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First published online November 30, 2007
Journal of Experimental Biology 210, 4428-4436 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.011288
Duration of socialization influences responses to a mirror: responses of dominant and subordinate crayfish diverge with time of pairing
Department of Biological Sciences, Brock University, 500 Glenridge Avenue, St Catharines, ON, L2S 3A1, Canada
* Author for correspondence (e-mail: amercier{at}brocku.ca)
Accepted 2 October 2007
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Summary |
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Key words: crayfish, dominance, agonistic behaviour, mirror image, Procambarus clarkii
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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; Edwards and Kravitz,
1997; Kravitz and Huber,
2003). When two crayfish are housed together, fighting between
them begins almost immediately. Fights include pushing, lunging, grasping and
striking (Bovbjerg, 1953;
Bovbjerg, 1956;
Hayes, 1975;
Tierney et al., 2000). A fight
will escalate until one member of the pair retreats and is deemed the loser of
the encounter. After a crayfish wins the first fight, it is more likely to win
successive fights, and the losing crayfish is less likely to win successive
fights. After a number of losses, the retreating crayfish will alter its
behaviour and begin to avoid the winning crayfish
(Issa et al., 1999;
Herberholz et al., 2001). It
is often at this point that an observer deems the winning crayfish as dominant
and the losing crayfish as subordinate. If more than two crayfish are housed
together, a linear dominance hierarchy is established, which is stable over
time (Lowe, 1956;
Issa et al., 1999;
Gherardi and Daniels,
2003).
Dominant and subordinate crayfish behave differently and have different
physiological characteristics (Yeh et al.,
1996; Yeh et al.,
1997; Krasne et al.,
1997; Edwards et al.,
2003). Dominant crayfish gain first access to shelter, food and
mates (Zulandt Schneider et al.,
2001; Herberholz et al.,
2003). Dominant crayfish frequently exhibit a threat display,
raising the body, extending the abdomen off the substrate and performing a
meral spread, in response to a conspecific
(Krasne et al., 1997;
Listerman et al., 2000). By
contrast, subordinate crayfish retreat, often via an escape tail
flip, in response to certain stimuli such as the presence of an opponent
(Huber et al., 1997;
Krasne et al., 1997;
Edwards et al., 2003).
Dominant crayfish also approach a conspecific more frequently than do
subordinate crayfish (Copp,
1986; Blank and Figler,
1996). When space is limited, burrowing, which creates a shelter,
is important and dominant crayfish burrow significantly more than do
subordinate crayfish (Herberholz et al.,
2003). Subordinate crayfish exhibit reduced excitability in the
lateral giant escape mechanism than do dominant crayfish
(Krasne et al., 1997). The
lateral giant tail flip escape is likely inhibited as a result of numerous
escapes performed during socialization.
Agonistic encounters between crayfish depend on visual, tactile and
chemoreceptive input (Rubenstein and
Hazlett, 1974; Bruski and
Dunham, 1987; Delgado-Morales
et al., 2004). Vision appears to play an important role in
fighting behaviours such as following and lunging
(Bruski and Dunham, 1987);
however, fighting occurs in the absence of vision
(Kellie et al., 2001;
Li and Cooper, 2002). Taction
appears to be important for striking, pushing and antennae tapping
(Bruski and Dunham, 1987).
Agonistic encounters also involve chemical cues
(Tierney and Dunham, 1982;
Hazlett, 1999;
Zulandt Schneider et al.,
1999; Zulandt Schneider and
Moore, 2000; Zulandt Schneider
et al., 2001; Breithaupt and
Eger, 2002), which play an important role in the intensity and
outcome of fights (Bergman et al.,
2003; Delgado-Morales et al.,
2004).
Responses to a reflective environment have been studied in many vertebrate
species (Schusterman et al.,
1967; Gallup,
1970; Pepperberg et al.,
1995; Craft et al.,
2003), but few studies have investigated reactions of
invertebrates to mirrors. The only crustaceans that have been reported to
respond to mirrors are hermit crabs
(Dunham et al., 1986), fiddler
crabs (McLean and Pratt, 2007)
and crayfish (Drozdz et al.,
2006; May and Mercier,
2006). Specific behaviours, such as turning and remaining in a
corner, are enhanced in crayfish by mirrors placed in an aquarium and even by
the reflection provided by the aquarium glass, but only in socialized crayfish
(Drozdz et al., 2006). Further
work has revealed that responses of crayfish to a reflection depend on
dominance rank (May and Mercier,
2006). In the latter study, crayfish were either paired or
isolated for two weeks and were subsequently observed in a test tank with
mirrors lining one half of the aquarium and a matte plastic lining the other
half. Dominant crayfish performed more cornering, turning and crossing on the
reflective side than on the non-reflective side. Subordinate crayfish did not
show differences with respect to these behaviours but performed more reverse
walking on the reflective side. Isolated crayfish exhibited no behavioural
differences between the two environments. In that investigation, crayfish were
paired for two weeks. It is possible, however, that the responses to the
reflective environment might develop sooner and that they might change over
time. Others have demonstrated that certain behaviours change with the length
of time during which crayfish are socialized
(Issa et al., 1999;
Kellie et al., 2001;
Gherardi and Daniels,
2004).
In the present work, crayfish were paired for 30 min, which is sufficient
to produce dominance ranks (Lowe,
1956; Herberholz et al.,
2001). Dominant and subordinate crayfish were observed for 30 min
independently in a test tank that consisted of an aquarium with one half lined
with mirrors and the other half lined with non-reflective plastic. A separate
group of crayfish was housed in pairs for 3 days, and a comparison group
consisting of crayfish housed in a large community tank were also observed in
the test tank. The frequency and duration of behaviours previously shown to be
enhanced by reflection were calculated for each crayfish group
(Drozdz et al., 2006;
May and Mercier, 2006).
Results indicated that responses of dominant crayfish paired for 30 min were
similar to those of dominant crayfish paired for two weeks. By contrast,
subordinate crayfish required 3 days of socialization for behaviour to
resemble that of subordinate crayfish paired for two weeks.
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Materials and methods |
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All crayfish were initially housed for at least one week in a large round community tank with a depth of 70 cm and a diameter of 120 cm. This tank typically housed 30 crayfish at one time and contained ample rocks and PVC tubing for shelter. Three groups were used in this study. One group consisted of 40 crayfish that were paired for only 30 min. The second group consisted of 40 crayfish that were paired for 3 days. The third group served as a control and included 20 crayfish that came directly from the same community tank as all the other crayfish and were treated in exactly the same way but were never paired.
All crayfish were fed ad libitum three times weekly with artificial crab meat obtained from local grocers. All containers were maintained in a controlled environment with a 12 h:12 h light:dark photoperiod with both water and room temperature from 18 to 21°C.
Dominance testing
For the 30 min group, 40 crayfish were taken randomly from the large
community tank, each placed into a separate plastic container (30
cmx17.5 cmx13 cm) and transported from the housing facility to a
testing room. The crayfish were then left isolated in containers for 30 min
prior to testing. Two crayfish of approximately the same size were moved,
using a plastic flower pot to reduce handling, into a new plastic container of
the same size, containing filtered, aerated water. Crayfish in each pair were
matched to within 10% of rostrum-to-telson length. There were no significant
differences between masses or lengths of the eventual dominant (mass
38.9±8.7 g, length 9.6±0.6 cm) and subordinate crayfish (mass
37.3±8.0 g, length 9.4±0.6 cm) paired for 30 min or between
dominant (mass 37.4±8.8 g, length 9.6±0.6 cm) and subordinate
crayfish (mass 35.1±8.3 g, length 9.2±0.7 cm) paired for 3 days.
Each pair remained together for 30 min and were observed by the researcher
during this period to determine the dominance rank. The small size of the
container encouraged contact between the crayfish, and fighting began almost
immediately. Typically, the encounters were initiated by one crayfish
approaching with raised chelae. The encounters escalated to include pushing,
lunging and striking. This behaviour led to retreat of the losing crayfish by
means of walking backwards and tail-flip escape behaviour, and eventually the
loser avoided the winning crayfish. These behaviours have previously been
described in detail (Bovbjerg,
1953; Copp, 1986;
Bruski and Dunham, 1987;
Huber and Delago, 1998;
Lundberg, 2004). A crayfish
that retreated from the first fight often retreated from subsequent fights.
After retreating from a number of fights, the losing crayfish always changed
its behaviour by no longer engaging in contact and by avoiding the other
crayfish. Such avoidance behaviour by the losing crayfish always appeared
within 30 min, and this crayfish was deemed subordinate; the winning crayfish
was deemed dominant. This method of determining dominance rank has been used
reliably and repeatedly (Guiasu and
Dunham, 1997; Goessmann et
al., 2000; Bergman et al.,
2003) and resulted in 20 dominant and 20 subordinate crayfish.
For the 3-day group, 40 crayfish were taken from the community tank, arranged in pairs according to size, and maintained in pairs for 3 days. Each pair was housed in a plastic container measuring 58 cm long x 30 cm wide x 35 cm high. Each container was filled with filtered, aerated water and contained one PVC pipe, measuring 10–15 cm in length, to serve as a shelter. Paper towels were placed between tanks to prevent visualization of other crayfish pairs. Each pair was observed for the first 30 min of socialization. Fights followed the same pattern, and dominance rank was determined as described above. Each pair was subsequently observed for the following two days to ensure that the rank remained stable. Dominant crayfish always occupied the shelter and gained first access to food. No rank reversals were observed during the course of this experiment.
Reflection testing
Each crayfish was tested for responses to reflective surfaces in a
specially constructed glass aquarium measuring 52 cm long x 25 cm wide
x 30 cm deep. Half of the tank's perimeter was lined with mirrors. This
included one end wall and half of each of the adjoining walls, including two
corners, which provided a reflective environment on one side of the aquarium.
A non-reflective environment was created by lining the other half of the
aquarium with a semi-transparent matte plastic. White paper was placed
underneath the tank to provide stronger contrast for videotaping. The aquarium
was filled approximately 15 cm deep with filtered, aerated water that was
replaced between trials.
Animals paired for 30 min were tested for responses to reflection immediately following the socialization period. Animals paired for three days were placed in a plastic container (30 cmx17.5 cmx13 cm) together and were transported from the housing facility to the testing room. They remained paired for 30 min to reduce the effects of any stress created during transportation and were tested immediately thereafter. Animals housed in the community tank were transported in the same manner and were given the same 30 min acclimation period before being tested.
Each crayfish was placed gently, using a flower pot to minimize contact, into the centre of the test aquarium, facing one of the midlines separating the two environments. The dominant or subordinate member of each pair was alternatively chosen to be tested first; thus, 50% of each group were tested first, and 50% were tested 30 min later. The experiment was also counterbalanced to remove the effects of any preference for one side of the room. The aquarium was turned between trials so that half of all crayfish tested experienced the mirrored environment on the left, and the others experienced it on the right.
Crayfish activity was videotaped for 20 min using a webcam (Logitech,
Freemont, CA, USA) mounted 30 cm above the aquarium. Video files were acquired
using Windows Movie Maker and burned to CD for later analysis.
Table 1 provides a full
description of all behaviours analyzed in the present report. Cornering,
turning towards corners, crossing and reverse walking have been described in
earlier reports on responses of P. clarkii to reflective environments
and were examined here because they have been shown to be enhanced by
reflection (Drozdz et al.,
2006; May and Mercier,
2006). These behaviours were also examined because we thought the
results might provide some insight into whether or not crayfish respond to the
mirror image as they would to a conspecific (see Discussion). Freezing,
defined as ceasing all visible movement (including all appendages) for a
minimum of 5 s, was not examined in earlier studies of reflective environments
but has been described in other reports
(Gherardi and Peraccini, 2004;
Lundberg, 2004).
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Turning towards corners
Turns can occur at a number of locations in the aquarium but commonly occur
at or near the corners. Turns towards the corner occurred when the crayfish
was within one body length of the corner and changed its direction to turn
towards a corner. Crayfish that were paired for 30 min turned more frequently
towards reflective corners than non-reflective corners
(Fig. 2) (paired
t-test; dominant, P<0.005; subordinate,
P<0.005). After 3 days of pairing, only dominant crayfish turned
towards corners more on the reflective side of the tank than on the
non-reflective side (P<0.05). Subordinate crayfish paired for 3
days (P=0.17) and group-socialized crayfish (P=0.6) showed
no preference for turning towards reflective corners.
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), the
following outcomes were considered: crossing towards a reflective wall
vs a non-reflective wall, and crossing away from a reflective wall
vs a non-reflective wall. Group-socialized crayfish did not cross
towards reflective walls any more often than towards non-reflective walls
(Fig. 3A) (paired
t-test, P=0.42). Following 30 min of socialization, dominant
crayfish crossed more often towards reflective walls (P<0.005),
but subordinate crayfish did not (P=0.11). This pattern remained the
same after 3 days of pairing, when again only dominant crayfish crossed
towards reflective walls more often than non-reflective walls
(P<0.05), and subordinates showed no preference
(P=0.24).
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Crossing away from reflective versus non-reflective walls was also quantified. Group-socialized crayfish crossed away from reflective walls more than non-reflective walls (Fig. 3B) (paired t-test, P<0.05). After 30 min of pairing, both dominant (P<0.005) and subordinate (P<0.05) crayfish also crossed away from reflective walls more than non-reflective walls. After 3 days of pairing, this pattern changed. Although dominant crayfish continued to cross away from non-reflective walls more frequently than reflective walls (P<0.005), subordinate crayfish did not (P=0.24).
Freezing
`Freezing' behaviour commonly occurred at or near the mid-line of the
aquarium but was observed anywhere in the tank. Both dominant and subordinate
crayfish paired for 30 min froze more frequently in the reflective environment
than in the non-reflective environment
(Fig. 4) (paired
t-test; dominant, P<0.0001; subordinate,
P<0.005). After 3 days of pairing, both dominant
(P<0.05) and subordinate (P<0.005) crayfish exhibited
freezing more often on the reflective side of the tank than on the
non-reflective side. Group-socialized crayfish did not show a preference for
freezing in either environment (P=0.06), but the results approached
statistical significance, with a trend suggesting more freezing on the
reflective side.
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Reverse walking
Reverse walking occurred infrequently but was not associated with any
external disturbance. Group-socialized crayfish performed reverse walking more
often on the mirrored side of the aquarium than on the non-reflective side
(Fig. 5) (paired
t-test, P<0.005). Neither dominant nor subordinate
crayfish paired for 30 min exhibited a preference for reverse walking on
either side of the aquarium (dominant, P=1.0; subordinate,
P=1.0). By contrast, after 3 days of pairing, both dominant
(P<0.05) and subordinate (P<0.05) crayfish reverse
walked more frequently on the mirrored side of the tank.
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Overall activity
Overall activity level was assessed in two ways. First, the number of times
each crayfish crossed the midline of the tank, leaving one environment and
entering another, was measured. Second, the number of occurrences of all
behaviours examined (cornering, turning, crossing and reverse walking) were
combined and were used to determine if there was a difference between crayfish
groups with regard to overall activity level. An ANOVA revealed a difference
between groups for the number of times crayfish crossed the aquarium midline
(Fig. 7) (P=0.012). A
Tukey HSD post-hoc analysis revealed that dominant crayfish paired
for 3 days crossed the midline more frequently than did dominant crayfish
paired for 30 min (P<0.05) and subordinate crayfish paired for 30
min (P<0.05). When behaviours on both sides of the tank were
combined, an ANOVA found no difference in activity level between crayfish
groups, but the results approached statistical significance (P=0.06).
The total behavioural events for each crayfish group were as follows:
group-socialized, 33.2±18.1; 30 min dominant, 29.4±9.5; 30 min
subordinate, 28.5±15.5; 3-day dominant, 40.0±14.6; and 3-day
subordinate, 38.2±14.9.
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) are
included for comparison. After 30 min of pairing, when social rank is first
emerging, dominant and subordinate crayfish respond with remarkable similarity
to a reflective environment. Their responses are essentially the same for
seven of the nine behavioural features examined in the present work
(Table 2), and their responses
are also nearly identical to those of dominant crayfish paired for 2 weeks,
reported previously (May and Mercier,
2006). Thus, the first few agonistic encounters appear to produce
a similar pattern of behaviours with respect to reflection in all crayfish
regardless of whether they win or lose, and this pattern is essentially the
same as that exhibited by dominant crayfish after 2 weeks of pairing. After 30
min of pairing, seven of nine responses of dominant and subordinate crayfish
to the reflective environment differ from those of group-socialized crayfish
(which were not paired). Thus, pairing appears to cause the emergence of a
dominant pattern of responses to reflection in all crayfish initially. After 3
days of pairing, several results indicate that responses of dominant and
subordinate crayfish diverge (Table
2). First, dominant crayfish show nearly the same pattern as at 30
min of pairing, with eight of nine behaviours the same, and the pattern is
also nearly identical to that of dominants after 2 weeks
(May and Mercier, 2006).
Second, subordinates on day 3 respond differently from subordinates paired for
30 min in eight of nine behaviours. Thus, the behaviour of subordinates
changes between 30 min and 3 days. Third, the responses of dominant and
subordinate crayfish after 3 days differ in seven of the nine behaviours
examined. Thus, the subordinate pattern of responses to a reflective
environment develops more slowly than the pattern observed in dominant
crayfish (May and Mercier,
2006).
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In previous work (Drozdz et al.,
2006; May and Mercier,
2006), isolated crayfish were used as a comparison group. In those
studies, however, the experimental groups were isolated for 2 weeks prior to 2
weeks of pairing, in an attempt to reduce or extinguish effects of
socialization in the community tank before pairing occurred. Those paired
crayfish responded differently to the reflective environment of the
mirror/matte tank than did crayfish that had been isolated for a total of 4
weeks (Drozdz et al., 2006;
May and Mercier, 2006). In the
present work, no attempt was made to reduce or extinguish effects of
socialization in the community tank prior to pairing. Crayfish housed in a
community tank were used as a comparison group because their holding
conditions and social environment were identical to those of the experimental
groups except for the fact that they were not paired. In fact, they came from
the same holding tank as the paired groups.
Crayfish housed in large groups naturally develop a stable dominance
hierarchy over time (Bovbjerg,
1953; Lowe, 1956).
Once the hierarchy is established, fighting behaviour and preludes to fighting
behaviour decrease over time (Copp,
1986; Goessman et al., 2000;
Bergman et al., 2003).
Group-socialized crayfish, therefore, would constitute a mix of varying
degrees of dominance states. Results in the present work are consistent with
this interpretation. If the different patterns of response to reflection
exhibited by dominant and subordinate crayfish after 3 or 14 days
(Table 2) represent two
extremes, one would predict that a group of crayfish with mixed dominance
ranks would behave differently from those two extremes, since they would not
all be dominant or subordinate. The group-socialized crayfish differed from
3-day dominant crayfish in six of the 10 behaviours examined and from 3-day
subordinate crayfish in four of 10 behaviours
(Table 2). Group-socialized
crayfish also differed from 14-day dominant crayfish in six of nine behaviours
and from 14-day subordinate crayfish in five of nine behaviours
(May and Mercier, 2006) (see
Table 2).
The patterns of responses to reflection were very similar between dominant and subordinate crayfish after 30 min of pairing, but they were not identical (Table 2). At this time, dominant and subordinate crayfish both performed more cornering and spent more time cornering in the reflective environment. They both turned towards and away from reflective corners more than non-reflective corners and crossed away from reflective walls more frequently. Both groups also spent more time on the mirrored side of the tank compared with the matte side. All eight of these features are also exhibited by dominant crayfish after 14 days of pairing. Several of these features suggest that the crayfish either seek the reflection or show a preference for it. Such features include more cornering on the mirrored side, more time cornering on the mirrored side, more turning towards reflective corners than non-reflective corners and more time spent in the reflective environment. The only differences between dominant and subordinate crayfish after 30 min of pairing were that subordinates did not perform more turns at the side in the reflective environment and did not cross more frequently towards reflective walls than non-reflective walls.
After 3 days of pairing, dominant crayfish continued to behave in a manner suggesting preference for the reflective surfaces. They performed more cornering, spent more total time cornering and spent more total time on the reflective side, and they turned more frequently towards reflective sides and crossed more frequently towards reflective sides. At the same time, subordinate crayfish showed no preference for any of these behaviours with regard to reflection. The only features shared by dominant and subordinate crayfish after 3 days were freezing and reverse walking.
Hazlett found that hermit crabs required 3 days of socialization to
effectively alter their behaviour
(Hazlett, 1966). In those
experiments, subordinates moved away or retreated into their shell in response
to a conspecific after 3 days. Although the responses to a reflective
environment after 3 days of pairing are very similar to those reported
previously for crayfish paired for 14 days
(May and Mercier, 2006), the
patterns are not identical (Table
2). After 3 days, subordinate crayfish turned significantly more
at the side of the tank in the reflective environment than in the
non-reflective environment, but no such difference occurred for subordinate
crayfish after 14 days of pairing. After 14 days of pairing, subordinate
crayfish spent more time in reflective corners and turned away more frequently
from reflective corners, but after 3 days of pairing, neither of these
responses was observed. Although dominant crayfish did not perform more
reverse walking in either environment on day 14, they did perform
significantly more reverse walking in the reflective environment on day 3.
These few differences between responses on days 3 and 14 cannot be attributed
to differences in the testing environment because the test tank and testing
procedure were identical to those used in the earlier study
(May and Mercier, 2006). Some
behaviours associated with the reflective environment might be labile and may
take more time to stabilize as dominance rank is established. Dominance rank
itself can be labile, as indicated by reversals in rank reported by others
(Goessmann et al., 2000;
Delgado-Morales et al., 2004;
Song et al., 2006). Although
no rank reversals occurred in the present investigation, responses to the
reflective environment might be more labile than dominance rank.
Alternatively, some behaviours (e.g. reverse walking) are infrequent, which
might give rise to sampling errors. The current results indicate that many
behaviours should be monitored when attempting to distinguish responses to
reflection between different groups of crayfish.
It is important to assess overall activity of crayfish reported in this study to determine whether or not differences between dominant and subordinate crayfish merely reflect differences in activity level. For example, did subordinate crayfish fail to respond to reflection simply because they were inactive? Although the data approached statistical significance (P=0.06), there was no significant difference in total events for all behaviours when data from reflective and non-reflective sides were combined. The largest differences in activity occurred between different days, and this was true for both dominant and subordinate crayfish. On any given day, however, dominant and subordinate crayfish exhibited very similar activity levels. This observation suggests that dominance rank is associated with changes in the distribution of the various behaviours on the reflective and non-reflective sides rather than a change in the total number of behavioural events in the tank. Dominant crayfish paired for 3 days crossed the midline between reflective and non-reflective environments more frequently than either dominant or subordinate crayfish paired for 30 min (Fig. 7). Occasionally, crayfish would walk back and forth across the midline without travelling far into either side. This type of movement required little locomotion but increased the frequency of midline crossing. Such behaviour might explain why dominant crayfish on day 3 crossed the midline more frequently than other crayfish and yet spent more time in the reflective environment.
Several results suggest but do not prove that crayfish perceive their
mirror image as a conspecific. After 3 days of pairing, subordinate crayfish
no longer corner more frequently in the reflective environment or spend more
time in reflective corners, and they no longer cross more frequently towards
reflective walls or turn more frequently towards reflective corners. These
observations are consistent with the notion that subordinate crayfish avoid
the reflection or no longer seek it out. A crayfish that loses its first
agonistic encounter is more likely to lose subsequent encounters and will
retreat from and avoid a winning crayfish
(Bruski and Dunham, 1987;
Huber and Kravitz, 1995;
Guiasu and Dunham, 1997;
Tierney et al., 2000). This
has been referred to as the `loser effect'
(Goessmann et al., 2000). Once
rank has been established, subordinate crayfish initiate contact with
conspecifics less frequently than do dominant crayfish, and subordinates do
not approach dominant crayfish as frequently as they approach naive crayfish
(Rubenstein and Hazlett, 1974;
Copp, 1986). If crayfish
perceive the mirror image as a conspecific, subordinate crayfish should avoid
reflective surfaces more frequently the longer they are paired. In the present
study, subordinate crayfish spent more time in the reflective environment
after 30 min of pairing but did not do so after 3 days of pairing. However,
they did not spend significantly more time in the non-reflective
environment.
An alternative explanation for the results reported here is that crayfish
respond to movement seen in the mirror and not to the image per se.
It is possible that dominant crayfish seek out the movement in the mirror and
subordinate crayfish do not. This difference may be a result of learned
submissiveness in subordinate crayfish. Further work is required to answer the
question of whether the crayfish views its mirror image as a conspecific. This
question could be addressed by comparing responses to reflection with
responses to a live conspecific viewed through a transparent barrier. Under
such conditions, one could also test for other known responses to conspecifics
such as increased urination (Breithaupt and
Eger, 2002) and an increase in heart rate
(Listerman et al., 2000).
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Acknowledgments |
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References |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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