Ra
,
ia
d
t
m
y
moving between burrows, crabs are at risk of predation by birds and they sometimes run to objects that
,
eprovide temporary cover. Thus, the female preference for burrows with hoods may help females to avoid
predators. Could selection for predator avoidance produce a directional preference for especially large
hoods? To examine this possibility, we made multiple replicas of two kinds of hood models with exagger-
ated dimensions (super models and wall models) and a single model with average dimensions (average
models). Super models were four standard deviations taller and two standard deviations wider than aver-
age-size natural hoods. Wall models were of average height but near the maximum width of natural hoods;
when males ?overbuild?, they construct wall-like hoods. We replaced males? hoods with these models and
measured their effects on male attractiveness. Males with exaggerated models neither encountered nor at-
tracted more females than did males with average models or natural hoods. Females did not show a direc-
tional preference for larger hoods. The attractiveness of hoods may plateau with increasing size because
discrimination between average and larger hoods may result in fatal hesitation, preventing the evolution
of a directional preference for an exaggerated form of this courtship signal. Males build hoods from pure sand,
so males may build unusually large hoods because they are more durable, not because they are better signals.
The Association for the Study of Animal Behaviour. Published by Elsevier Ltd.
Darwin (1871) observed that many male sexual signals
and behavioural displays are so exaggerated that they
must reduce survival and cannot therefore be favoured
by natural selection. Nevertheless, he argued, they may
still be bene?cial if females have an aesthetic sense that
leads them to prefer to mate with males who use the
brightest, loudest, most vigorous and otherwise extreme
signals and displays. It is now clear that aesthetic sensibil-
ities, as determined by properties of female sensory re-
sponse systems, do often affect mate choice (Bradbury &
Vehrencamp 1998; Endler & Basolo 1998; Ryan 1998; Ba-
solo 2000), and most studies of directional preferences
support Darwin?s prediction; when females show a bias
they prefer courtship signals of greater quantity as mea-
sured along nearly every stimulus dimension (Ryan &
Keddy-Hector 1992). Females may even harbour unex-
1951), those with stimulus values that exceed the upper
limits of current signal variation (e.g. Andersson 1982).
In general, directional, open-ended biases in female mate
preferences are thought to govern the evolution of ever
more exaggerated male courtship signals until they are
checked by natural (mortality) selection (Andersson
1994).
Courting males of at least 18 species of the approxi-
mately 100 species of ?ddler crabs (genus Uca; Rosenberg
2001) build structures of mud or sand at the entrances
to their burrows (17 structure-building species are listed
in Christy (1988a) and Christy et al. (2001), to which we
add Uca uraguayensis; P. Ribeiro, personal communica-
tion). Males of structure-building species use claw waving
and other visual and acoustic (seismic) displays to attract
females into their burrows for mating. In most species,No preference for exaggerated co
JOHN H. CHRISTY* & PAT
*Smithsonian Tropic
ySchool of Botany and Zoology
(Received 20 May 2005; init
?nal acceptance 14 November 2005; publishe
Courting male ?ddler crabs, Uca terpsichores, construc
burrows to which they attract females for mating. Fe
they stay with one and mate in his burrow, and the
ANIMAL BEHAVIOUR
doi:10.1016/j.anbpressed preferences for supernormal signals (Tinbergen
Correspondence: J. H. Christy, Smithsonian Tropical Research Institute,
Apartado 0843-03092, Balboa, Anco?n, Panama?, Repu?blica de Pan-
ama?; from the USA, Unit 0948, APO AA 34002 (email: christyj@si.
edu). P. R. Y. Backwell is at the School of Botany and Zoology,
Australian National University, Canberra, ACT 0200, Australia.
123
0003?3472/06/$30.00/0 The Aurtship signals in a sensory trap
ICIA R. Y. BACKWELL*?
l Research Institute
Australian National University
l acceptance 21 July 2005;
online 14 April 2006; MS. number: A10166)
unusually large sand hoods at the entrances to their
ales sequentially visit several courting males before
preferentially approach burrows with hoods. While
2006, 71, 1239?1246
hav.2005.11.008males attract females either by approaching them to
within a few centimetres and leading them with claw wav-
ing and other displays back to their burrows, or by staying
close to their burrows and directing vigorous waving to fe-
males from a distance. Receptive females visit a dozen or
more males in quick succession before they choose one
and stay in his burrow (deRivera & Vehrencamp 2001).
9
ssociation for the Study of Animal Behaviour. Published by Elsevier Ltd.
1240Male courtship structures might attract females at any of
three sequential steps leading to mating: (1) from a dis-
tance, affecting the rate that males encounter and court
females, (2) from nearby, affecting the rate that females
approach the males that court them, (3) at the burrow,
affecting the rate that females stay and mate with the
males they visit. Studies of the functions of the mud pil-
lars built by male Uca beebei (Christy 1988a, b) and the
sand hoods built by male Uca terpsichores (previously
U. musica) have shown that these structures attract females
only at the second step, after a male has directed claw
waving to a female. To con?rm that females visually orient
to structures as they approach courting males, we removed
males? hoods and replaced them with hood models posi-
tioned a few centimetres to the side of the burrow open-
ings. About 40% of the time, females moved to the
hood models, not to the courting males, especially when
males did not closely approach and lead the females to
their burrows. We also showed that females preferentially
moved to hoods over unadorned burrows when they ran
from a simulated predator (Christy et al. 2003a) and that
hoods effectively mimic pieces of wood, stones and shells
to which females run for cover (Christy et al. 2003b). Fi-
nally, females were just as attracted to courting males
with these natural objects at their burrows as they were
to males with hoods (Christy et al. 2003b). We concluded
that hoods elicit landmark orientation (Herrnkind 1983;
Langdon & Herrnkind 1985) and co-opt for mate choice
a behaviour that is selected by predation. Hoods can be
said to catch females in a ?sensory trap? (West-Eberhard
1984; Christy 1995) because they elicit a response that
mediates mate choice but is selected for another function,
predator escape. Females who are caught in this ?trap? may
bene?t directly; by preferentially moving to hoods, they
may move relatively quickly and without error from one
male?s burrow to the next and thereby minimize their ex-
posure to predators while they are between burrows and
without a safe refuge.
Hoods are, relative to male size, the largest structures
built by courting male ?ddler crabs (Crane 1975; Christy
et al. 2001). Here we report the results of ?eld experiments
to determine whether female U. terpsichores prefer hoods
that are as large as or larger (supernormal) than the largest
hoods. Our objective was to discover whether a directional
bias in the preference might have contributed to the exag-
geration of this signal and whether this bias is open-
ended.
METHODS
Study Site
We conducted this study on intertidal sand beaches,
bars and ?ats on the west bank of the Paci?c entrance to
the Panama Canal between the Bridge of the Americas and
Rodman Naval Station, about 1 km north. Uca terpsichores
lives in mixed-sex colonies with one crab per burrow,
except for mating pairs. Although the sexes are spatially
ANIMAL BEHAVIOUR, 71, 5intermixed, usually only vigorously courting males oc-
cupy burrows at the drier upper limit of the distribution.Models
We tested the attractiveness of two kinds of hood
models (super models and wall models) whose dimensions
were exaggerated relative to those of the average hood
(Fig. 1). On average (X
 SD, N ? 100; Christy et al. 2001),
hoods in the study area were 23.3
 3.17 mm high,
33.3
 5.35 mm wide at their base, and wider (0.57 
 0.061
of total width) on the side opposite the male?s large
claw (Fig. 2a, b). Supernormal models, ?super models?,
exaggerated hood height and overall size; they were
35.5 mm high, about four standard deviations higher
than average, taller than any natural hood that we mea-
sured, and 45 mm wide, about two standard deviations
wider than average (Figs 1b, 2a). Super models were
nearly symmetrical, with 0.54 of their width on the
wider side (Fig. 2b). Wall models also were 45 mm
wide but only 22.8 mm high, just less than average
(Figs 1c, d, 2a), and they were very asymmetrical
(0.69; Fig. 2b). We extended the walls of the wall models
to the left or right. In the experiments described below,
we matched the typical asymmetry between males and
their hoods by giving ?left-handed? models to right-
handed males and vice versa. Hood models were based
on a single concrete cast of a real hood of average di-
mensions that we built up with modelling clay. We
made latex moulds of the exaggerated models and cast
numerous replicas in grey concrete, to which we glued
sand from U. terpsichores habitat so that they appeared
realistic to us. We also used replicas of the average-sized
hood (Figs 1a, 2a, b) as controls for possible model ef-
fects, although no model effects have been found previ-
ously (Christy et al. 2002).
Tests of Attractiveness: Super Models
The ?ddler crab eye has an equatorial zone of acute
vertical resolution that the crab keeps aligned with the
horizon (Zeil et al. 1986; Zeil & Al-Mutairi 1996). Fiddler
crabs are best able to detect objects that are tall enough
to be imaged in their acute zone. This may explain, in
part, why they preferentially orient to tall vertical objects
(Langdon & Herrnkind 1985) and it may put a premium
on the height of the structures that males build to attract
females. Compared to no hood at all, hoods of average
height are not more likely to attract females from a dis-
tance or to increase male?female encounter rates (Christy
et al. 2002). However, the attractiveness of unusually tall
hoods was not tested. In this study, we determined
whether super models increase male?female encounter
rates, the ?rst step leading to mating, and whether super
models are subsequently more attractive to females as
they approach the males that court them. This second
step is when males with hoods are demonstrably more at-
tractive than are males without hoods.
Encounter frequencies
Although we can recognize when an encounter occursby observing that a male directs courtship to a female, we
cannot determine when an encounter could have
41occurred but did not. For direct measures of the effect of
courtship structures on male?female encounter frequen-
cies, one needs to establish an objective criterion for when
an encounter could take place and then record the
number of times that the male and female do and do
not encounter each other. We have not been able to do
this. Therefore, we measured the effects of courtship
structures on male?female encounter frequencies indi-
rectly by measuring the frequencies that females stopped
at burrows with different structures (visit frequency) and
the frequency that females moved to the males from each
structure category that courted them (approach fre-
quency). Visit frequencies are the product of encounter
frequencies and approach frequencies. Hence, we mea-
sured two of these frequencies and solved for the
unknown third. We determined whether the different
male and hood model combinations affected encounter
frequencies by comparing (G test of goodness-of-?t with
William?s correction Sokal & Rohlf 1995) the relative visit
frequencies to each male and structure combination with
the relative frequencies that are expected based only on
the approach frequencies to the same combinations (see
also Christy et al. 2002). This tests the null model of no
effect of structure type on encounter frequencies.
We found previously (Christy et al. 2002) that removing
we assume that models of different size do not affect male
behaviour differently and that differences in the responses
of females to male?model combinations are determined
by differences in their responses to the structures.
Visit frequencies
On each of 4 days, about 30 min before low tide, we
marked 120 hooded burrows with small sticks placed
about 15 cm from each burrow. For every three adjacent
males, we left the natural hood on the ?rst burrow and re-
placed the hoods on the second and third with average
and super models, respectively. Thus, 40 males of each
male?model combination were approximately evenly dis-
tributed across the observation area. We changed the loca-
tion of our observations daily to minimize the chance of
observing the same males on subsequent days. Beginning
at low tide and continuing until about 30 min before the
rising tide covered the area, we tallied the number of times
females visited males with the different hoods. We contin-
uously scanned the area to record the responses of as
many mate-searching females as possible. We did not
identify individual females, so our tally probably included
several visits by each female. We noted the time when
males lost their burrows, mated or ceased active courtship
Figure 1. Model hoods used to test the effects of hood dimensions on male attractiveness. Models were made of concrete and were coated
with sand. Models are on the left and a natural hood and the male fiddler crab, Uca terpsichores, that built it are on the right of each frame. (a)
Average model; (b) super model; (c) ?left-handed? wall model (asymmetrical to the right when viewed from the front), as might be built by this
right-handed male; (d) ?right-handed? wall model.hoods does not affect male courtship and that females vi-
sually orient to hoods as well as to the courting male. HereCHRISTY & BACKWELL: BIG SIGNAL, NO PREFERENCE 12for other reasons or had a badly damaged hood (if a natural
one), and we subsequently excluded them from the pool
1of males who could receive visits. We used a G test of
goodness-of-?t to determine whether the males with dif-
ferent structures received the relative number of visits ex-
pected based on the relative amount of time that they
were available for visits.
Approach frequencies
We recorded the frequency that females approached
courting males without hoods and courting males with
natural hoods, average models and super models. On each
of 9 days, about the time of low tide, we replaced natural
hoods of 15?30 males with models of each kind. We
watched individual mate-searching females for several
courtship interactions and recorded whether they passed
or approached each male. Each courtship was a unique
male?female pair, and we treated each as a statistically
independent observation. Use of more than one observa-
tion per female does not bias conclusions about mate
preferences in this species (Christy et al. 2002). We used G
tests of independence to determine whether the frequen-
Average Wall
Super
14
16
18
20
22
24
26
28
30
32
34
36
15 20 25 30 35 40 45 50 55
Width (mm)
H
ei
gh
t 
(m
m
)
(a)
Wall
Super
Average
0.5
0.55
0.6
0.65
0.7
0.75
15 20 25 30 35 40 45 50 55
Width (mm)
A
sy
m
m
et
ry
 
(b)
Figure 2. Dimensions of natural hoods (B), average model hoods
(,), wall model hoods (>) and super model hoods (6) used to
test the effects of hood dimensions on male attractiveness. (a)
Hood height in relation to width. (b) Hood asymmetry in relation
to width. Asymmetry is the larger proportion of the total width of
a hood on either side of the burrow opening.
ANIMAL BEHAVIOUR, 71, 5242cies with which females approached males depended on
the kind of structure they had.Tests of Attractiveness: Wall Models
Wall models were about the same height as natural
hoods, so we had no reason to expect, based on our
previous research and our understanding of the visual
system of ?ddler crabs, that they would be more attractive
to females from a distance. Hence, we did not measure the
possible effect of wall models on visit rates or encounter
rates. However, when males make exaggerated courtship
structures, they build them wider, like the wall models,
not wider and taller, like the super models. If a directional
female preference has shaped hood design, then wall
models should be more attractive to females than natural
hoods once females are courted by males.
We followed the same general, analytical and statistical
methods described above to test whether wall models
affected the frequency that courted females approached
males. When measuring the attractiveness of super
models we found, as in previous studies (Christy et al.
2002), that hood builders that had their hoods removed
were signi?cantly less attractive to females than were
hood builders that had hoods on their burrows. Therefore,
we excluded hoodless hood-building males from this ex-
periment, leaving the comparisons between males with
natural hoods and those with average, super and wall
model hoods.
We de?ne a sexually receptive female as one that
chooses a mate during a given activity period and a non-
receptive female as one that does not chose a mate. Both
receptive and nonreceptive females move on the surface,
approach courting males and stop at male burrows.
Receptive females move more quickly and directly be-
tween males, feed little and seldom investigate empty
burrows or attempt to take over burrows from other
females. We made most of our observations during peak
days of the biweekly courtship cycle and we made every
effort to record the responses only of receptive females.
Nevertheless we probably watched some nonreceptive
females. However, the resulting errors should be small,
because receptive and nonreceptive females do not differ
in their responses to courting males with hoods or in their
responses to courting males without hoods (Christy et al.
2002), suggesting that their responses to hoods of differ-
ent sizes may be similar as well.
We followed the procedures in Cohen (1988) to conduct
power analyses for all nonsigni?cant statistical tests of
hypotheses.
RESULTS
Visits to Super Models
Over the 4 days of this experiment we accumulated
66 411 male-minutes of observation on males with natural
hoods (proportion of total ? 0.302), average models
(0.347) and super models (0.351). We had slightly fewer
observation minutes of males with natural hoods because
some hoods disintegrated during each observation period
but none of the concrete models did. We saw 944 visits.
We used these proportions to calculate the expected visit
frequencies to males with the different structures under
43the null hypothesis that the kind of structure that a male
had did not affect the frequency that he was visited. The
differences between the observed and expected visit fre-
quencies under the null model were extremely small and
nonsigni?cant (G2 ? 0.1276, P ? 0.938; Table 1). With
N ? 944, the power to detect a small effect (w ? 0.10)
was about 80%.
Approach to Super Models
Over 9 days we observed 1596 interactions between 203
females and hood-building males without hoods, with
natural hoods, with average models and with super
models. The frequencies with which females approached
these males differed signi?cantly (G3 ? 128.9505,
P ? 0.000; Table 2); females approached males without
hoods less often than males with hoods. There was no sig-
ni?cant difference in the approach frequencies of females
to males with natural hoods, average models or super
models (G2 ? 0.9437, N ? 604, P ? 0.624; power to detect
a small effect approximately 58%).
Encounter Rates of Males with Super Models
There was no signi?cant difference in female encounter
rates of males with natural hoods, average models and
super models (G2 ? 1.3114, N ? 944, P ? 0.519; power to
detect a small effect approximately 80%).
Approach to Wall Models
Over 13 days we observed 485 interactions between 106
females and courting males with natural hoods, average,
super and wall model hoods. There was no signi?cant
difference between the frequencies that females ap-
proached males with these different hoods (G3 ? 1.0527,
P ? 0.789; Table 3). Wall models, models of the largest
hoods that males build, were not more attractive (power
to detect a small effect approximately 36%).
DISCUSSION
Female preferences for more intense, frequent or other-
wise extreme male courtship signals are fundamental for
Table 1. Female visit frequencies to courting hood-building male
Uca terpsichores with natural or model hoods
Type of
hood
Observed
visits
Expected
visits
Observed
expected
Natural hood 280 285 5
Average model 331 328 ?3
Super model 333 331 3
Total 944
The expected frequencies were calculated from the proportions of the
total observation time (66 411 male-minutes) that males with the
different hoods were available for visits; natural hoods: 0.302; average
models: 0.347; super models: 0.351.the operation of good genes (indicator) and Fisher?s
(1958) two-step mechanisms of sexual selection (Ander-
sson 1994; Bradbury & Vehrencamp 1998; Kokko et al.
2003). In contrast, directional preferences are not re-
quired for the evolution of signals that elicit responses
governed by biases in sensory systems, such as those me-
diating sensory traps (Christy 1995; Endler & Basolo
1998). Biases exist at all levels in sensory response sys-
tems and they can evolve for many reasons (Basolo
2000), not only because individuals bene?t from
responding to stimuli of greater quantity. Courtship sig-
nals that are tuned to these biases will be most effective,
whether or not they are exaggerated. However, signal
degradation during transmission often may favour signals
of greater quantity simply because they are easier to
detect. This pertains especially to signals that attract re-
ceivers to senders. Thus, sensory response biases may
often produce preferences based on perceived signal
intensity (Parker 1983) or ?ef?cacy? alone (Guilford &
Dawkins 1991; Johnstone 2000; Ryan & Cummings
2005), leading to signal exaggeration. We therefore ex-
pected that female U. terpsichores would prefer larger
hoods. Our sample sizes were, in general, large enough
to detect with reasonable con?dence even a small
directional preference.
Female U. terpsichores did not prefer hoods of a size near
the upper extreme of the current distribution of hood sizes
(wall models), nor did they prefer hoods that were larger
than those males ever make (super models). Compared
to natural hoods and average models, super models were
not more attractive to females from a distance, nor were
super models or wall models more attractive to females
as they viewed and interacted with courting males. Com-
parisons of the time budgets of males with and without
structures (Christy 1988b; Christy et al. 2002) and experi-
ments in which males were given supplemental food
(Backwell et al. 1995; T. Kim & J. Christy, unpublished
Table 2. Approach frequencies by 203 females to courting hood-
building male Uca terpsichores without hoods or with natural, aver-
age or super model hoods
Type of hood Pass Approach Totals % Approach
None 584 408 992 41
Natural hood 95 239 334 72
Average model 44 90 134 67
Super model 42 94 136 69
Table 3. Approach frequencies by 106 females to courting hood-
building male Uca terpsichores with natural hoods or average, super
or wall model hoods
Type of hood Pass Approach Totals % Approach
Natural hood 37 72 109 66
Average model 38 66 104 64
Wall model 36 52 88 59
CHRISTY & BACKWELL: BIG SIGNAL, NO PREFERENCE 12Super model 32 52 84 62
124data) indicate that structure building depends on male
condition. Indeed, individual males build hoods, court
most vigorously and feed relatively little on only 1?2
days during each 8?10-day biweekly reproductive cycle
(Christy et al. 2001). The presence of a hood and perhaps
its size (although this has not been shown) both may in-
dicate a male?s phenotypic condition. Yet females are
not more likely to stay and mate with males with hoods
after they have reached their burrows and they are not
more attracted to males with larger hoods. These results il-
lustrate that one cannot deduce whether or how females
may bene?t from a mate preference from the observation
that production of the preferred signal depends on male
condition.
One possible reason why unusually large hoods were
not more attractive is that females did not perceive the
size difference between average and larger hoods. This
seems unlikely. The ?ddler crab U. pugilator responds to
objects as small as 1?1.3  angular size moving at 1 /s at
or above their visual horizon (Land & Layne 1995; Layne
et al. 1997). As U. terpsichores females approach a male?s
hood, they see the expanding upper edges of the hood
above their visual horizon (hoods are taller than crabs?
eyes) and we assume that they respond similarly. Uca terp-
sichores density averages about 12 hood-builders/m where
we observed them (Christy et al. 2001). Assuming an even
distribution of male burrows, the mean distance between
nearest neighbours would be about 30 cm (Clark & Evans
1954). Females orient to hoods when they are moving be-
tween males? burrows. Assuming females begin orienting
to hoods while they are about 20 cm away and that they
approach at about 5 cm/s, a moderate walking speed, an
average hood will increase in apparent height at about
2.18 /s, and a super hood will loom larger at about
3.34 /s, well within the female?s likely perceptual and
response thresholds.
Larger hoods may be more salient visual cues to the
locations of males? burrows but they did not elicit
a stronger orientation response. It is likely that small
hoods that are lower than the female?s visual horizon are
less conspicuous and do not readily elicit approach, but
this remains to be determined. There are signi?cant
positive but weak correlations between male body size
and hood height (R2 ? 0.0585) and width (R2 ? 0.0754,
N ? 100; Christy et al. 2001); either small males build rel-
atively large hoods or large males build relatively small
ones, or both. This pattern is inconsistent with direc-
tional selection for medium- and large-sized males to
build as large a hood as possible. One possibility is that
hoods are categorical signals of species identity that are
constrained to certain distinctive dimensions. Several ob-
servations indicate that this is not the case. First, hoods
and the pillars of U. beebei are not necessary for mate
choice. They affect the frequency with which females
will approach males but not whether they will stay and
mate (Christy 1988b; Christy et al. 2002). Second, struc-
ture building is notoriously variable within and between
populations (Crane 1975), making structures, at best,
unreliable signals of species identity. Third, structure pref-
ANIMAL BEHAVIOUR, 71, 54erences are not species speci?c (Christy et al. 2003a).
Fourth, U. terpsichores typically live on pure sand ?atsand bars with few or no other ?ddler crabs close by. At
the lower edge of its distribution, female U. terpsichores
occasionally encounter courting male U. stenodactylus
and U. deichmanni, neither of which build structures
and both of which have highly distinctive claw-waving
displays totally unlike that of U. terpsichores. Uca terpsich-
ores females readily distinguish and reject (with a speci?c
body-bobbing display) courting males of both species on
the basis of their behaviour alone (Zucker & Denny
1979). In summary we ?nd no support for the idea that
females have a categorical preference for hoods as a spe-
cies recognition signal.
Unlike biases that are incidental consequences of how
sensory systems develop, the responses that produce
sensory trap mate preferences have a history of selection
for some other function. This should constrain sensory
trap signals to conform to the stimuli that elicit the
response for the other function (Christy 1995); the re-
sponse should produce signals that are better mimics of
their models. This prediction has been con?rmed in stud-
ies of (1) the orange colour spots on the tails of male gup-
pys, Poecilia reticulata, that mimic fruits from the
cabrehash tree, Sloanea laurifolia (Rodd et al. 2002), (2)
the yellow terminal tail stripe of some males of some spe-
cies of Goodeinae ?sh that probably mimic damsel?y lar-
vae (Mac??as-Garcia & Ramirez 2005), and (3) the white
silk-wrapped nuptial gifts that male nursery web spiders,
Pisaura mirabilis, display to females that mimic females?
egg sacs (Sta?lhandske 2002).
However, sensory trap responses may not always pro-
duce directional selection for ever-closer conformity of
male signals to model stimuli. Females respond to the
mimetic signal as they do the model but the stimulus
properties of the signal may be relatively free to vary
depending on the rules that females use to recognize
models. In a previous study we tested the prediction that
females should approach hoods as they do other objects
that they use for landmark orientation. We found no
differences in the rates that mate-searching females
approached males with natural hoods or males whose
hoods were replaced with average models, stones, shells or
pieces of wood (Christy et al. 2003b). Females respond
categorically to these objects despite their many physical
differences. The present study extends these results by
showing that large hood size also does not affect the
strength of the female approach response. We suggest
a simple functional explanation. When a predator is hunt-
ing near a crab that does not have immediate access to
a burrow, including a female as she moves between the
burrows of potential mates, the crab may do best to run
to the nearest object, press its body against it and remain
still. This movement, combined with the crab?s typically
mottled colour pattern, should make it less conspicuous
because it will no longer be seen as a distinct object on
the surface of the sand or mud ?at. It may be vital to
make this response quickly. If a crab discriminated be-
tween potential objects on the basis of size, colour or tex-
ture, and perhaps passed one in favour of another, the
delay in its run to cover might well be fatal. Hence, selec-
tion to reduce predation risk may result in a general orien-
tation response to nearby objects of some minimum size,
CHRISTY & BACKWELL: BIG SIGNAL, NO PREFERENCE 1245not one based on selective discrimination between poten-
tial safe sites. This may explain why the preference that re-
sults from co-option of landmark orientation for mate
choice is expressed to a relatively broad range of objects
and why the mimetic relationship between hoods and
other objects need not be exact.
Our results do not explain why male U. terpsichores build
relatively large courtship structures. One simple possibility
is that structures with nearly vertical walls built of ?ne
moist sand become unstable as the sand dries. As we ob-
served, some natural hoods crumbled or blew away as
the activity period advanced. Hence, males may build
large hoods to make them durable, not because females
prefer them.
Acknowledgments
We thank the Smithsonian Tropical Research Institute for
continued support and the Authoridad de la Regio?n
Interocea?nica and the Servicio Maritimo Nacional of the
Repu?blica de Panama for permission to access our study
site through the Rodman Naval Station.
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