The role of ultraviolet wavelengths in the mate-choice decisions of female three-spined sticklebacks
The University of Edinburgh, Institute of Cell and Animal Population Biology, West Mains Road, Edinburgh EH9 3JT, Scotland
* Author for correspondence (e-mail: pip.boulcott{at}virgin.net)
Accepted 2 March 2005
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Summary |
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Key words: mate choice, three-spined stickleback, Gasterosteus aculeatus, ultraviolet, UV, light vision
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Introduction |
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Under competition for potential mates, whilst controlling for factors such
as courtship vigour and size, female three-spined sticklebacks have been found
to prefer males displaying blue rather than silver irises, and in particular,
more intensely coloured red throats
(McLennan and McPhail, 1990).
Yet, the function of the red signal should not be regarded as a universal
feature across the Gasterosteus aculeatus species complex since
black-throated populations are found to exist
(Fitzgerald, 1993
). Even in
those populations where red-throated males are present there exists
considerable room for ambiguity, with female preference for one male over
another depending on the degree of difference in the signals displayed by two
competing males (Braithwaite and Barber,
2000
), her motivational state
(Rowland, 1995
), or prior
experience of the female (Bakker and
Milinski, 1991
). Evidence also suggests that female choice may not
be related to the specific optical properties of the red colour patch per
se, but may instead relate to the high contrast appearance of the red
signal when viewed in some environments
(Baube et al., 1995
). Hence,
whilst the red throat of the male may be utilised in the mate-choice decisions
of the stickleback, its importance may be overstated. In contrast, relatively
little is known about the role that the ultraviolet waveband plays in the
visual communication of the stickleback despite growing evidence that this
region of the spectrum is used by many vertebrates (for a review, see
Tovée, 1995
).
Data suggesting that the three-spined stickleback is sensitive to
ultraviolet light does exist. As early as the 1930s, Merker, using a single
population of three-spined sticklebacks, found that each of the optical
components of the stickleback's eye cornea, vitreous humour and lens
were found to be transparent to ultraviolet wavelengths (Merker,
1932,
1934
,
1937
). Given the absorption
characteristics of the three widely acknowledged cone pigments in the
stickleback retina (peak absorptions 445, 530, 605 nm), it is possible that
ultraviolet sensitivity is mediated by a secondary (beta) absorption peak
belonging to pigments that are maximally sensitive in the human-visible region
of the spectrum (Dyer, 2001
).
However, in addition to the possibility of beta peak sensitivity, recent
microspectrophotometric evidence suggests that the stickleback also possesses
a fourth, independent cone photoreceptor, maximally absorbent in the UV region
of the spectrum at 360 nm (Rowe et al.,
2004
). That such a visual system is capable of perceiving
ultraviolet light is supported by behavioural experiments carried out under
high intensity ultraviolet illumination
(Merker, 1939
), and under
full-spectrum conditions (P. D. Boulcott and V. A. Braithwaite, manuscript
submitted for publication).
Despite growing evidence that sticklebacks can perceive ultraviolet
wavelengths, our understanding of their mate-choice decisions has been
exclusively derived from experiments using techniques appropriate to the human
visual system. In adopting such an approach, it is likely that colour choice
has been misclassified in this species, and the functional significance of the
ultraviolet waveband overlooked (Bennett et
al., 1994). Moreover, it has become apparent that the use of
ultraviolet wavelengths during mate choice may be quite a common behaviour
amongst vertebrates. Ultraviolet reflectance has been found to influence the
mate-choice decisions of birds (Bennett et
al., 1996
), lizards (Fleishman
et al., 1993
), butterflies
(Arikawa et al., 1987
) and fish
(Kodric-Brown and Johnson,
2002
; Smith et al.,
2002
). Given that the three-spined stickleback is capable of
responding behaviourally to ultraviolet wavelengths, and that colour plays an
important part in the mate-choice decision of this species, we hypothesised
that females may use ultraviolet-based cues in their selection of a mate.
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Materials and methods |
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Experimental set-up
The experimental apparatus consisted of five rectangular tanks (50 cm
x40 cm x30 cm), each divided by an opaque plastic partition into
three sections: a central viewing chamber and two adjacent choice chambers
(Fig. 1). In each experiment,
the presentation of the two choice-chambers to the test fish differed in terms
of two interchangeable light-filter treatments placed in front of each viewing
chamber. These filters were either ultraviolet transmitting (UV+) or blocking
(UV); Fig. 2A. For an
animal with the appropriate visual system, identical objects viewed through
these filters will differ in terms of their hue and brightness.
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To prevent the use of olfactory cues during the trials, the two choice chambers were sealed from the viewing chamber by a single sheet of UV transmissive Perspex. The tank walls were then covered with black plastic screen to minimise outside disturbance. Test tanks were illuminated by a single 5500°K full spectrum fluorescent tube (Arcadia Ltd, UK) suspended 40 cm above the tank (Fig. 2B).
Subjects
Adult three-spined sticklebacks Gasterosteus aculeatus L. were
collected from the Balmaha Pond, Loch Lomond, in March 2002. Whilst in the
laboratory, fish were held in two main stock tanks (92 cm x39 cm
x30 cm), each illuminated by a full spectrum fluorescent tube in
addition to eight ceiling 80 W fluorescent tubes, and were fed twice daily on
a diet of live and frozen Tubifex worms. To encourage the onset of
reproductive condition for subjects in Exps 3 and 4, the ambient temperature
of these two stock tanks was maintained at approximately 20°C. For similar
reasons the light regime was set to a 16 h:8 h light/dark cycle
(Hoar, 1962). Once the fish
had reached a sexually reproductive condition, males were temporarily removed
to smaller holding tanks (30 cm x22 cm x20 cm) prior to testing.
Temperature and light regimes in these smaller tanks, and the test tanks, were
the same as the stock aquaria.
Assessing preference
Preference was quantified in terms of the proportion of time the fish spent
orientated towards the choice chambers within a 5 cm zone of either of the two
filter types. Each test fish was only judged to have entered a new zone once
its head and pectoral fins had crossed the zone boundary. For the data to be
included in any subsequent analysis, the test fish must have entered both test
zones during the first 2 min of the trial period. Such a measure of preference
has proved a useful estimate of preference in previous mate-preference
experiments (Rowland,
1995).
At the outset of each trial all test individuals were allowed a 10 min settling period. During this period the two choice chambers were screened off using an opaque screen. This screen could be raised remotely, signifying the beginning of a trial, and was only lifted when the test fish was situated centrally in the non-preference zone. Each trial lasted 20 min, during which observations were recorded via a video camera (JVC, GX-NFE) suspended 1 m above the tank. All trials were carried out between 10:00 h and 16:00 h.
Experiment 1 filter preference of non-reproductive individuals
Exp. 1 was designed to determine if fish in non-reproductive condition
exhibited a preference for either filter type. Ten test fish were screened
individually. A fish was selected at random from either of the two stock tanks
and placed centrally in the non-preference zone for a 10 min period of
acclimation, after which fish preference was recorded for 20 min.
Experiment 2 ultraviolet perception and shoaling behaviour
Exp. 2 examined whether ultraviolet wavelengths are used in the visually
mediated shoaling decisions of the three-spined stickleback. This experiment
was identical to Exp. 1, with the exception that two groups of
non-reproductive stimulus shoals, each containing three individuals, now
occupied the two choice chambers. Both shoal fish and test fish used in each
of the ten trials were naïve to the set-up. Shoal members were size
matched in order to minimise size selection bias during the shoaling. All fish
in any given trial had been pooled in the same stock tank for a minimum of 14
days and had, therefore, a similar degree of familiarity with each other. This
time period has been found to be sufficiently long for the stickleback to
establish shoaling preferences (Barber and
Wright, 2001).
Experiment 3 filter preference of gravid females
The experimental procedure in Exp. 3 replicated Exp. 1, except that ten
gravid females were selected as test individuals. As it was important to
ensure that these gravid females were sexually responsive, females were chosen
only if they exhibited the characteristic `head-up' display in response to a
sexually reproductive male. To avoid the problems associated with prior
experience, the stimulus male used in this procedure was not used in the later
mate-choice experiment.
Experiment 4 ultraviolet perception and mate choice
Exp. 4 examined the role that ultraviolet wavelengths might play during
mate-choice decisions of the female three-spined stickleback. Sexually
responsive males were placed in each of the two choice chambers, which were
furnished with suitable nesting materials, and encouraged to build nests. Once
both males had constructed a nest and were observed to court a stimulus female
vigorously, they were judged ready for test trials. To minimise any influence
of the male's nest on the female's assessment, the water filter in each
viewing chamber was placed in front of the nest
(Braithwaite and Barber, 2000;
Barber et al., 2001
).
Preference measures were made with ten gravid females, all of whom were
naïve to the test apparatus. In order to assess male behaviour, the
number of zig-zag displays performed by each male during the trial was
recorded. Such display rates have been found to be a useful measure of male
sexual tendency in this species (Rowland,
1984
).
Pairing stimulus males
Males used in the trial did not develop the same intensity of nuptial
colouration, despite being reared in identical conditions. Evidence suggests,
however, that females may select males based on their degree of colouration
and also on the extent of the differences between two males
(Braithwaite and Barber,
2000). In view of this, each pair of males was matched for size,
general morphology and, as far as practicably possible, colour. To obtain an
estimate of the degree of intensity of the red belly of each fish, males were
scored from 15 for colour by two observers, and the average score taken
(Rowland, 1984
). No trials
were undertaken where the males differed in redness by more than one point.
Whilst this technique is necessarily subjective, it does reduce the influence
of factors other than ultraviolet content that are known to affect female
choice. Once paired, males were then randomly assigned to one of the two
filter treatments: UV transmitting or UV blocking.
Statistical analyses
Female preference, measured as the proportion of time spent within either
of the two preference zones, was analysed using a General Linear Model (GLM).
A GLM was initially fitted to the data and included all explanatory variables
and their interactions. Terms were then removed by stepwise deletion. Where
possible, minimal models for the main factor are reported for each of the four
experiments in the form of a student t-test. Prior to analysis, all
proportional data was arcsine square-root transformed. An
AndersonDarling test was performed on the data to assess normality.
Male display rates in Exp. 4 were analysed using a Wilcoxon matched
signed-pairs ranked test.
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Results |
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Experiment 2
Females did not prefer to shoal with conspecifics viewed through either the
UV+ or UVfilter (two-tailed t-test: t=1.27,
d.f.=15, P=0.22; see Fig.
3). No effect of the holding tank was found (GLM:
F4,5=0.96, P=0.503). When comparing the time each
female spent in the preference zones across Exps 1 and 2, there were no
differences (two-tailed t-test: t=1.88, d.f.=12,
P=0.08).
Experiment 3
Gravid females did not exhibit a preference for either the UV+ or
UVfilter (two-tailed t-test: t=1.52, d.f.=17,
P=0.15; see Fig. 3).
Similarly, no effect of the holding tank was revealed (GLM:
F4,5=2.09, P=0.219). Hence, gravid females, in
common with their non-gravid counterparts in Exp. 1, did not express a
preference for either filter type.
Experiment 4
Throughout all mate-choice trials, males were seen to perform the zig-zag
display. The rate of this display was not found to differ significantly
according to the filter type (Wilcoxon matched signed-pairs ranked test:
Z=0.255, P=0.79). Similarly, all females were seen to
display the classic head-up posture while in the preference zone. No effect of
the holding tank was found (GLM: F4,5=0.41,
P=0.79). Gravid females did, however, exhibit a significant
preference for those males viewed through the UV+ filter compared to those
viewed through the UVfilter (two-tailed t-test:
t=3.88, d.f.=12, P=0.002; see
Fig. 3). Gravid females in Exp.
4 also spent more time facing the preference zone when males were present
compared to Exp. 3, where no conspecifics were present (two-tailed
t-test: t=4.46, d.f.=17, P<0.001).
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Discussion |
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The two ultraviolet treatments used in this experiment did not influence
shoaling choice. Given that previous studies have shown three-spined
sticklebacks prefer to associate with conspecifics with a familiar appearance
(Barber and Ruxton, 2000), it
would seem that the females we tested did not regard the individuals viewed
through the ultraviolet blocking filter as abnormal. Furthermore, this
indicates that preference decisions taken by the female in experiment 4 were
based on a sexually motivated mate preference, rather than a natural tendency
to associate with individuals, male or female, displaying across the
ultraviolet and human-visible region of the spectrum.
Male behaviour
Female sticklebacks, following the onset of sexual receptivity, develop a
dark bar-like melanated pattern down their silver flanks
(Wootton, 1976). Reflectance
spectra obtained from these colour patches reveal that the silver flanks
possess a peak in the ultraviolet region of the spectrum, providing a contrast
with the relatively low UV reflectance of the flanks
(Boulcott, 2003
). Since the
addition of the ultraviolet waveband will serve to enhance the contrast of
this signal, it is possible that this characteristic pattern might be used by
males when assessing female receptivity
(Rowland et al., 1991
).
Furthermore, as female mate choice in the stickleback has been found to be
affected by the vigour of the displaying male's courtship
(Rowland, 1995
), it could be
argued that the female's choice in experiment 4 is driven by the two males
responding to the female differentially, since they viewed her under different
spectral conditions. Such a mechanism would operate despite the random
assignment of filter type to each pair of males. Our results, however, suggest
that male display rate does not differ significantly with filter, and that
each male courts the female vigorously throughout the period. It is,
therefore, likely that female preference in experiment 4 was driven by the
visual appearance of the males alone, and not by a difference in display
activity. This finding differs from similar mate-choice studies carried out in
the guppy Poecilia reticulata, where the addition of the ultraviolet
waveband was found to enhance the attractiveness of potential mates in both
males and females (Kodric-Brown and
Johnson, 2002
; Smith et al.,
2002
).
Mate choice and signal display
Work carried out by Rowe et al.
(2004) suggests that the red
throat colour of the reproductively active male is encoded by an orthogonal
opponent mechanism that is largely insensitive to input from the ultraviolet
photoreceptor. Hence, the addition of ultraviolet wavelengths to the
appearance of the displaying male in our experiment would not alter the red
signal appreciably. However, as the mosaic pattern red throat, blue
iris and dark green melanted flank of the male is a high-contrast
signal (Milinski and Bakker,
1990
), females viewing a displaying male under full spectral
conditions may find that both hue and luminance contrast are enhanced
thereby increasing total contrast. Both the silver flanks
(Boulcott, 2003
) and the blue
iris of coloured males (Rowe et al.,
2004
) reflect short wavelength light strongly, and would contrast
against the red throat and dark flank, which only weakly reflect light in the
ultraviolet waveband. This will be true even if the ultraviolet photoreceptor
does not contribute to the colour visual system because differences in
contrast can be detected by brightness perception alone
(Kevan et al., 2001
).
Furthermore, if a displaying male is viewed against a background reflective to
ultraviolet light, a situation which is likely if the fish is viewed against a
sandy substrate, visual contrast would be increased given the relatively poor
ultraviolet reflectance of the cryptic flank
(Rowe et al., 2004
).
The addition of the ultraviolet region would seem to enhance total contrast
of the male's nuptial signal and might, therefore, be responsible for the
manipulated change in female preference observed during experiment 4. Indeed,
the importance of red colouration in this species complex may be overstated,
and there is increasing evidence to suggest that female preference for the
male signal is more likely to be related to its efficiency in generating
visual contrast. In an elegant series of experiments, Baube and coworkers
found that 100% red dummies used to elicit mate-choice behaviour in females
brought no greater reaction than 100% beige coloured dummies
(Baube et al., 1995). However,
dummies that possessed some degree of visual contrast produced the greatest
reaction, and this response was found to be independent of the area of
shading. Such findings suggest that not only are abnormal signals disregarded,
and that key stimuli must have context
(Tinbergen, 1951
), but that
the stickleback's visual system is tuned to detect signals in terms of form
and colour that produce a high contrast target within the aquatic environment.
The function of the mosaic signal as a high-contrast optical signal is
supported in the field by studies investigating mate choice in populations of
black-throated three-spined sticklebacks (McDonald et al., 1995). In this
instance signal contrast, rather than the red throat colour itself, was found
to be the important determining factor of female choice during courtship.
Although our results show that females utilise ultraviolet wavelengths during their mate-choice decision, key questions relating to the evolutionary implications of this finding arise. Given the diversity of the three-spined stickleback species in terms of evolutionary history and habitat, it is unclear as to what extent the ability to perceive ultraviolet wavelengths varies. If variation exists, it may lead to selective pressure on male colouration in this species.
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Acknowledgments |
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