Behavioral Ecology Vol. 12 No. 6: 681-685
© 2001 International Society for Behavioral Ecology
Female preferences for single and combined traits in computer animated stickleback males
Abt. Verhaltensökologie, Zoologisches Institut, University of Bern, Wohlenstrasse 50a, CH-3032 Hinterkappelen, Switzerland
Address correspondence to R. Künzler. E-mail: reto{at}kuenzler.ch . C.M. Bakker is now at Institut für Evolutionsbiologie und Ökologie, University of Bonn, An der Immenburg 1, D-53121 Bonn, Germany.
Received 14 September 2000; revised 6 January 2001; accepted 6 January 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In many animal species, males have more than one secondary sexual character. Apart from theoretical considerations about the evolution of multiple traits, there are almost no empirical studies on female mate choice decisions based on combinations of traits as opposed to decisions based on single traits. Because three-spined sticklebacks are exceptionally well suited to be tested with computer animation technique, which itself fills the gap of adequate test paradigms for multiple traits, we tested female sticklebacks for their preferences for both single and combined male traits. We used virtual stickleback males that differed either in red throat coloration, courtship intensity, body size, or in combinations of these traits. The virtual male with increased redness was found to be preferred by females, whereas the male courting more intensely was not. The tests for combinations of traits revealed the more pronounced female preferences, the more traits were available to the females to judge male quality.
Key words: computer animation, courtship, redness, female mate preference, Gasterosteus aculeatus, multiple traits.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In species with female mate choice, males often have a number of morphological and behavioral traits in order to attract potential mates (e.g., Buchanan and Catchpole, 1997
;
Dale and Slagsvold, 1996;
Morris, 1998). These
multicomponent displays often combine different sensory modalities
simultaneously. The evolution of multiple secondary sexual traits is explained
by several hypotheses (Johnstone,
1997; Møller and
Pomiankowski, 1993; and references in both). Empirical studies
clearly lag behind these developments, which may be due to the difficulties of
performing genetical studies (Bakker and
Pomiankowski, 1995) or to technical limitations of available test
paradigms. Video editing and computer animation techniques open the
possibility of precise experimental manipulation of single male traits and
combinations thereof through the creation of virtual stimuli.
There are critical attitudes concerning the usage of video techniques in
behavioral ecology in general (e.g.,
Oliveira et al., 2000).
Television and computer systems are designed to imitate color and motion for
the human eye. It is obvious that such images may be perceived differently by
an animal species that differs in aspects of visual processing. For an animal,
a screen may be a flickering, light emitting object. The frequency at which a
flickering stimulus starts to appear continuous, known as the critical
flicker-fusion frequency (CFF), and the number and characteristics of
light-sensitive pigment cells varies greatly across species
(D'Eath, 1998). However, the
most important aspects of the visual system of three-spined sticklebacks
(Gasterosteus aculeatus L.) are very similar to those of humans (see
below). For the following reasons, as we have shown in
Künzler and Bakker
(1998), our experimental setup
using virtual stickleback males is a valid test paradigm for the measure of
female preferences for single and combined male traits. We use flicker-free
computer animation movies presented at 30 frames per second (fps) on a 120 Hz
Trinitron computer display. Sensitivities of stickleback photoreceptors match
those of humans (see Methods section), which means that colors are correctly
reproduced for sticklebacks, additional depth cues like water turbidness and
shadows help in simulating a third dimension, and last but not least,
ready-to-spawn female sticklebacks react with a sexual response: they court
the virtual males by showing the typical head-up posture, and they try to swim
towards their preferred mating partner (see Methods section below for more
detailed presentation; see Künzler and Bakker
[1998] for a precise
description of the method).
In the present study, we focused on stickleback females' preferences in an
experimental system, where mate choice decisions could be based on one
isolated trait or on combinations of traits. Stickleback males have several
secondary sexual characters in addition to red throat coloration, which may be
of importance to the females. Some of these traits are known for sticklebacks
to be under sexual selection pressure by female mate choice, for example
courtship intensity (Rowland,
1995; stabilizing selection), others such as body condition, body
size, and eye color still lack experimental verification. The methodological
set-up we used allows the alteration of the expression of a single trait
independently of other traits within a virtual stimulus individual. We expect
stronger preferences towards the high-quality male when indicated by multiple
traits compared to preference tests based on only one male sexual
characteristic.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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First, we present the most important arguments to underline the exceptional situation concerning three-spined sticklebacks and our computer animation set-up.
Screen refresh rate, presentation rate
The usage of a personal computer with a computer display (PC) instead of
television equipment (TV) has several advantages. Although many authors in the
field generalize and use the term "video" for all kind of systems
(but see Rosenthal, 2000), PC
equipment has important technical improvements compared to TV equipment. A
standard PAL TV screen uses a scheme, called interlace scanning, to refresh
the screen in two top-to-bottom passes, so that the lines scanned in one pass
are positioned between lines drawn in the previous pass. These passes are
called fields, the first field containing all the odd numbered lines, and the
second field containing all the even numbered lines. A field therefore
contains one half of the screen resolution. Together, the first and the second
field make an entire image, called a frame. Interlace scanning limits image
quality, introduces artifacts and constrains visible detail, due to overlap of
subsequent fields caused by a memory effect of the light-emitting elements of
a screen, called phosphors. The often-mentioned 50 Hertz for TV systems (60Hz
for NTSC) corresponds to 50 fields per second. Focusing on a single pixel of a
PAL screen, it is in fact redrawn only 25 times per second (30 for NTSC), so
this is the value to be compared with refresh rates of other systems or with
the CFF of a given animal species. These limitations are also valid for higher
quality video systems such as Hi8 or SuperVHS, because these systems are also
based on interlaced recording/playback and scanning.
A computer display uses progressive scanning for screen refresh and renders
all lines in a single top-to-bottom pass. This requires that the horizontal
scanning rate be doubled to scan twice the number of lines compared to
interlace scanning. "Line doubling," "deinterlacing"
and "noninterlaced refresh" are synonyms for this scanning mode.
Computer displays have even higher refresh rates. The SONY monitor we use in
our experimental set-up (see below) rescans at 120 full frames per second,
which is far above CFF both for humans
(Fleishman and Endler, 2000)
and most fish (e.g., D'Eath,
1998). Together with the computer-generated presentation rate
(term from Fleishman and Endler,
2000) of 30fps—which means full frames, not fields—we
can be sure that our female sticklebacks perceive a flicker-free movie of
continuous and smooth motion, because the threshold for motion perception for
fish is supposed to be much lower than 30 images per second
(Fleishman and Endler, 2000;
Oliveira et al., 2000). Note
that every frame of the animation movie is refreshed exactly four times by the
computer display (120Hz divided by 30fps).
Pixel spacing
A pixel of a color screen is built up by three so-called subpixels, which
are phosphor elements of the three basic colors red (R), green (G), and blue
(B). The grid pattern of these primaries in standard PC or TV screens (with
classical Shadow Mask) can cause spatially random variation in perceived color
(see Fleishman and Endler,
2000: Figure 7). This is not the case for SONY Trinitron computer
displays (with Aperture Grille), because their phosphor elements are arranged
much closer to each other in uninterrupted columns. Because of this, even a
screen area as small as the size of a single pixel contains equal proportions
of the three primaries.
Color perception
Color perception is probably the most debated issue. Animals in general are
indeed not suited to be tested with any PC or TV equipment, because species
differ in number and kind of photoreceptor cells and their spectral
sensitivity (D'Eath, 1998;
Goldsmith, 1990), and
therefore the colors rendered on a screen do not reflect the natural
appearance of a given object. Color reproducing devices designed for humans
are capable of fooling the visual system of Homo sapiens ssp. because
they stimulate the three cone classes R, G, and B at the appropriate ratios,
that is identical to the ratios produced by a given stimulus spectrum. This is
possible because for each of a screen's three primaries for example, the
phosphor is selected to match the sensitivity of the corresponding human cone
class, and because the spectrum emitted by a given phosphor does almost only
stimulate the receptors of the desired cone class. Because sticklebacks have
three cone classes with sensitivity maxima (B 452/G 529/R 604nm;
Lythgoe, 1979; B 435/G 530/R
570-610nm; Baube CL, personal communication, and cited by
Rowland et al., 1995) very
similar to humans (B 435/G 534/R 560nm;
Boynton, 1979), a SONY
Trinitron computer display (B 430-470/G 510-530/R 610-620nm;
McDonald et al., 1995) can
also be used for our study species. From Fleishman et al.
(1998; based on
Brainard, 1995), RGB values may
be corrected for species with slightly different cone sensitivities compared
to humans, in order to make colors appear correctly for those animals.
Precalculations (Künzler, unpublished data) for
sticklebacks showed that the differences between uncorrected and corrected
colors would be difficult to detect for humans and therefore needless. To our
knowledge, there are no indications of UV or IR vision in sticklebacks. The
presence of an ultraviolet visual pigment, if it is discovered, would suggest
that the blue coloration of the eye of the fish may not be rendered in natural
colors by a computer image, however the red coloration, which was the ornament
manipulated in this study, would likely be rendered accurately
(Cuthill et al., 2000).
Depth cues
It is clear that a computer display lacks the third dimension. It is
unknown whether fish do have and use depth vision at all
(Zeil, 2000). Computer
animation technique offers several possibilities to simulate or indicate
elements of a 3rd dimension on a two-dimensional screen. Water
turbidness can be simulated with a fog effect. This means that approaching
virtual fish become sharper, more contrasted and easier to distinguish from
the background. The virtual sun offers another key to depth perception: the
distance between an object and its shadow on the ground changes relative to
the movement of the object towards or away from the viewer.
Testing procedure
Female sticklebacks were hand-netted from a small water channel in Roche
(Switzerland, 42°26' N, 06°55' E) in spring 1999 before
and during the breeding season. They were kept in large group tanks in a
climatized fish room (18°C) on a 16:8 h light:dark cycle and fed daily ad
libitum with frozen red Chironomus larvae in the later afternoon.
Ripe females were selected every morning and transferred to the climatized
test room (18°C). Before the female entered the experiment, her standard
length and wet weight were measured. Female condition was calculated as
100*body mass in g/standard length3 in cm
(Bolger and Connolly, 1989) and
included eggmass before spawning. Females were gently put into 1-l containers
and allowed to acclimatize to temperature and light conditions of the test
set-up for 30 min in a pre-test compartment without computer display. Shortly
before the start of the experiment, the container with the female was put in
front of the computer display. The following test procedure is detailed in
Künzler and Bakker
(1998). Briefly, we offered a
simultaneous choice between two copies of a realistic model of a courting
stickleback male, that was built upon a typical male differing either in
courtship, redness, or in a combination of both. Male positions were
alternated between tests and all females were used only once. Preference data
of the choosing females were recorded on videotapes (2 min 20 s per test)
taken from above to allow naïve analysis with
respect to which male was on which side of the display. A preference index was
calculated as the proportion of the female's total time oriented towards one
male divided by the time directed to both males. Only females that courted and
followed a live male to its nest shortly after the test were used.
The original bright red male model (see
Künzler and
Bakker, 1998) had a colored throat area which covered 16.1% of the
total body area on a lateral view (measured without fins and eye). This throat
area was enlarged by 191% for the a priori high-quality male, resulting in a
throat area that covered 31% of the body. For the low-quality male, the
original throat area was reduced in size by 57%, leaving a larger proportion
of the body nonred (throat area 9% of total body area). Using standard color
chips, we carefully matched the redness of the virtual males to those on
standardized photographic slides taken of the throat of courting males in the
field. For this study, we selected red and dull males from the tails of the
distribution of throat coloration based on densitometer-analysis
(Bakker, 1993). For the
high-quality male, we chose a bright red (hue/saturation/brightness, HSB
18°/84%/92% as determined with Adobe Photoshop V4.0.1), and for the
low-quality male a faint pale red (HSB 28°/45%/65%). With the combination
of throat area and coloration, we could generate a maximum difference between
the red and dull male model. Courtship behavior was modified in order to
achieve a maximum difference between the two males presented for the second
trait under investigation as well. While one male courted on the original
zigzag path (the a priori high-quality male), the low-quality male approached
the female directly on a straight path without any zigzags or stops. The
swimming speed of the straightly courting male was constant and adjusted in
order to have both males starting from the nest at the same time and reaching
the frontmost position again at the same time. The tracks back to the nest of
both males were identical.
With the four different male models (Figure 1) described above, a total of six test movies could be produced. Besides the movies for preference tests for single traits, that is either degree of redness with two levels of courtship (both males zigzag or both males court in a straight line) or courtship with two levels of redness, this design allowed two tests for combined traits: A bright red and zigzagging male was presented together with a dull and straightly approaching male, which meant that two "concordant" traits pointed to the high-quality male. The last possibility was a test between a bright red and straightly courting male versus a pale and zigzagging competitor, a situation with two "contradictory" traits.
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For an extra test, we added male body size as a third trait. Larger males were found to be preferred by females in another experiment (Künzler, unpublished data). A seventh movie was produced with a concordant combination of all three traits, meaning that a bright red, zigzagging large male was opposed to a dull, straightly courting small male. Body sizes were either enlarged to 120% or reduced to 80% compared to the original male model.
Variables were normally distributed according to Shapiro-Wilk W test. P-values are two-tailed unless stated otherwise. Analyses were performed using JPM-IN (SAS Institute, v3.2.1) statistical software.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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A total of 140 females were tested. Except for one treatment, we could formulate a priori expectations about which male should be preferred (red over dull, and zigzagging over courting in a straight line). In these cases, an index greater than 0.5 indicates a preference for the high-quality male, smaller than 0.5 a preference in the non-expected direction, respectively. As for the one contradictory trait combination, we assumed coloration to be more important than courtship (see Milinski and Bakker, 1990
) and defined the red, straightly courting male to be
of higher quality than the dull, zigzagging one. Although the females of the six treatments differed in mean condition and the number of days they had been kept in the laboratory prior to the test (ANOVA, F5,114 = 2.69, p =.025 and F5,114 = 2.43, p = 0.039, respectively), there was no significant linear relationship between these and other potential covariates and the measured preference index (all p >.15, Table 1). We therefore performed all further analyses without correcting the preference indices.
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The preference index was significantly different between all six treatments (ANOVA, F5,114 = 2.51, p = 0.034), although all but one of the indices were higher than 0.5 (Figure 2). We performed a series of paired t tests within treatments for a comparison of the female's time spent oriented towards the preferred against the time toward the nonpreferred male (Table 2). In both tests with different red coloration but equal courtship behavior females significantly preferred the redder male. We found no significant preference in females of the two tests with different courtship behavior but equal redness. In the test for the concordant combination of traits females showed a highly significant preference for the red, zigzagging male. In the contradictory trait combination test, the mean preference index was slightly but not significantly below 0.5.
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The comparison of preferences for one and combinations of two and three
concordant traits (two levels of redness pooled, N = 40 females;
redness and courtship, N = 20; redness, courtship and body size using
the extra movie, N = 20) revealed a stronger preference for the
high-quality male, the more traits were available for females to base their
choice on (ordered heterogeneity test, Rice and Gaines
[1994]; based on ANOVA,
F2,77 = 1.75, P =.18; rs = 1,
pc =.82, pdirected =.045;
Figure 3).
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Because females had to spawn with a real male immediately after the test, they could only be tested once. All females were used only once, yielding independent data across all seven treatments that do not allow tests for nonadditive, that is, interaction effects.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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This study provides the first experimental evidence that female sticklebacks show stronger preferences for males that differ in concordant combinations of traits than for males that differ in only one trait or in a contradictory combination of traits. The combination of bright and extensive red throat coloration with high courtship intensity was strongly preferred by female sticklebacks. The mean preference index in this experiment was higher than in all four single-trait tests (Figure 2). As in earlier studies (e.g., Künzler and Bakker, 1998
; Milinski and
Bakker, 1990), the redder male was clearly preferred over the dull
male, independently of whether both males zigzagged or both males courted in a
straight line. In both tests for courtship intensity, no significant
preference for zigzagging over courting in a straight line could be found (as
in Milinski and Bakker, 1990;
but see Rowland, 1995), no
matter whether both males were red or both males were dull. It is of course
possible that females would respond to tactile, chemical, or visual aspects of
courtship other than the ones we tested, such as the zigzagging angles or the
proportion of the time the male spends very close to the female in front of
the scene. In the experiment with the contradictory combination of traits, females seemed to be puzzled by the fact that one trait was advertising high quality of one male while the other trait indicated the other male to be of better quality. The individual choices of the 20 females for either the redder (and not zigzagging) or zigzagging (and dull) male were not correlated to any of the additional female variables that we measured, leaving open the possibility that we did not measure the crucial variable.
Courtship intensity and red throat coloration may be correlated in real
males (Bakker and Milinski,
1991). The results of our preference tests suggest that at least
the interpretation of red coloration by female sticklebacks depends on male
courtship: on average the redder male was preferred when courting more
intensely than his rival but not so when his duller rival was courting more
intensely, while more vigorous courtship was not preferred when both males had
equal nuptial coloration. We suggest an additive mixture of color (stronger
effect) and courtship (weaker effect) to be responsible for the presented
results. With our simplification of the courtship movements for the straightly
courting male, we did not only change the shape of the courtship path, but
also decreased absolute swimming speed, which may be an important aspect of
courtship itself.
The addition of male body size as a third concordant male trait revealed an
even stronger preference toward the high-quality male in the choosing females
compared to the test with two concordant traits. Larger males are reported to
be preferred in several species (e.g.,
Backwell and Passmore, 1996;
Kraak et al., 1999) and
differences in body size are often linked to dominance also in sticklebacks
(Rowland, 1989). Because both
the enlarged and the size-reduced male model had the same proportion of the
throat area colored bright red or dull respectively, the absolute difference
in red throat area was considerably greater than in the other experiments. We
can therefore not rule out that the stronger female preference in that case
was partly based on the absolute amount of red.
Given that the comparison of one, two, and three (concordant) secondary sexual traits revealed a more pronounced female preference for every additional character, preference tests using computer animations are not only able to detect qualitative choice behaviours, but also appear to be suited for assessing female mate preferences quantitatively. The seemingly linear increase in the strength of the preference with the number of traits that distinguishes the males (Figure 3) would probably become less strong with a fourth and fifth male trait being added, and asymptotically approach the maximal possible preference index of 1.0.
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ACKNOWLEDGEMENTS |
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We thank Marc Zbinden, Dominique Mazzi, and Julian Rauch for assistance, Innes C. Cuthill, Simone S. Maurer, and three anonymous referees for helpful comments on the manuscript, and the Swiss National Science Foundation and the Bernese Hochschulstiftung for financial support.
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