Conservation, Biology, and Natural History of Crayfishes from the Southern US
2010 Southeastern Naturalist 9(Special Issue 3):231–244
Agonistic Interactions Among Size-matched Form I and
Form II Male Procambarus suttkusi (Choctawhatchee
Crayfish)
Paul M. Stewart1,*, Amanda D. McKenzie1, Thomas P. Simon2,
and Amanda M. Baker1
Abstract - This study observed agonistic interactions and contests between sizematched
form I vs. form I, form II vs. form II, and form I vs. form II males to
establish baseline dominance orders and behavioral dynamics for Procambarus suttkusi
(Choctawhatchee Crayfish). Three types of initiation behaviors, or attacks, were
observed during each interaction: ambivalent contact, claw raise, and lunge. Feeding
competition studies were also performed to assess the relationship between the winners
of the agonistic interactions and feeding contests. Only form I vs. form I contests
showed a decline in the number of interactions observed over time, but the duration
of each individual agonistic interaction declined for all three types of behaviors during
the thirty-minute recording period. Winners and losers did not preferentially use
the same initiation behaviors as used in other studies. Previous studies also documented
form I dominating form II males, but form I males did not dominate form II
males in the present study. Winners of the agonistic contests won significantly more
feeding contests. The results of the current study suggested that behavioral dynamics
and form dominance may be species specific and that careful consideration of size
differences are necessary in order to examine form dominance.
Introduction
Aggressive behaviors are important to many organisms (Lowe 1956) and
have been studied extensively in crustaceans, including lobsters (Karavanich
and Atema 1998, Karnofsky et al. 1989), crabs (Hazlett 1997), prawns (Barki
et al. 1992, Evans and Shehadi-Moacdieh 1988), and crayfish (Capelli and
Hamilton 1984, Hazlett et al. 1992, Quinn and Janssen 1989). Crustaceans
compete for resources using aggressive interactions, often termed agonistic
interactions. As a result of these behaviors, hierarchical relationships are
thought to be formed (Copp 1986, Issa et al. 1999), giving the organism with
the higher rank access to the best resources, such as shelter, reproductive success,
and feeding access (Klocker and Strayer 2004, Krebs and Davies 1987).
The subordinate organism is often excluded from these resources, especially
when resources are limited (Edsman and Jonsson 1996, Söderbäck 1991).
Hierarchical relationships begin to form when two crayfish first encounter
each other and exhibit intricate, stereotyped behavior, usually escalating
into an agonistic interaction (May and Mercier 2006, Tierney et al. 2000).
1Department of Biological and Environmental Sciences, Troy University, Troy, AL
36081. 22364 East Linden Hill Drive, Bloomington, IN 47401. *Corresponding author
- mstewart@troy.edu.
232 Southeastern Naturalist Vol. 9, Special Issue 3
Interactions occur with and without the presence of resources (Issa et al.
1999). As dominance orders become established, the intensity and frequency
of agonistic competitions decrease as one of the competitors avoids the other
and exhibits submissive instead of dominant behaviors (Gherardi and Daniels
2003, Herberholz et al. 2003). This sequence of behaviors signifies the
establishment of dominance order (Herberholz et al. 2003).
Visual communication, such as claw and antennae postures, and physical
contact are major aspects of agonistic interactions and are important in
determining the intensity of subsequent contests (Baird et al. 2006, Smith
and Dunham 1990). Agonistic interactions between pairs of crayfish involve
threat behaviors such as attacks and chelae interactions and avoidance behaviors
such as walking away and tailflips (Lundberg 2004). The attacks,
often called initiation behaviors, were further divided by Guiasu and Dunham
(1997b, 1998, 1999) into categories of ambivalent contact, claw raise,
and lunge. The claw raise and the lunge are more aggressive behaviors, and
the ambivalent contact and avoidance behaviors are submissive behaviors
(Guiasu and Dunham 1997b, 1998, 1999; Issa et al. 1999). The intensity
and extent with which a particular agonistic behavior is used differs among
crayfish species (Tierney et al. 2000).
Asymmetrical morphological differences between two opponents often
influence success in interactions (Barki et al. 1992, Rubenstein and Hazlett
1974). Asymmetric contests are shorter and less intense, with larger individuals
winning agonistic contests and gaining access to more advantageous
resources (Bergman and Moore 2003). For example, chelae size is important to
the aggressive strategies of crayfish (Schroeder and Huber 2001). Orconectes
rusticus (Girard) (Rusty Crayfish) with larger chelae usually win intraspecific
interactions (Gherardi et al. 2000, Snedden 1990). However, size-matched opponents
have longer agonistic interactions (Bergman and Moore 2003).
Reproductive morphology may also influence the outcomes of agonistic
interactions. Form I males are reproductively active and usually have a
larger carapace to chelae size ratio than non-reproductively active form II
males (Stein 1976). Some species have a synchronous alternation between
reproductive forms (Orconectes species) and other species (Cambarus species)
molt and breed many times of the year (Guiasu and Dunham 1998).
Procambarus suttkusi Hobbs (Choctawhatchee Crayfish) has a synchronous
reproductive cycle, from May to September, but form II males of reproductive
size can be found in the population during the mating period (Baker et
al. 2008). During this time, form I males may compete with form II males for
valuable stream resources. Guiasu and Dunham (1998) suggested that form
I males dominate form II males.
Extrinsic asymmetries also influence agonistic interactions. These include
previous wins in agonistic contests, prior residence when competing
for shelters, and differences in fight strategies (Bergman and Moore 2003).
For example, winners of previous interactions behave in the same manner
as in preceding contests when competing with familiar and unfamiliar
2010 P.M. Stewart, A.D. McKenzie, T.P. Simon, and A.M. Baker 233
opponents, thus increasing the likelihood of successive wins or successive
losses (Bergman et al. 2003, Gherardi and Daniels 2003). The effects of
extrinsic asymmetries are time dependent, variable, and frequently species
specific (Bergman et al. 2003).
Many studies have documented aggressive behaviors in crayfish (Figler
et al. 1999, Guiasu and Dunham 1998, Rorer and Capelli 1978), but no
studies have focused on species that are indigenous to the southeastern
coastal plains ecoregion of the United States. The subject of this study was
Procambarus suttkusi, a species widely distributed and dominant in the
Choctawhatchee drainage system of Alabama and northern Florida (Heath
et al. 2010, Hobbs 1953) where, because of its limited range, it has been
listed as a species of concern (Bouchard 1976, Fitzpatrick 1990). Form I and
form II individuals co-exist temporally in the Choctawhatchee watershed,
but information for P. suttkusi is limited, and the dominance order among
reproductive forms of male P. suttkusi has not been documented.
The current study analyzed agonistic interactions between form I vs. form
I, form II vs. form II, and form I vs. form II P. suttkusi males to establish baseline
dominance orders and agonistic dynamics for this species. Three types
of initiation behaviors, including lunge, claw raise, and ambivalent contact,
demonstrated by the winners and losers of each individual interaction were
quantified. These same pairs of individuals competed for food in another
trial to see if the winners of the agonistic contests would also win the feeding
contests. These data may be used to compare the agonistic behaviors of this
species to others to better understand competitive exclusion mechanisms.
Methods
Collection sites
Crayfish were collected from wadeable streams of the Choctawhatchee
watershed, previously known to contain P. suttkusi (Heath et al. 2010). All
streams sampled in the current study were moderately meandering streams
that have not been channelized or dredged, except for Persimmon Branch. The
low-gradient, wadeable streams of this watershed are characterized by sandy
substrates, woody debris, leaf packs, root wads, and instream vegetation.
Laboratory preparations
Form I and form II P. suttkusi males were collected using wire minnow
baskets and a Smith Root Model 12-B ® backpack electrofisher. Identifications
were confirmed in the Troy University laboratory using taxonomic
keys (Hobbs 1981). Each crayfish, after being kept dry for two minutes, was
weighed to the nearest tenth of a gram. Using digital calipers, the cephalothorax
length and the length and width of the largest chelae were measured for
each crayfish. Sixteen form I vs. form I, 20 form II vs. form II, and 13 form I
vs. form II male pairs were size-matched, and each form I vs. form I and form
II vs. form II pair differed by no more than 2 grams in body weight and a 10%
difference in chelae length (Nakata and Goshima 2003, Tierney et al. 2000).
234 Southeastern Naturalist Vol. 9, Special Issue 3
Form I vs. form II pairs were size-matched in order to eliminate the effects
of size advantage on the outcome of the agonistic contests and differed by no
more than a 10% difference in chelae length. Numbers were placed on each
pair using correction fluid. Each crayfish was held in an isolated area of the
holding tank for at least one week in preparation for the agonistic contest to
reduce the impacts of any previous interactions (Gherardi and Daniels 2003).
The isolated areas of the holding tank for each crayfish consisted of a 10.2-cm
diameter, 15.5-cm long PVC pipe. The PVC pipe was covered by a wire mesh
material, with an aeration stone placed in the middle of the holding area. To
reduce the effects of hunger on motivational differences, crayfish were last
fed Lumbricus sp. (night crawler) pieces 24 hours prior to the agonistic contest.
The experimental tank, 51 cm (length) x 25 cm (width) x 29 cm (height),
was divided into two equal halves using a vinyl plexiglass divider and contained
gravel substrate, about 3 cm in depth. All agonistic contests took place
in the same experimental tank. Holding and experimental tanks contained
water collected and sieved from the same sites at which the crayfish were
collected. All crayfish were housed in the laboratory at the natural ambient
photoperiod. The same crayfish pairs used in the agonistic contests were later
used in the feeding contests. Individuals from these pairs were not used in any
other agonistic or feeding contest.
The agonistic contest
This study followed procedures established by Guiasu and Dunham
(1997b). Contests were performed between 0800 and 1500 hours. Prior to
initiating the contests, trials were performed to determine if there was a difference
between diurnal and nocturnal trials. No differences were noted for
interactions observed during these time periods (P > 0.05); however, previous
studies have suggested an increase in activity and frequency of agonistic
interactions during nocturnal periods in other species of crayfish (Fero et al.
2007, Issa et al. 1999). Before being placed in the experimental aquarium, a
red or yellow marking was randomly placed on the cephalothorax of each opponent
to easily distinguish the two crayfish. Before each agonistic contest,
two crayfish of the intraspecific matched pair were placed on opposite sides
of the divider and allowed to acclimate for 20 minutes. The divider was removed,
and individual interactions between the two crayfish were recorded
for 30 minutes using a Panasonic Digital Video Camcorder (PV-GS19). Crayfish were not disturbed during the recording period. Three types of initiation
behaviors were quantified: lunge, claw raise, and ambivalent contact. The
crayfish starting each agonistic interaction, the initiation behavior performed,
and the outcome of the individual interactions were also recorded for each
contest. The crayfish that retreated, tail-flipped, ran, or used other avoidance
mechanisms was considered the loser of the individual agonistic contest, and
the opponent was declared the winner. The duration of each interaction was
recorded from the time that the two crayfish came in physical contact with
one another until one crayfish either backed away, walked away, or tail flipped
away from its opponent. The 30-minute observation period was divided into 6
2010 P.M. Stewart, A.D. McKenzie, T.P. Simon, and A.M. Baker 235
five-minute time intervals to see if the mean duration of each agonistic interaction
and the number of individual interactions changed as time progressed.
The feeding contest
Crayfish pairs, tested previously in the agonistic contest, were isolated
for one week, using the same isolation method as in the agonistic contest.
To reduce the effects of feeding on motivation, crayfish were not fed for 72
hours prior to the feeding contest. Practice trials, performed in this current
study, revealed that a 72-hour waiting period was the minimum time needed
to practically ensure that individuals of this species of crayfish would show
interest in feeding. Before each feeding contest, the intraspecific pairs were
placed on opposing sides of the dividers and were allowed to acclimate
for 20 minutes. The divider was removed and a night crawler, attached to
a string and weight, was placed in the middle of the tank. Crayfish were
recorded until a definitive winner of the food contest was established. The
definitive winner of the feeding contest was the individual that successfully
obtained the food item. If both subjects obtained a portion of the food item,
the crayfish that obtained 60% of the food item, based on visual observations,
was declared the winner. If neither obtained a larger portion, then the
contest was recorded as a tie.
Statistical tests
All statistical analysis for the agonistic contests followed those performed
in previous studies of this type (Guiasu and Dunham 1997b) using
SPSS® 11.0.1. Overall winners of the agonistic interaction were determined
using the percentage of overall individual wins. Due to the low sample
number (n < 20), nonparametric tests were used. The Mann-Whitney U test
for two independent samples (α = 0.05) was used to compare differences
between the winners and losers depending on the type of initiation behavior
used. A Mann-Whitney U test was also used to determine if the crayfish
that initiated the first interaction would also be declared the overall winner
during the 30-minute recording period and to compare size-matched pairs
regarding size measurements (P = 0.05). The winners of the feeding contest
were compared, using a Mann-Whitney U test, to the winners of the same
pairs of crayfish in the agonistic interactions to see if the winner of the agonistic
interaction also won the feeding contests. The Kruskal-Wallis H test
was used to compare the number of interactions during the 6 time intervals
of the 30-minute observation period (P = 0.05).
Results
Number of interactions observed
Eight hundred and forty-one individual interactions among the 16 form
I vs. form I, 20 form II vs. form II, and 13 form I vs. form II male pairs
were observed during a total of 24.5 hours (1470 min) of observation. Three
hundred and thirty-seven agonistic interactions occurred between form I
vs. form I males, constituting 40% of the individual interactions observed
236 Southeastern Naturalist Vol. 9, Special Issue 3
among the 3 form categories, 281 interactions (33% of the total interactions)
were observed between form II vs. form II males, and 223 interactions (27%
of the total interactions) were observed between form I vs. form II males.
Results of the agonistic contests were based on all of the individual interactions
that occurred during the 30-minute recording periods. There was
a significant difference among the time intervals for the form I vs. form I
contests (Kruskal-Wallis H: P = 0.01), with the most interactions occurring
in the second time interval and the least occurring during the fifth and sixth
intervals (Fig. 1). There were, however, no significant differences found
among the 6 time intervals for the form II vs. form II and form I vs. form II
contests (Kruskal-Wallis H: P = 0.15 and 0.13, respectively).
Mean duration of agonistic interactions
There was a significant difference in the duration of interactions among
the 6 time intervals for all 3 form categories (Kruskal-Wallis H: P = 0.001, for
all types; Fig. 2). During form I vs. form I and form I vs. form II competitions,
Figure 1. Box-andwhisker
plot representing
the total number
of interactions
observed for form I
vs. form I agonistic
contests during each
of the six time intervals
of the 30-minute
recording periods.
Circle indicates a
possible outlier.
Figure 2. Mean (± S.E.) duration of each individual interaction (recorded in seconds)
observed for form I vs. form I, form II vs. form II, and form I vs. form II agonistic
contests during each of the six time intervals of the 30-minute recording periods.
2010 P.M. Stewart, A.D. McKenzie, T.P. Simon, and A.M. Baker 237
there was an increase in the duration during the second interval, but an overall
decrease thereafter. For form I vs. form I contests, longer interactions
occurred during the first time interval than the fifth and sixth intervals, with a
decline after 21 minutes. A gradual decline in the duration of each interaction
was observed over time for Form II vs. Form II contests, especially during
the fourth, fifth, and sixth intervals. In form I vs. form II contests, agonistic
interactions during the first time interval lasted longer than those of the last
interval, with a prolonged decline beginning after 26 minutes.
Winner and loser initiation behavior comparisons
Overall winners of form I vs. form I, form II vs. form II, and form I vs.
form II contests initiated 68%, 52%, and 61% of all individual interactions,
respectively. Overall winners of both form I vs. form I and form I vs. form
II contests did not significantly initiate more interactions than losers (Mann-
Whitney U: P = 0.25 and 0.39, respectively). However, the overall winners
of form II vs. form II contests initiated significantly more interactions than
losers (Mann-Whitney U: P = 0.002).
Winner and loser use of each of the 3 types of initiation behaviors (ambivalent
contact, claw raise, and lunge) were analyzed for the 3 form categories.
During form I vs. form I contests, there were no significant differences found
for the 3 types of behaviors displayed by the winners (Kruskal-Wallis H: P =
0.07). Losers, however, used significantly more claw-raise behaviors (Mann-
Whitney U: P = 0.01) than either lunge or ambivalent-contact behaviors
(Mann-Whitney U: P = 0.05, for both types). Winners of form II vs. form II
contests, initiated significantly more interactions with claw raise behaviors
than ambivalent contact behaviors (Mann-Whitney U: P = 0.005), but no significant differences were found between the claw-raise and lunge initiation
behaviors or the ambivalent-contact and lunge initiation behaviors (Mann-
Whitney U: P = 0.17 and 0.54, respectively). No significant differences were
found among all 3 types of behaviors for the losers of the form II vs. form
II contests (Kruskal-Wallis H: P = 0.433). Form I vs. form II contests were
similar to form I vs. form I contests in that there were no significant differences
found for the 3 types of behaviors used by the winners (Kruskal-Wallis
H: P = 0.35). Losers, however, used significantly more claw-raise behaviors
than either lunge (Mann-Whitney U: P = 0.01) or ambivalent-contact behaviors
(Mann Whitney U: P = 0.005).
Within each form category, the dominant initiation behavior used by winners
was compared to the dominant behavior used by losers. During form I
vs. form I contests, winners used claw-raise initiation behaviors significantly
less than losers (Mann-Whitney U: P = 0.05). Form II vs. form II winners
did not use lunge or claw-raise initiation behaviors significantly more than
losers (Mann-Whitney U: P = 0.22 and 0.16, respectively), and winners did
not use ambivalent-contact behaviors less than losers (Mann-Whitney U:
P = 0.22). Results were similar for form I vs. form II contests—winners did
not use more lunge or claw-raise behaviors than losers (Mann-Whitney U:
P = 0.08 and 0.42, respectively), and winners did not use ambivalent-contact
behaviors less than losers (Mann-Whitney U: P = 0.41).
238 Southeastern Naturalist Vol. 9, Special Issue 3
The winner of each overall contest was compared with the crayfish that
initiated the first interaction to see if the crayfish that initiated the interaction
would also win a majority of the total interactions. The crayfish that initiated
the first interaction won 71% of the individual interactions during form
I vs. form I contests, 68% during form II vs. form II contests, and 62% of
the form I vs. form II contests. There were no significant differences found
between the winners and losers that won the first agonistic interaction and
the eventual status for all 3 form categories (Mann-Whitney U: P = 0.32, for
all 3 categories). After pooling the data to observe if the crayfish initiating
the first interaction was the overall winner of the contest and to increase the
degrees of freedom in the statistical analysis, overall winners were found
to initiate more of the first interactions than did the overall losers (Mann-
Whitney U: P = 0.03).
Comparison between form I and form II males
Interactions between form I vs. form II males were observed to document
possible hierarchical formations and agonistic behaviors between these
size-matched pairs. Although, there were no significant differences found
between the number of individual agonistic interactions won by form I or
form II males (Mann-Whitney U: P = 0.34), the most commonly used initiation
behavior by form I males was the claw raise. Form I males used the
claw-raise behavior significantly more than either the lunge or the ambivalent
contact behavior (Mann-Whitney U: P = 0.003 and 0.005, respectively;
Fig. 3A). There were no significant differences found among the 3 initiation
behaviors for form II males (Kruskal-Wallis H: P = 0.713; Fig. 3B). When
comparing the behaviors of the 2 reproductive types, form I males used the
claw-raise behavior to initiate more interactions than did Form II males
(Mann-Whitney U: P = 0.021). There were no significant differences in use
of either lunge behavior or ambivalent-contact behavior between the 2 reproductive
types (Mann-Whitney U: P = 0.698 and 0.327, respectively).
Agonistic and feeding contests comparison
Crayfish pairs, from each form category, competed in both agonistic and
feeding contests to see if the winners of the agonistic contest would also
win the feeding contest. Two pairs were deleted from this analysis because
either one or both of the crayfish of the agonistic pair died prior to the feeding
contest or there were no interactions observed between the crayfish pair.
Winners of the agonistic interactions won significantly more of the corresponding
feeding contests than did the losers of form I vs. form I, form II vs.
form II, and form I vs. form II contests (Mann-Whitney U: P = 0.027, 0.029,
and 0.017, respectively).
Discussion
Differences in behavior, the number and duration of contests, and outcomes
of agonistic and feeding contests were observed among the 3 form
categories and between winners and losers. There was a significant decrease
2010 P.M. Stewart, A.D. McKenzie, T.P. Simon, and A.M. Baker 239
in the number of form I vs. form I contest interactions after 21 minutes,
which suggests the development of hierarchical relationships among P.
suttkusi form I individuals. There were no significant differences among the
number of interactions that occurred during different time intervals for form
II vs. form II and form I vs. form II contests, suggesting that hierarchical
relationships did not form among individuals of these groups during the
30-minute observation periods.
Some of the results observed for P. suttkusi in our study differed from
the results found for other studies of this type. During form I vs. form I
Cambarus robustus Girard (Big Water Crayfish) contests, the number of
interactions began to decline in the 21- to 30-minute time interval, which
is similar to results of this study (Guiasu and Dunham 1997b). However, a
gradual decline in the number of interactions were observed over time for
form I vs. form I C. robustus (Guiasu and Dunham 1997b), while observations
of form II vs. form II contests for C. robustus suggested a decline in
the number of interactions only in the last 30 minutes of the observation
period (Guiasu and Dunham 1997a). During form I vs. form II contests, the
number of interactions was significantly greater in the second time interval
Figure 3. Box-and-whisker
plots representing the
total number of initiation
behaviors displayed by
A) form I and B) form II
males during form I vs.
form II agonistic contests.
240 Southeastern Naturalist Vol. 9, Special Issue 3
than the last, with a decline suggested after 51 minutes (Guiasu and Dunham
1998). The recording period for our study may have not been long enough
to reveal a significant decline in the number of interactions for form II vs.
form II and form I vs. form II contests. Because some decline was suggested,
and the form I vs. form I individual interactions declined at the same time
interval for studies of this type by Guiasu and Dunham (1997a), it is likely
that a 60-minute recording period would have revealed a significant decline
for these paired interactions as time progressed. The decline in agonistic
interactions may be linked to the establishment of the dominant-subordinate
relationship between the two interacting individuals.
In the present study, a significant decrease in the mean duration of each interaction
was observed for all 3 form categories. Declines were observed after
11 minutes for form I vs. form I, 16 minutes for form II vs. form II, and 21 minutes
for form I vs. form II contests. Our results were similar to those provided
by other studies for C. robustus, in which there was a decline found at 21 minutes
for form I vs. form I contests (Guiasu and Dunham 1997a), 31 minutes for
form II vs. form II contests (Guiasu and Dunham 1997b), and 51 minutes
for form I vs. form II contests (Guiasu and Dunham 1998). There were distinguishable
differences observed among the Guiasu and Dunham studies (1997a,
1997b, 1998), possibly suggesting that the point at which interactions begin to
decline is species and form specific. Regardless of exactly when the period of
time engaged in agonistic interactions declined, there appears to be a clear decrease
in the duration of these interactions over time.
Other studies, using different species, have documented winners using
more lunge and claw-raise behaviors than ambivalent behaviors during form I
vs. form I and form II vs. form II contests (Guiasu and Dunham 1997b, 1998),
which supports the findings on the claw-raise behavioral preference by winners
of form II vs. form II individuals of this study. However, loser preference
of ambivalent contact behaviors in other studies (Guiasu and Dunham 1997b,
1998) did not support loser preference of claw-raise behavior in this study,
suggesting that this behavior in form I vs. form I contests may be species specific in P. suttkusi males. For form I vs. form II contests (Guiasu and Dunham
1997a), winners used the claw-raise behavior more than any other type, but
there were no significant differences among the behaviors of the losers. During
form I vs. form I and form I vs. form II contests, there were no significant
differences found among the initiation behaviors used by the winners, but the
losers preferred to use the claw-raise initiation behavior more than any other
initiation behavior. Winners of form II vs. form II contests in the current study
used claw-raise initiation behaviors more than any other type of behavior, but
they did not significantly perform more claw-raise behaviors than the losers
of these interactions. Because the winners and losers in the current study did
not prefer the same initiation behaviors as other studies, the types of initiation
behaviors demonstrated by crayfish may be species specific. Tierney et al.
(2000) compared the intraspecific agonistic behaviors of 4 crayfish species
and found that each species preferred to use certain initiation behaviors more
2010 P.M. Stewart, A.D. McKenzie, T.P. Simon, and A.M. Baker 241
than others. There were also different use patterns observed among the 3 form
categories in the current study and other behavioral studies suggesting that
winner and loser preferences for lunge, claw-raise, and ambivalent-contact
behaviors may also be form specific.
Form I males did not win significantly more contests than form II males
in the present study; however, during form I vs. form II C. robustus male
contests, form I males won significantly more agonistic contests than form
II males (Guiasu and Dunham 1998). Guiasu and Dunham (1998) found that
form I C. robustus males used the claw-raise behavior more than any other initiation
behavior. Similar results were observed for the current study, form I P.
suttkusi males used the claw-raise behavior more than any other type of initiation
behavior, but no preference was found for form II males in this species.
In this study, crayfish were size-matched with no significant difference
found among all measurements, which may have caused results to
be different from the studies by Guiasu and Dunham (1998). Guiasu and
Dunham (1998) found winners to be significantly larger than losers in all
measurements, except for cephalothorax length. In addition, differences in
observational periods between the two studies may also have caused these
differences because Guiasu and Dunham (1998) observed interactions for
60 minutes, while interactions were only observed for 30 minutes in the
present study. In the present study, form I males did not dominate form II
males; however, this study only documented size-matched form I vs. form
II P. suttkusi pairs. Further research using pairs that are not size-matched
may be needed to better understand agonistic interactions between the
two male reproductive forms of this species. Guiasu and Dunham (1997a,
1997b, and 1998) compared form differences compounded by size differences
for several variables. Any study examining size differences must use
the same form, and conversely, any study comparing form differences must
use size-matched pairs as in the present study.
Winners of the agonistic contests also won significantly more of the
feeding contests than losers. Dominance has often been linked to the ability
of crustaceans to obtain valuable resources (Goessmann et al. 2000).
Orconectes rusticus, an invasive species, dominated native Orconectes sp., in
both agonistic contests and shelter trials (Klocker and Strayer 2004). Another
study of O. rusticus revealed that this species competed for food using aggressive
interactions, and that these interactions escalated as these resources
became limited (Capelli and Hamilton 1984). Procambarus clarkii (Girard)
(Red Swamp Crawfish) was able to aggressively dominate a sympatric species,
P. zonangulus Hobbs and Hobbs (Southern White River Crawfish), in
shelter competitions, indicating an aggressive advantage for P. clarkii (Blank
and Figler 1996). However, some studies suggest that, while dominant status
influences behavioral decisions, it has less of an impact on feeding and mating
success in crayfish (Fero et al. 2007). Results of the above-mentioned studies
and the present one disagree with Fero et al. and provide further evidence that
winners of agonistic interactions may dominate valuable resources.
242 Southeastern Naturalist Vol. 9, Special Issue 3
Because some of the results on initiation behaviors and behavioral preference
between winners and losers and among form categories in this study
disagree with findings of previous studies, behavioral dynamics in P. suttkusi
males may be species specific. Although reproductive form in P. suttkusi
males was not found to significantly influence dominance, winners of agonistic
contests were more likely to win feeding contests. When the data was
pooled, this study suggested that P. suttkusi individuals initiating agonistic
interactions are more likely to become dominant in size-matched contests
regardless of reproductive form. Thus, aggressive P. suttkusi males might
gain improved access to food resources. In this species, when size-matched
pairs were examined, dominance may be related to the internal motivational
state of the organism and not related to form. A more aggressively motivated
individual, regardless of the form, also tended to win the feeding competition.
Aggressiveness may be limited by survivability, as overly aggressive
individuals may be subject to increased predatory losses.
Acknowledgments
The authors thank Dr. Stephen Landers and Dr. Allen Tubbs for editorial
assistance. Thanks to Raymond McCall, Troy Baker, and Robert Yoder for field
assistance, and to Stuart Welsh for publication support. This project was supported
by the ALFA Fellowship at Troy University. The publication of this manuscript was
supported, in part, by the US Geological Survey Cooperative Research Unit Program,
including the West Virginia Cooperative Fish and Wildlife Research Unit.
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