Ultraviolet (UV) vision is well documented for many species of vertebrates. UV cues are known to be used in foraging, navigation and in mate choice. We conducted a series of behavioral experiments to investigate the role of UV perception in mate choice in both female and male guppies (Poecilia reticulata). In our experiments the visual appearance of potential mates was manipulated using either UV transmitting (UV+) or UV blocking (UV-) filters. Female guppies significantly preferred UV+ males. Male guppies tended to prefer UV- females, but their preferences were marginally nonsignificant. Further experiments investigating the role of luminance, indicate that UV wavelengths are probably being used for color discrimination rather than for detecting differences in brightness. These experiments raise the possibility that UV is used in mate assessment in different ways by male and female guppies. This may reflect the different strategies that the two sexes have in order to maximize reproductive success. To our knowledge, these are the first data showing that UV is used by any fish species in mate selection.
The functional significance of vertebrate ultraviolet (UV) vision has been the focus of an increasing number of research studies over the last decade (for reviews see Bennett and Cuthill, 1994; Bennett et al., 1994; Cuthill et al., 2000; Jacobs, 1992; Losey et al., 1999). This study investigates the role of UV vision in mate choice in a teleost, the guppy (Poecilia reticulata Peters, 1859).
Although photons with wavelengths between approximately 400-700 nm excite the sensation of vision in humans (Campbell and Dwek, 1984), many animals additionally perceive UV wavelengths in the range 300-400 nm (Tovée, 1995). However, photo-oxidative tissue damage is a function of wave-length, with UV being particularly damaging (e.g., van Norren and Schellekens, 1990; Zigman and Bagley, 1971; see also, Cockell and Knowland, 1999), thus UV vision is costly. Sensitivity in the UV must, therefore, confer advantages over visual systems that exclude these wavelengths (Bennett and Cuthill, 1994). Behavioral and physiological studies have provided evidence that UV perception is used by many species of vertebrates in foraging (Browman et al., 1994; Church et al., 1998; Loew et al., 1993; Viitala et al., 1995), navigation (Hawryshyn and McFarland, 1987) and in birds at least, in mate choice (Andersson and Amundsen, 1997; Andersson et al., 1998; Bennett et al., 1996, 1997; Hunt et al., 1997, 1998, 1999; Maier, 1993).
The guppy is a small freshwater poeciliid fish native to Trinidad and parts of South America (Rosen and Bailey, 1963), and has been used extensively in laboratory based research, particularly in studies of sexual selection (for a review see Houde, 1997). However, previous experiments investigating mate choice in guppies have not explored the possibility that they utilize UV cues in mate assessment. Guppies primarily occur in clear shallow water streams and are thus exposed to irradiance that includes the UV (Baker and Smith, 1982; Downing et al., 1986; Dunne and Brown, 1996; Loew and McFarland, 1990; McFarland, 1986). Guppy courtship activity peaks at dawn and dusk when light is relatively rich in short- and long-wave photons and poor in middle wavelengths (Endler, 1987, 1991, 1993; Loew and McFarland, 1990). Reflectance spectrophotometry (300-700 nm) indicates that both male and female guppies have UV reflectance (Smith EJ, unpublished data; see also Methods). Furthermore, guppies have UV transparent ocular media (Douglas and Hawryshyn, 1990; see also Douglas and McGuigan, 1989) and cone photoreceptors sensitive to UV (Archer et al., 1987; Archer and Lythgoe, 1990). It is highly plausible, therefore, that guppies are visually sensitive to the UV, and that UV perception plays a role in mate selection. In this article, we investigate for the first time whether UV cues are used by guppies in mate choice decisions.
Guppies are livebearing, sexually dimorphic fishes and have a promiscuous, non-resource-based mating system in which males court one female after another and male mating success is largely determined by female acceptance (Baerends et al., 1955; Endler, 1983; Liley, 1966). Research has shown that female choice of males plays a significant role in sexual selection in this species (see Houde, 1997). Although females receive no obvious direct benefits from mating, they are choosy most likely because indirect (genetic) benefits may be conferred, leading to increased offspring fitness (Reynolds and Gross, 1992). Female guppies assess male quality by visual observation of both behavior (especially courtship display: see Methods) and morphological traits. Mate choice based upon male color pattern is well documented for wild populations (see Endler and Houde, 1995, for a review). Body color patterns of males are heritable (Haskins and Haskins, 1951; Houde, 1992, 1994; Winge and Ditlevsen, 1947) and comprise various combinations of different colored spots and patches including carotenoid (orange, red, and yellow pigments), melanin (black or brown) and highly reflective structural colors such as blue, green, and silver iridescence (Endler, 1983; Houde and Endler, 1990).
In contrast to the large amount of work on female mate choice in guppies, male mate choice has been less extensively studied. While males might seem to have no benefit to choosiness there are limits to the number of effective matings that can take place as males have a refractory period between copulations to refill sperm ducts (see Houde, 1997). Therefore, a male's ability to inseminate females is probably limited enough to make choosing pay. This is particularly the case for some natural populations in which female-biased operational sex ratios are found (see Endler, 1995; Jirotkul, 2000).
MATERIALS AND METHODS
The fish we used were laboratory reared male and female guppies (all approximately 6-8 months old) descended from individuals that had been collected between 1992-1994 from the Lower Tacarigua River, Trinidad. Guppies in this river are subject to high-predation pressure (see Magurran and Seghers, 1994).
To the human observer, color patterns of experimental males comprised three specific elements: orange (carotenoid) patches, black (melanin) spots, and structural colors—areas of green, purple, and white iridescence. Female guppies are cryptically colored and are dull gray-brown, and typically attain a larger body size than males at sexual maturity. However, reflectance spectra (300-700 nm) of the color pattern elements and body color of males reveals that both structural colors and orange spots/patches have a UV component (Smith EJ, unpublished data). Females also reflect UV wave-lengths. There exists, therefore, the possibility of variation in natural reflectance in the UV for both male and female guppies.
The test males and females used in this study were raised to maturity in mixed-sex aquaria (45 cm L × 30 cm W × 30 cm H) and were maintained at 25 ± 1°C under a 12:12 h light:dark illumination cycle provided by fluorescent tubes (True-Lite, Duro-Test, USA, 75/85 Watt) which have a spectral emission that simulates natural skylight, including the UV. Downward irradiance from these lights provides a ratio of UV quantal flux (300-400 nm) to total quantal flux (300-700 nm) of 7% (Hunt et al., 1999) compared to 6% for direct sunlight (Wyszecki and Stiles, 1982). Aquaria were covered with acrylic lids that transmitted human visible and UV wavelengths (see Figure 2, transmission spectra of UV transmitting filter). Polystyrene tiles were placed between all holding and rearing aquaria, to prevent visual contact between fish held in adjacent tanks. All fish were fed Tetramin tropical fish food (Tetra Werke, Melle, Germany) once daily.
The apparatus used to test mate choice decisions (Figure 1) was similar in design to that used in many previous studies (see Houde, 1997). The choice chambers were constructed from UV transmitting filter (see Figure 2), and were lightly frosted on each of the three sides. These choice chambers were placed within a transparent acrylic rectangular container (which transmitted human visible and UV wavelengths) positioned within the main aquarium. This prevented the transmission of olfactory cues, which are known to be used in mate choice decisions (Crow and Liley, 1979; Meyer and Liley, 1982). The main aquarium was surrounded with polystyrene tiles to reduce external visual disturbances. The entire apparatus was filled to a depth of 15 cm with aerated water and maintained at a temperature of 25 ± 1°C. Even overhead illumination was provided by two True-Lite fluorescent tubes suspended 60 cm above the experimental apparatus.
General experimental protocol
This study comprised five experiments designed to investigate the role of UV vision in mate choice in both female (experiments 1, 2, and 5) and male (experiments 3, 4, and 5) guppies. Experiments 1 and 3 were conducted to determine whether UV perception plays a part in mate choice decisions. Experiments 2 and 4 were performed to ascertain whether the UV is used for hue discrimination or for perception of achromatic brightness differences. Experiment 5 was carried out to ensure that the mate choice apparatus measures heterosexual preferences.
Mate preference was quantified as the amount of time spent within 5 cm of the treatment filters, a measurement used by many previous workers (Bischoff et al., 1985; Kennedy et al., 1987; Kodric-Brown, 1985, 1989, 1992, 1993; Kodric-Brown and Nicoletto, 1996; Stoner and Breden, 1988). Test fish had to enter both preference zones during the mate assessment phase for the data to be included in subsequent statistical analyses (see Houde, 1997).
The females that we used in our experiments were “experienced,” i.e., they had been raised to maturity in the presence of males (different individuals from those used in the experiments). To ensure that they were receptive to males during experimental trials, test females were caught at random from their rearing aquaria 21 days before experiments 1, 2, and 5 commenced, and were placed in a single-sex aquarium where they were held as a group until trials began.
The males we used were also non-virgins. One week before the male-preference trials of experiments 3, 4, and 5 began, 10 test males were taken at random from mixed-sex rearing aquaria and transferred to two single-sex tanks (five males in each). We recorded the standard length and color pattern of each male on an outline drawing of a guppy to allow individuals to be identified during each of the experiments.
Experiment 1: ultraviolet vision and female mate choice
Male guppies perform a characteristic sigmoid display to persuade receptive females to mate (Baerends et al., 1955; Houde, 1997; Liley, 1966). Some researchers have found that male display rate is an important cue utilized by females in mate choice (Farr, 1980; Kodric-Brown, 1993; Kodric-Brown and Nicoletto, 1996). To control for this variable, preliminary trials were conducted to match pairs of males as closely as possible for courtship display. Previous studies have controlled for differences in male display by excluding from statistical analyses any trials in which male pairs differed by more than three sigmoid displays per 10 min (Kodric-Brown, 1985; Sheridan and Pomiankowski, 1997). We decided to record the duration of bouts of courtship during which a male was paying close attention to a female and performing sigmoids, rather than the number of sigmoid displays as it proved difficult to determine when any one individual display ended and another commenced.
Preliminary trials were conducted in the experimental apparatus described above and held between 1 and 3 h after lights on in the morning. Treatment filters (see below) were not used in preliminary trials and thus the observing female fish saw the males under full-spectrum illumination (UV and human visible wavelengths). Males taken from mixed-sex stock aquaria were placed individually in either the left or right choice chamber. One female, chosen at random from a “pool” of experienced females which were used exclusively in preliminary trials, was placed within the central area of the apparatus. All fish underwent a 10 min period of acclimatization, with opaque partition (G in Figure 1) in place, prior to a 20 min test period during which the female was able to view both males. During this test period an observer recorded the duration of all courtship display behavior performed by both males using two stopwatches.
To enable identification after the pre-trial, males were placed individually within holding aquaria (45 cm L × 30 cm W × 30 cm H) divided into 10 equal compartments. The position of males within these holding aquaria was determined randomly. To minimize the effects of isolation, compartments were constructed from fine plastic mesh, thus allowing visual observation between adjacent animals.
Preliminary trials were conducted until 10 pairs of males were obtained for use in experiment 1. Males were paired if: (1) they differed by no more than 5% in display behavior (i.e., total duration per 20 min); (2) were matched for body coloration patterns (males having asymmetric color patterns were not used in this study due to potentially confounding effects [Sheridan and Pomiankowski, 1997]); and (3) differed by no more than 2 mm in standard and total body length.
Experiment 1 comprised ten trials with all fishes (10 females, 20 males) being used once only. One test female and one pair of matched males were used in each trial. Females had never previously seen the males they encountered during experimental trials.
For each trial two treatments were created by the vertical positioning of a randomly allocated UV+ or UV- filter (ICI Perspex) within each choice chamber. UV+ transmitted ultraviolet and human visible wavelengths, while UV- blocked ultraviolet wavelengths (Figure 2). Filters were randomly assigned in a balanced way so that UV+ and UV- treatments occurred an equal number of times to the left and to the right side of the experimental apparatus. Trials were conducted between 1 and 3 h after morning lights on and comprised three 20-min phases: control 1; test (mate assessment); control 2. Individual test females were present in the central area of the experimental apparatus throughout the trial. Males were present only during the test phase (one male was present in each choice chamber) for mate assessment. Each test female underwent two control phases to ensure that any preference demonstrated was due to the appearance of the males undergoing mate assessment, rather than a more general preference for a UV+ or UV- view independent of mate choice (Bennett et al., 1996). On introduction to the experimental apparatus, all fishes underwent a 10-min period of acclimatization; females prior to trial commencement and males prior to the mate assessment phase.
All trials were recorded on videotape using a Sony Hi8 video camera recorder (CCD-TRV65E) suspended 70 cm above the experimental apparatus. Female behavior, the amount of time spent within 5 cm of each filter during each of the three phases, was transcribed from videotape blind to the positioning of filter type in each trial. Preliminary trials had shown that it was not possible to accurately transcribe male display behavior from videotape, therefore, the duration of male courtship display was recorded directly during the mate assessment phase. Behavioral observations of males were made from behind a dark screen. Because it was necessary for the observer to be present in the laboratory during the mate assessment phase, the observer was also present during both control periods.
Experiment 2: ultraviolet vision and female mate choice; the role of luminance
Treatment filters used in experiment 1 differed in quantal flux (UV+ to UV-: 8-10% reduction (Bennett et al., 1996; Church et al., 1998)) and thus, likely, the luminance of the stimuli. A second experiment was, therefore, conducted to determine whether UV wavelengths are used for hue discrimination or for detecting differences in brightness. Neutral-density (ND) filters (Figure 3) were used to manipulate luminance (achromatic brightness) independent of hue. Experiment 2 was a replica of experiment 1 except that all females were tested twice, once under UV+ conditions (the two treatments being ND1 and ND2) and once under UV- conditions (ND3 and ND4). ND2 and ND4 filters produced a reduction in quantal flux equal to that of experiment 1.
A further 30 fishes (10 females, 20 males, paired as described above) were used in this experiment. The design was counterbalanced so that half of the females were exposed to UV+ in the first trial and half to UV-. Females viewed a different pair of males in each trial and were tested with males that they had never previously seen. Although all fishes underwent two trials, no fish was tested twice on one day. To enable identification, females were held individually between trials. In all other aspects experiment 2 was identical to experiment 1.
Experiment 3: ultraviolet vision and male mate choice
This experiment was, in essence, a direct replica of experiment 1 but instead investigated male mate preferences. The stimulus females used in this experiment were unfamiliar to the test males and had previously been kept in mixed-sex stock aquaria. Female pairs were matched for standard length and body depth (± 1 mm), tail size and body color.
The experimental procedure was identical to experiment 1, except that: (1) trials were conducted between 1 and 6 h after morning lights on; and (2) observations of courtship display performed by individual test males, were not made during the mate assessment phase. The laboratory was, therefore, vacated during each phase of all male-preference trials.
Experiment 4: ultraviolet vision and male mate choice; the role of luminance
Male guppies are known to display less frequently in the presence of predators and/or at high light intensity (Endler, 1987; Reynolds, 1993; Reynolds et al., 1993). As the treatment filters used in experiment 3 differed in quantal flux (luminance), we conducted experiment 4 to investigate the possibility that males may preferentially court females viewed under conditions of reduced illumination. This experiment was a replica of experiment 2, except that males were now able to choose between pairs of females presented under varying degrees of luminance.
Stimulus females were obtained and paired as described above. Male-preference trials were again conducted between 1 and 6 h after lights on, and the laboratory was vacated during each trial.
Experiment 5: does our mate choice apparatus measure heterosexual preferences?
Experiment 5 was designed to confirm that our apparatus measures heterosexual mate preferences, rather than, for example, a natural tendency to preferentially associate with other individuals regardless of sex.
A further 20 test fish (10 males, 10 females) were selected at random from rearing aquaria for use in this experiment. Twenty stimulus fish (10 males, 10 females) were caught at random from those used in our previous experiments (experiments 1-4), and were randomly assigned to form 10 opposite sex pairs.
Each test fish underwent one UV+ and one UV- trial during which a pair of stimulus fish (one female and one male) were available for assessment. In this experiment UV+ and UV- filters were chosen to yield equal quantal transmittance despite the difference in transmitted wavelengths. All trials were conducted between 1 and 3 h after morning lights on. On introduction to the mate choice apparatus all fishes underwent a 10 min acclimatization period with opaque partition (G) in place. This was followed by a 20 min test period, during which an observer recorded the amount of time spent by each individual test fish within 5 cm of each filter using two stopwatches. All observations were made from behind a dark screen. The design was counterbalanced so that five test males and five test females were exposed to UV+ in the first trial, while five test fish of each sex received a UV- trial first.
Individual test fish and stimulus fish pairs were randomly assigned to trials, so that each test male and female viewed a different pair of conspecifics in each of the two trials. No fish was used twice on one day, and test fish had never previously seen the stimulus fish they encountered during each trial. The position of male and female stimulus fish to the left or right choice chamber was determined randomly. Stimulus fish were held individually throughout the experiment, and test fish were held individually between trials.
Male and female behavioral data, specifically the amount of time spent within 5 cm of each treatment filter during each of the three phases, were analyzed using repeated-measures ANOVAs. Data were tested for normality before statistical analyses.
Ultraviolet vision and female mate choice
Nine test females met the criterion to be included in subsequent statistical analyses, namely that they visited and thus potentially assessed both males during the test phase.
The amount of time spent by females within 5 cm of the treatment filters was significantly higher during the test phase, when males were available for mate assessment, than in either of the control phases (F2,16 = 7.42, p =.005). This indicates that the females were attracted to the choice chambers by the presence of the males.
There was a highly significant two-way interaction between filter and phase (F2,16 = 11.07, p <.001), therefore, we divided the data according to phase and conducted a separate ANOVA for the control 1, test (mate assessment) and control 2 periods. These results showed that females had no preference for UV+ or UV- treatments during control phases 1 and 2 (F1,8 = 0.00, p =.954; and F1,8 = 0.16, p =.697 respectively; data for control 2 were log-transformed before analysis) but had a significant preference for the UV+ treatment during the mate assessment phase (F1,8 = 20.46, p <.001; Figure 4).
Two test females failed to visit both males during a mate assessment phase and thus all data for these females were excluded from statistical analyses.
Results indicated that females spent more time within 5 cm of ND treatment filters during the test phase in both UV+ and UV- conditions (F2,14 = 7.73, p =.005). However, there were no other significant two- or three-way interactions (luminance × phase, F2,14 = 1.26, p =.313; filter × luminance, F1,14 = 1.61, p =.245; filter × phase, F2,14 = 2.62, p =.108; filter × luminance × phase, F2,14 = 1.90, p =.186). Figure 5a,b illustrates the results for UV+ and UV- conditions respectively. These graphs show that females exhibited no significant preference between luminance treatments either in the presence or absence of UV wavelengths.
Ultraviolet vision and male mate choice
All test males fulfilled the requirement to visit and hence assess both females during the test phase.
Results showed that males spent significantly more time within 5 cm of the treatment filters during the test phase when females were present than in either of the control periods (F2,18 = 10.75, p <.001), therefore, an association preference is evident.
A significant two-way interaction between filter and phase (F2,18 = 5.07, p =.018) prompted us to perform separate ANOVAs for each of the three phases. However, when analyzed separately, results showed no significant effect of filter indicating that males had no significant preference for UV+ or UV- treatments during control phases 1 and 2 (F1,9 = 0.18, p =.682; and F1,9 = 0.08, p =.785 respectively) when females were absent, nor during the test phase (F1,9 = 4.58, p =.061) when females were available for mate assessment. Although formally, the test phase result is nonsignificant at the 5% level, males tended to prefer UV- females (Figure 6). Indeed, eight out of the 10 males tested showed a preference for females viewed through the UV- filter treatment.
One test male failed to visit both females during a test phase, hence all data for this male were discarded.
When testing data for normality, we found that the residuals were significantly different from a normal distribution. Three forms of transformation were attempted (log, square-root, and rank), but the residuals were still not normalized. Although ANOVAs assume normality and homogeneity of variance, they are robust statistical tests and can be used when these conditions do not apply, especially the normality assumption (see Howell, 1997). As the three transformations showed the same pattern of results, we decided to use the most conservative ANOVA, carried out on the untransformed data.
Results indicated that males spent significantly more time within 5 cm of ND treatment filters during the test phase when females were available for assessment in both UV+ and UV- conditions (F2,16 = 18.50, p <.001). However, there were no significant differences between treatments for any phase in either condition (luminance × phase, F2,16 = 0.24, p =.791; filter × luminance, F1,16 = 1.42, p =.268; filter × phase, F2,16 = 0.16, p = 0.857; filter × luminance × phase, F2,16 = 0.06, p =.945). Figure 7a,b illustrates that males show no preference based on luminance regardless of the presence or absence of UV wavelengths.
Time of day analyses for experiments 3 and 4
Most previous researchers investigating mate choice in guppies have conducted experimental trials 1-3 h after dawn/laboratory lights on. Due to time constraints, we were unable to perform and complete all male-preference trials within this time period. To investigate whether our results were influenced by differences in trial starting time, we conducted Pearson correlation analyses between the start time of each trial and the total amount of time spent by males within 5 cm of the treatment filters during the test (mate assessment) phase.
No correlation was found between trial time and the amount of time spent by test males in mate assessment for experiment 3 (r =.229, df = 8, p >.10), or experiment 4 (UV+ condition, r = -.300, df = 7, p >.10; UV- condition, r =.529, df = 7, p > .10). All values are non-significant, therefore, trial time seemed not to strongly influence male preferences in either experiment.
Does our mate choice apparatus measure heterosexual preferences?
All test fish strongly preferred to associate with individuals of the opposite sex, rather than with same sex conspecifics (stimulus × sex, F1,18 = 101.04, p <.001). Results also showed that male and female preferences for opposite sex individuals were not affected by either the presence or absence of UV wavelengths (stimulus × filter × sex, F1,18 = 0.24, p =.630). This indicates that test fish preferred opposite sex stimulus fish irrespective of the light environment in which they were tested.
The results of this experiment confirm that our apparatus measures heterosexual mate preferences.
Ultraviolet vision and female mate choice
Females exhibited a strong preference for UV+ males during the test (mate assessment) phase of experiment 1. As females had no preference for UV+ or UV- treatments during control phases 1 and 2 when there were no stimulus males present, we can discount the possibility that females have a general preference for UV+ unrelated to mate choice. Experiment 1, therefore, strongly suggests that UV perception plays a part in female mate choice decisions, with males seen in the presence of UV wavelengths being preferred. Of course, there may be other resources (e.g., food) for which UV wavelengths are also an important part of perception.
The possession of UV sensitive receptors (in addition to ocular media that transmit UV) does not necessarily mean that they form part of a color vision mechanism, as UV wavelengths may only contribute to achromatic or brightness discrimination. We considered the possibility that females in our first experiment were perhaps responding to differences in quantal flux (luminance), and hence the brightness of the males. Experiment 2, in which pairs of males were presented under varying degrees of luminance in both UV+ and UV- conditions, established that females had no preference between ND filter types in either condition. Thus, the preference for UV+ exhibited by females in experiment 1 is not a simple preference for mate assessment to occur in conditions of increased luminance. UV is, therefore, probably being used for hue discrimination rather than for detecting differences in brightness.
Ultraviolet vision and male mate choice
Males exhibited no significant differences between treatments during any phase of experiment 3, however, there is a suggestion of a possible trend for male mate assessment of females occurring under UV deficient conditions (p =.061). Eight out of ten males used were found to have a preference for females viewed in UV- conditions, rather than UV+ conditions as found in their natural habitat.
To minimize the risk of predation, guppies in wild populations court most intensively shortly after dawn and shortly before dusk, when levels of illumination are reduced (Endler, 1987). We considered, therefore, that as luminance differed between the two filter types, males may choose, not the absence of UV, but decreased levels of illumination in which to carry out mate assessment. We conducted experiment 4 to test this hypothesis. We expected that males would prefer females viewed through ND treatment filters which reduced achromatic brightness, particularly as the guppies we used were descendants of those caught in a high-predation zone in Trinidad. The results of this experiment showed, however, that males exhibited no preference for females viewed at lower (or higher) light intensity, regardless of filter type.
A possible confounding factor that was not controlled during the male-preference trials, was the behavior of the stimulus females. Non-virgin females were used which may have varied in their state of receptivity. Thus some females may have behaved in a way that was more attractive to the male, and that this was influential in mate choice. However, stimulus females were allotted randomly to the choice chambers in all trials, therefore, receptive females should have been evenly distributed. In addition, it has been suggested that females may indicate their receptivity through visual signals which are only effective in accordance with pheromonal cues (Crow and Liley, 1979; Farr, 1980). Therefore, our experimental apparatus was designed to prevent the transmission of olfactory cues. Furthermore, as test females exhibited a strong preference for UV+ males in experiment 1, we would expect stimulus females positioned behind UV+ filters to respond preferentially toward the test male.
The results of our study demonstrate that guppies are capable of perceiving UV wavelengths and that UV is used in mate assessment. However, results also suggest that male and female preferences differ. Females strongly prefer UV+ males while males tend to prefer UV- females. Furthermore, mate choice decisions of both sexes seem to be based upon hue discrimination rather than perception of achromatic brightness differences.
Differences in preferences between the two sexes may reflect the different strategies that males and females have in order to maximize reproductive success. Females in most natural guppy populations are the limiting sex and can, therefore, select which males they accept as mates (but see Magurran and Seghers, 1994; Matthews and Magurran, 2000; Pilastro and Bisazza, 1999). As indirect (genetic) benefits may be conferred, leading to increased offspring fitness (Reynolds and Gross, 1992), it is advantageous for females to select the highest quality male(s). Therefore, they require as much information as possible regarding male quality, including visual information that exists in the UV.
Interestingly, the pike cichlid (Crenicichla alta) which occurs sympatrically with guppies from high-predation sites and is considered the most dangerous predator of guppies (Endler, 1991), lacks a blue-sensitive cone and thus has poor sensitivity to short wavelengths (Endler, 1991). Therefore, UV photosensitivity in this predator is extremely unlikely. This suggests that the UV waveband may provide a private channel for sexual signaling by guppies in high-predation localities, although this requires further investigation.
In contrast to the female guppy, the optimal strategy for a male is probably to choose a female that will survive the duration of pregnancy. This is particularly pertinent in guppy populations as females are known to be a greater target for predation as they make more profitable prey being, generally, larger than males at sexual maturity (Pocklington and Dill, 1995). Males among this population of guppies tended to prefer UV- females and this may be an adaptation to a high-predation locality. Females viewed without UV wavelengths may appear less conspicuous and thus potentially suffer smaller predation risk. We do acknowledge, however, that although test males tended to prefer UV- females, conclusive evidence of UV- preferences in males awaits further study.
Although these experiments raise the possibility that UV is used in mate choice in different ways by male and female guppies, as Endler and Houde's 1995 study exemplifies, it is not possible to extrapolate findings from one guppy population to others. We need to investigate mate choice preferences of guppies from other populations, including those inhabiting low-predation localities. Interestingly, the diurnal freshwater prawn, Macrobrachium crenulatum, which co-exists with guppies inhabiting low-predation areas, possesses a UV visual pigment (maximally sensitive at ca. 350 nm; Endler, 1991). The diurnal fish predator, Rivulus hartii, which also inhabits low-predation zones, may also be visually sensitive in the UV. This species has a “ violet” cone (maximally sensitive at ca. 410 nm), although the transmission spectra of the ocular media have yet to be measured (Endler, 1991). Despite the possibility that Macrobrachium and Rivulus may both possess UV photoreception, neither species is particularly dangerous to guppies (see Houde, 1997). Hence, it is feasible that UV perception plays a role in mate assessment in guppies from both high- and low-predation systems.
Finally, we do not claim that UV is in anyway “special” in the context of mate choice in guppies, simply that UV perception plays an integral part of mate assessment in this species and has been overlooked in previous studies. It is important to remember that there are major differences between the visual systems of humans and other animals, UV perception being just one. Such differences need to be kept in mind when designing tests of visual behavior if such tests are to be ecologically or evolutionarily relevant.
We thank Ben Seghers (University of Oxford, UK) for the original stock population of fish; also, many thanks to Sarah Hunt and Sean Rands for their comments on earlier versions of this manuscript. This study was funded by the Biotechnology and Biological Sciences Research Council.