The ability to perceive spatial detail (visual acuity) places a physiological limit on a viewer’s ability to discriminate signals of different sizes. We used three methods to quantify visual acuity in male and female green swordtails. First, we mapped the density of retinal ganglion cells (RGCs), the final stage of spatial processing in the retina. We also used two behavioural methods: one used an innate response called an optomotor assay (for which we developed a 3D-printable, low-cost apparatus that we describe in a methods publication, Caves, Troscianko and Kelley 2020, Methods in Ecology and Evolution), and the other used a conditioned choice assay. We found that acuity as predicted by peak RGC density is the same in males and females (3 cycles per degree, cpd), and is characterized by a horizontal region of higher acuity, which may be an adaptation for assessing horizontally-oriented stimuli. Behaviourally, however, both assays showed lower acuity in males (~ 1cpd) than in females (~ 3cpd), a sexual dimorphism that has previously been demonstrated primarily in invertebrates. These results lend key insight into how swordtails perceive one another during mate choice; provide crucial data on how measures of acuity generated using different methods relate to one another, which is lacking in the majority of taxa; and are one of the first demonstrations of sexual dimorphism in visual acuity in any vertebrate. This work has been presented at the virtual annual Poeciliid Fishes Forum and the University of Exeter Friday Talk series, and a manuscript detailing these findings is in preparation for the Journal of Experimental Biology.
We then used two-choice tests, in which a female is given a choice between two male stimuli, and quantified the amount of time she spent with each stimulus, an indicator of mate preference. Using over 500 behavioural trials with 11 pairs of males differing from one another in size by different proportional and absolute amounts, we demonstrated that the proportional difference in size between males is a better predictor of female preference than is the absolute difference. Females exhibit stronger preferences for the larger male in a pair when both males are small, compared to when both males are larger. This work has been presented to the University of Exeter ExBase Symposium and will be presented at the 2021 meeting of the Animal Behaviour Society. Additionally, a manuscript describing these findings will be submitted to Science.
Finally, we performed a comprehensive literature review of published data regarding visual acuity in animals, including humans. This resulted in the largest and most comprehensive database of visual acuity to date, with data on nearly 1000 species. This database has yielded several novel insights. First, within a species, acuity can vary by an order of magnitude; previously, outside of humans, the question of whether certain individuals of a given species can be “near-sighted” or “far-sighted” has received very little attention. Second, the database provides information on how acuity measures in the same species, but gathered using different methods, relate to one another. Lastly, we are using the database to examine acuity in certain taxonomic groups more closely. For example, using data on 92 species of birds, we have shown that avian acuity is correlated with eye size, and that the way in which birds forage as well as their foraging habitat, correlate with acuity. Work on avian acuity was presented at the 2020 virtual meeting of the Animal Behaviour Society, and data analysis is nearly complete for a manuscript to be submitted to The American Naturalist.
