Trying to See Red Through Stickleback Photoreceptors: Functional Substitution of Receptor Sensitivities

Computerized stimuli for studying oddity effects

Visually hunting predators must overcome the challenges that prey groups present. One such challenge is the confusion effect where an overburdened visual system means predators are unable to successfully target prey. A strategy to overcome confusion is the targeting of distinct, or odd, individuals (the oddity effect). In live prey experiments, manipulation of group member phenotypes can be challenging and prey may differ on more than the single feature one intends to define as odd. The use of highly controllable computerized stimuli to study predator–prey interactions is increasingly popular in the field of behavioral ecology. However, to our knowledge, the validity of computerized stimuli to study the oddity effect has not been established. Predator choice experiments were conducted using naive stickleback predators to ascertain whether the oddity effect could be demonstrated in the absence of live prey. We found evidence for both the oddity effect and preferential targeting of group edges and low-density regions, as would be predicted if predators targeted prey individuals to minimize confusion. The oddity effect was evident at a low threshold, above which dots were no longer perceived as odd and no longer attacked more often than expected by chance. We conclude that computerized stimuli are an improved, practical method for studying oddity effects while further validating the use of similar methods for studying other aspects of visual predation. In addition to higher control of “prey” appearance, the replacement of live prey animals with digital stimuli is ethically beneficial and reusing code improves experimental efficiency.

Color-induced changes in oxygen consumption and swimming performance of juvenile bighead carp (Aristichthys nobilis)

Fish physiology and behavior are affected by exposure to light of varying colors, but little is known about the effects on swimming performance and metabolism. The purpose of this study was to measure the effects of light color on the swimming performance of bighead carp (Aristichthys nobilis), a planktivorous fish species widely used in aquaculture. Stepped velocity testing was conducted in a modified Brett-type swim tunnel respirometer to determine critical swimming speed (U_crit), oxygen consumption (MO₂), and excess post-exercise oxygen consumption (EPOC) for juvenile bighead carp under red, yellow, blue, and green lights at 25 °C. U_crit was significantly lower for fish swimming under yellow and green lights compared with red and blue light. Cost of transport (COT), a measure of swimming efficiency, also varied with color. The results, obtained under controlled conditions, add to our knowledge on the effects of artificial color light on fish physiology and behavior and inform decisions on the use of artificial color light in aquaculture and fishway design.

Sexual isolation promotes divergence between parapatric lake and stream stickleback

Speciation can be initiated by adaptive divergence between populations in ecologically different habitats, but how sexually based reproductive barriers contribute to this process is less well understood. We here test for sexual isolation between ecotypes of threespine stickleback fish residing in adjacent lake and stream habitats in the Lake Constance basin, Central Europe. Mating trials exposing females to pairings of territorial lake and stream males in outdoor mesocosms allowing for natural reproductive behaviour reveal that mating occurs preferentially between partners of the same ecotype. Compared to random mating, this sexual barrier reduces gene flow between the ecotypes by some 36%. This relatively modest strength of sexual isolation is surprising because comparing the males between the two ecotypes shows striking differentiation in traits generally considered relevant to reproductive behaviour (body size, breeding coloration, nest size). Analysing size differences among the individuals in the mating trials further indicates that assortative mating is not related to ecotype differences in body size. Overall, we demonstrate that sexually based reproductive isolation promotes divergence in lake-stream stickleback along with other known reproductive barriers, but we also caution against inferring strong sexual isolation from the observation of strong population divergence in sexually relevant traits.

The sexual selection paradigm: have we overlooked other mechanisms in the evolution of male ornaments?

Extravagant male ornaments expressed during reproduction are almost invariably assumed to be sexually selected and evolve through competition for mating opportunities. Yet in species where male reproductive success depends on the defence of offspring, male ornaments could also evolve through social competition for offspring survival. However, in contrast to female ornaments, this possibility has received little attention in males. We show that a male ornament that is traditionally assumed to be sexually selected--the red nuptial coloration of the three-spined stickleback--is under stronger selection for offspring survival than for mating success. Males express most coloration during parenting, when they no longer attract females, and the colour correlates with nest retention and hatching success but not with attractiveness to females. This contradicts earlier assumptions and suggests that social selection for offspring survival rather than for sexual selection for mating success is the main mechanism maintaining the ornament in the population. These results suggest that we should consider other forms of social selection beyond sexual selection when seeking to explain the function and evolution of male ornaments. An incorrect assignment of selection pressures could hamper our understanding of evolution.

Role of the iridescent eye in stickleback female mate choice

Many vertebrates exhibit prominent body colours that are used in courtship and territorial communication. Some fishes also have an eye whose iris becomes iridescent during the mating season, as in the threespine stickleback. Behavioural studies in this species have focused on the redness of the throat/jaw as the primary determinant of female mate choice. Unlike the iridescent eye, however, the red throat/jaw is not present in all stickleback populations, suggesting that the colour of the eye may be equally important for female mate choice. Here, we used data on photoreceptors and environmental light to assess body conspicuousness and the colour contrast of courtship signals for stickleback populations living in a range of waters, from clear (mesotrophic) to red light shifted (dystrophic). This analysis indicated that the redness of the throat/jaw is expressed to enhance the contrast of the eye. To test the importance of eye colour as a courtship signal, we carried out mate choice experiments in which females were presented with identical videos of a courting male but for the colour of the eye and/or the throat/jaw. Females did not choose based on differences in throat/jaw redness between videos, but preferred males with the highest contrast between the eye and the throat/jaw. This result points to the blue iridescent eye as the primary courtship signal in stickleback female mate choice.

Naturalistic color discriminations in polymorphic platyrrhine monkeys: Effects of stimulus luminance and duration examined with functional substitution

X-linked photopigment polymorphism produces six different color vision phenotypes in most species of New World monkey. In the subfamily Callitrichinae, the three M/L alleles underlying these different phenotypes are present at unequal frequencies suggesting that selective pressures other than heterozygous-advantage operate on these alleles. Earlier we investigated this hypothesis with functional substitution, a technique using a computer monitor to simulate colors as they would appear to humans with monkey visual pigments (Visual Neuroscience21:217–222, 2004). The stimuli were derived from measurements of ecologically relevant fruit and foliage. We found that discrimination performance depended on the relative spectral positioning of the substituted M and L pigment pair. Here we have undertaken a systematic examination of two simulation parameters—test field luminance and stimulus duration. Discriminability of the fruit colors depended on which phenotype was simulated but only at short stimulus durations and/or low luminances. Under such conditions, phenotypes with the larger pigment peak separations performed better. At longer durations and higher luminances, differences in performance across different substitutions tended to disappear. The stimuli used in this experiment were analyzed with several color discrimination models. There was limited agreement among the predictions made by these models regarding the capabilities of animals with different pigment pairs and none predicted the dependence of discrimination on changes in luminance and stimulus duration.