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SIGNALLING BEHAVIOUR IN THE FIDDLER CRAB, UCA VOCANS
Introduction
Fiddler
crabs inhabit intertidal sand and mud flats, living in dense colonies. The male
fiddler crabs have one massively enlarged claw, which is used in fighting with
other males and for signalling to females. Pope (1999) investigated claw waving
in male fiddler crabs in response to both rival male signals and receptive
females. In agreement with an original study by Salmon and Stout (1962), Pope
found that males waved significantly more in the presence of females regardless
of whether other males were present. Hyatt and Salmon (1997) investigated the
behavioural response of male combat in fiddler crabs, furthering work (Crane
1966, 1975) that identified 21 combat acts. Studies by Hazlett (1968) and Schein
(1975) indicate that a combatent who is larger has an advantage in winning
fights. Research by Land has shown that larger rivals are perceived as a threat
by male crabs which will retreat into their burrows, whereas similar or smaller
crabs evoke normal competitive claw waving behaviour.
Aims
This
study aims to examine a range of possible features of waving claws that may
influence the responses of target males in their waving behaviour. This will be
achieved by manipulating the appearance of a model claw in the following ways:
1.
Size of model claw- slow, medium and large
2.
Speed of waving model claw- slow, medium and fast
3.
Distance of model claw to the subject – (e.g. 2cm, 1cm, 0cm). This will
be manipulated to allow a match in the apparent size of different sized model
claws at different distances (e.g. using a medium sized claw at a close distance
to appear the same size as a large claw at a greater distance). This will assess
whether the crabs use this simple rule to judge the apparent threat of a rival
as a simple combination of size and distance.
Method
A
sample of fiddler crab claws will be assessed to establish the natural range in
claw sizes in the population. The extreme values in this data will be used to
determine the two marginal manipulation values, with the third one being
equidistant between the two. Model claws will be taken from dead crabs or those
found released by autotomy. A container will be made out of wood to prevent any
reflections that would occur from using a glass tank. A sample crab will be
place in the middle of the container with enough room to escape backwards and
about 1m of space infront for the model to be manipulated. Once the sample crab
has acclimatised to its new surrounding its response will be recorded when
stimulated by the models. The model claws will be glued onto a stick and pushed
to a measured distance in front of the crab where waving response will be
recorded. Each male crab will be
marked with a number on its carapace allowing the results to be repeated. Each
of the 27 possible treatment combinations will be presented to each subject in a
randomised order with sufficient time before hand to allow the crab to
acclimatise.
Crabs
respond more aggressively to a similar or smaller sized claw at a closer
distance. If the crab is stimulated
with a larger claw then it will retreat unless the claw is at 0cm when the crab
must attack as a last resort. None of the crabs showed any habituation to the
stimulus.
The
percentage waving responses of a target male crab to each experimental treatment
will be analysed in a within-subjects design. The effects of the model claw
size, distance and waving frequency will be analysed using a three-way repeated
measure ANOVA, or in a number of smaller models if all possible treatment
combinations are unavailable for too many subjects. Additional variables could
be used to explain additional amounts of the variation in responses within these
models.
A final year
dissertation report will be completed by Paul Rollinson, University of Bangor by
May 2003.