control group (mean = 23.32,
SEM = 2.75 ng/ml, P = 0.0001); winners and the control
group were not significantly different (P = 0.2410).
Plasma CORT levels (ng/ml) were not significantly
correlated to plasma lactate levels (mM) in the control
group (r 2 = 0.003, F 1,20 = 0.068, P = 0.797), winners
G.W. Schuett, M.S. Grober / Physiology & Behavior 71 (2000) 335±341
Fig. 1. Effects of fight outcome on (A) mean plasma lactate and (B) mean
plasma CORT. Histogram bars denote the mean ‹ SEM. Sample sizes for
both histograms are provided in (B). Letters above error bars represent
significant differences, P < 0.05.
(r 2 = 0.158, F1,9 = 1.690, P = 0.226), or losers (r 2 = 0.256,
F1,9 = 3.099, P = 0.112).
4. Discussion
We have shown, in male copperheads, that losers of
fights have significantly higher post-fight levels of plasma
lactate and CORT when compared to winners and either of
two types of controls. The levels of plasma lactate we report
are within the ranges reported in previous studies of
vertebrates [8], and, as shown for other species of snakes
(e.g., Crotalus viridis) [22], are a reliable indicator of
whole-body lactate levels. Thus, the differences in plasma
lactate levels in winners and losers may demonstrate the
influence of aggressive interactions and stress responses on
energy metabolism. Studies on aggression in fish [1,16],
demonstrated a greater metabolic cost of agonism for subordinate relative to dominant individuals. We are not aware
of prior studies that have examined the interaction of lactate
and CORT in snakes.
The binary outcome of male fights in copperheads and
other snakes [23] may be a key factor with respect to the
differences in post-fight levels of plasma lactate and CORT
in winners and losers. Agonistic interactions in other squamates (e.g., lizards) generate a more complex social hier-
339
archy, rather than a binary outcome. This basic difference
may be an emergent property of the solitary habits of
copperheads relative to the frequent social interactions of
many lizards [15]. Our study, to the best of our knowledge,
is the first of its kind in a species of vertebrate that is not
fighting for territorial reasons, but solely for priority of
access to females during the mating seasons. Accordingly,
fighting in male copperheads (and other snakes) is probably
uncommon, but significant with respect to reproductive
success. This scenario may provide insights into the long
time course of behavioral recovery (i.e., courtship behavior
and aggression) in copperheads [23,26]. In species that
experience frequent agonistic encounters (e.g., Anolis lizards and cichlid fish), selection should favor rapid recovery, while there would be less opportunity for selection to
favor rapid recovery in animals where interactions are
relatively rare, such as in copperheads [23].
Previous work on the highly social green anole lizard
(Anolis carolinensis) provides an interesting comparison to
our study. Wilson and Gatten [32] showed that territorial
intruders exhibited a significant increase in lactate, but there
was no change in lactate levels in territory owners. The
mechanism underlying these differences is unclear because
there were no significant differences in activity levels or
behavior between the two groups of males. This suggests
that the difference in lactate accumulation is not solely (or
partly) a response to exercise or oxygen debt. In contrast to
mammals, reptiles and amphibians are adept at converting
lactate to glycogen and glucose in muscle, and do not
transport lactate to the liver for conversion [8]. In bypassing
the liver, rapid recovery and complete glycogen restitution
can be achieved. Wilson and Gatten [32] showed that
agonistic interactions did not significantly affect lactate
levels in green anoles. Moreover, larger and more active
males had lower levels than those smaller and more sedentary, and a positive correlation was found between length of
the interaction and lactate levels; this was not the case for
copperheads in our study. Wilson and Gatten [32] suggested
that increased sympathetic activity, resulting from the agonistic interaction, generated increased catecholamine levels
that subsequently stimulated glycogenolysis in skeletal
muscle and the liver. These conclusions are consistent with
a recent study on tree lizards, Urosaurus ornatus [13], and
suggest that the neuroendocrine response to the agonistic
interaction may be differentially modulating plasma lactate
levels in winners and losers.
The above studies raise an intriguing question as to
whether the increase in plasma lactate and CORT is a
response to exercise, as suggested by Gleeson et al. [9], or
a pyschoneuroendocrine response to agonistic stress. We
suggest that the data are more consistent with the latter
hypothesis, since Gleeson et al. [9] found an increase in
CORT in all animals that were exercised, but we found that
only losers had both elevated plasma lactate and CORT at
60-min post-fight. Furthermore, in male copperheads, the
increase in plasma lactate and CORT were not correlated
340
G.W. Schuett, M.S. Grober / Physiology & Behavior 71 (2000) 335±341
with the duration of the fight and did not show observable
post-fight differences in behavior (e.g., activity) between
winners and losers after being returned to their respective
cages. Finally, during fights, male copperheads that ultimately win show higher frequencies of energetically demanding acts (e.g., hooking) than losers [26], which
suggests that winners, not losers, should have higher postfight levels of plasma lactate. Thus, in our study, higher
levels of post-fight plasma lactate in losers appear to result
from psychoneuroendocrinological factors rather than metabolic responses to exercise per se.
Our interpretation is consistent with a study by Neat et al.
[16] who showed, in a cichlid fish, that the duration of a
fight was not correlated with muscle lactate levels, and that
winners and controls had lower lactate levels than losers.
These authors concluded that the lack of a difference
between pre- and post-fight lactate levels in winners suggests that glycolysis is not important in providing energy for
agonistic interactions. We suggest that glycolysis may be
important for agonistic interactions, but that winners and
losers deal with lactate accumulation differently and these
differences may be mediated by hormonal responses to the
agonistic interactions. In support of this idea, Gleeson et al.
[9] found that CORT had no effect on lactate metabolism in
lizard skeletal muscle, but that epinephrine had a significant
effect on removal of lactate from muscle. They concluded
that elevated levels of glycogenolytic hormones (e.g.,
CORT) retard metabolic recovery because skeletal muscle
in lizards effectively converts lactate to glycogen. Therefore,
CORT may shunt lactate from muscle to blood in losers,
whereas in winners, with lower plasma CORT levels, lactate
remains in the muscle and is recycled to glycogen. Eros and
Milligan [6] showed that CORT has a rapid effect on lactate
release in fish skeletal muscle, and they suggest that this
process is mediated via membrane-bound receptors rather
than genomic mechanisms [17,27]. In trout, experimental
reduction in CORT levels yielded reduced blood lactate,
accelerated blood and muscle lactate removal, and accelerated glycogen replenishment. Therefore, increased CORT
levels appear to exert a negative influence on metabolic
recovery [14,18]. Our results with copperheads are consistent with this scenario and support the hypothesis that losers
are releasing lactate into circulation.
Finally, we did not detect a significant correlation between plasma lactate and CORT levels in winners, losers, or
controls. Given that fighting has similar effects on both
CORT and lactate, we suggest two alternative hypotheses to
explain a lack of correlation between these parameters: (1)
The absence of a positive relationship between fight duration and CORT levels indicates that the effect of fighting on
losers has a threshold function rather than a dose ± response
function. We propose that losing a fight, regardless of
duration, triggers a threshold increase in CORT. This
increase is sufficient to drive changes in lactate recovery
and release. Thus, this hypothesis does not predict a
correlation between the levels of CORT and lactate. (2)
The absence of a correlation between CORT and lactate
suggests that there may be other neuroendocrine factors that
effect the changes in both lactate and CORT. Possible
examples would be shifts in the levels of brain monoamines
[31,34], or a sympathetic cascade from behavior to CORT
and/or lactate [13].
By addressing some of the limitations of our study, future
studies can provide a more detailed explanation of the
hormonal and metabolic correlates of male agonistic behavior. First, our analysis would have been facilitated by an
additional control group that was exercised in a context not
involving social interactions (e.g., treadmill). Second, in
view of the information on retention and recycling of lactate
in the muscles of ectothermic vertebrates [8], a measure of
muscle lactate would have permitted us to look both at the
source of lactate and its primary metabolic processing. Last,
we did not collect samples immediately post-fight, so we
cannot examine the rate of plasma lactate accumulation in
losers. Based on prior studies, substantial increases in
plasma CORT were expected to be delayed; thus, we
sampled at 60-min post-fight to detect high and potentially
significant differences in CORT between winners and losers. Sampling at multiple times post-fight would permit
development of a more complete model of the energetics of
and recovery from agonistic interactions.
Acknowledgments
We thank D. Martin for his assistance in measuring
lactate levels, W. Murdoch and E. Van Kirk for performing
CORT analyses, H. Harlow for lab space and K. Malmos for
statistical advice. Funding was provided by the American
Museum of Natural History (Theodore Roosevelt Memorial
Fund), Sigma Xi (Grant-in-Aid of Research), The Graduate
School and Office of Research, University of Wyoming, to
GWS, and NSF (IBN-9723817) to MSG. This paper is
dedicated to the memory of Colleen Kelly, whose contribution to our understanding of the biology of copperheads will
be sorely missed.
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