Browsing by Subject "Visual ecology"

Understanding how animals see the world and how visual systems have evolved to meet the needs of particular animals are major goals of visual ecology research. The Poeciliidae are a diverse family of Neotropical freshwater fishes and are excellent models for visual ecology research given longstanding interest in visual signaling in this group. However, despite extensive research investigating the form and function of visual signals in the poeciliids, there remains a surprising paucity of research regarding poeciliid visual system function and evolution. To address this gap, my dissertation research sought to investigate: (1) how Trinidadian guppies (Poecilia reticulata) perceive visual stimuli that vary in spatial detail and contrast, (2) correlates of eye size and eye investment across P. reticulata populations that experience different threats from predation and, (3) visual signaling correlates of eye size across the family Poeciliidae.

The first chapter of this dissertation introduces the questions and the study system. In Chapter 2, I examine the ability of Trinidadian guppies (Poecilia reticulata) to perceive visual stimuli that vary in spatial frequency and contrast. Male P. reticulata bear complex body patterning made up of patches that vary in color, contrast, and size, and these visual signals that are known to be important in mate choice. However, the extent to which conspecifics are able to resolve the details of these patterns has historically been overlooked. I used an optomotor assay to measure the behavioral responses of eight individual P. reticulata (N = 4 males; 4 females) to rotating achromatic stimuli. Unsurprisingly, I found that P. reticulata are better able to perceive stimuli as they increase in contrast and decrease in spatial frequency. Moreover, I found that female P. reticulata may outperform males on optomotor tasks.

In Chapter 3, I investigate how predation environment contributes to eye size variation in P. reticulata. Eye size is an important predictor of visual abilities, and it varies widely across taxa. Moreover, eye size is known to be correlated with numerous ecological factors including habitat complexity, light availability, and predation risk. However, less is known about how differences in ecological parameters across populations influence variation in eye size within species. I measured the eye diameter and standard length of 45 females and 307 males from 21 populations of known geographic origin and predation environment. I found that eye diameter was correlated with predation environment after controlling for standard length, with fish from low-predation environments having eyes that are 5.5% - 7.9% larger in diameter than those from high-predation environments. I also found that sexual dimorphism in eye diameter appears to be driven by sexual dimorphism in standard length, as there was no significant effect of sex on eye diameter after accounting for standard length.

Finally, in Chapter 4 I examine variation in eye size across the Poeciliidae. The poeciliids are a diverse family of freshwater fishes to which Poecilia reticulata belongs, and the group exhibits substantial variation in the distribution and types of visual signals used in mate choice. I measured eye size and eye investment for 66 species of poeciliids and took a phylogenetic approach to test whether variation in eye morphology was correlated with aspects of visual signaling. I found that the presence of sexually selected visual signals was associated with greater eye investment and, in particular, that sexual dichromatism was associated with an approximately 6% increase in eye diameter investment compared to species without sexual dichromatism.

Cleaner shrimp are small, brightly-colored tropical crustaceans that attract reef fish clients to set locations on a reef called cleaning stations and then "clean" them by removing ectoparasites from their scales, gills, and mouths. Because clients benefit from the removal of potentially harmful parasites, and shrimp benefit from a meal, this interaction is considered mutualistic. The evolution of cleaner-client relationships is especially paradoxical, however, given that crustaceans make an easy meal for many reef fish. How did such an interaction arise and evolve? Additionally, how do cleaner shrimp and clients find and recognize each other, and why doesn't the client eat the cleaner? One hypothesis is that mutualistic partners evolve signals that identify them as beneficial partners. For the cleaner shrimp-client fish mutualism, it has been suggested that these signals are visual, and function to identify cleaners as helpers, rather than food, and perhaps to identify clients as seeking cleaning rather than a meal.

The broad goal of this dissertation was to examine the cleaner shrimp-client fish mutualism by combining visual physiology, visual ecology, and animal behavior. Throughout, I focus on visual acuity-the ability to perceive detail-an underexplored aspect of visual capability which should represent an important selective force on signals, as it determines what visual information can and cannot be resolved. To begin, in Chapter 2, I characterized the visual capabilities of cleaner shrimp for the first time, in particular examining spectral sensitivity, visual acuity, and temporal resolution of three species from the three primary genera where cleaning has arisen. This revealed that these cleaner shrimp have monochromatic, coarse vision, and thus that their color patterns likely do not serve an intraspecific signaling purpose, but rather may be part of signals directed at client fish. Thus, Chapter 3 examined visual acuity in fish by performing a literature synthesis of known visual acuity values across fishes and examining how acuity relates to certain aspects of morphology and ecology. Here, I found that acuity is higher in fish with larger eyes and in fish that live in spatially complex habitats.

Having examined the visual capabilities of both parties in the cleaner-client mutualism, I then explored signaling between cleaners and clients. In chapter (4), I used sequential analysis to demonstrate that certain stereotyped motions by cleaner shrimp are signals directed at clients, and provide the first evidence of potential signals on the part of client fish as well. Additionally, I developed an R package, AcuityView (Appendix A) which displays images with only the spatial information available to a receiver of given acuity from a given distance, and used it to show that cleaner and client signals are visible to their intended receivers. Lastly, in Chapter (5), I outlined what we know about acuity across species, provide primers on the anatomical basis for acuity and what factors can make acuity higher, and put forward specific predictions and hypotheses about how receiver acuity may influence signal form.

Researchers have long assumed that black widow coloration functions as a warning signal to avian predators. However adult female black widow coloration does not resemble typical warning coloration in two distinct ways. First, black widows are less colorful than most other documented aposematic species. Second, the hourglass shape of an adult female varies both between species and within a site. Here I examine the ultimate causes and consequences of North American black widow coloration.

In chapter two I present data that suggest that black widow coloration not only functions as an aposematic signal to avian predators, but has also been selected to be inconspicuous to insect prey. In choice experiments with wild birds, I found that the red-and-black coloration of black widows deters potential predators: wild birds were ~3 times less likely to attack a black widow model with a red hourglass than one without. Using visual-system appropriate models, I also found that a black widow's red-and-black color combo is more apparent to a typical bird than typical insect (Euclidean color distance ~2.2 times greater for birds than insects). Additionally, an ancestral reconstruction revealed that red dorsal coloration is ancestral in black widows and that at some point some North American black widows lost their red dorsal coloration (while maintaining the ventral hourglass). Behaviorally, differences in red dorsal coloration between two North American species are accompanied by differences in microhabitat that affects how often a bird will view a black widow's dorsal region. All observations are consistent with a cost-benefit tradeoff of being conspicuous to potential predators while being inconspicuous to prey. I suggest that avoiding detection by prey --- combined with Müllerian mimicry --- may help explain why red-and-black aposematic signals occur frequently in nature.

In chapter three, I examine the variation in hourglass shape. Classical aposematic theory predicts near uniformity in warning signal appearance because a uniform signal is easier to learn to avoid than a variable signal. However the shape of the hourglass of North American black widows appears to vary both within and between sites in ways that are inconsistent with classical aposematic theory. Using 133 black widows of three different species from nine sites across the United States, I quantified the variation in hourglass shape and examined how Müllerian mimicry, species type, and condition each influenced hourglass shape. A principle components analysis revealed that 84.5% of the variation in hourglass shape can be explained by principle components 1, 2, and 3, which corresponded to hourglass size (PC1), the separation between hourglass parts (PC2), and the slenderness of the hourglass (PC3). Both a black widow's condition and species significantly predicted hourglass shape; however I found no support for localized Müllerian mimicry within different geographical regions. My results suggest a relaxed role for selection on hourglass shape. I discuss several hypotheses that could explain the variation in hourglass morphology including that potential predators may avoid any red markings rather than an exact shape (categorical rather than continuous perception).

In chapter four I expand on my work from chapter two to examine the eavesdropper's perspective on private communication channels. Private communication may benefit signalers by reducing the costs imposed by potential eavesdroppers such as parasites, predators, prey, or rivals. It is likely that private communication channels are influenced by the evolution of signalers, intended receivers, and potential eavesdroppers, but most studies only examine how private communication benefits signalers. Here, I address this shortcoming by examining visual private communication from a potential eavesdropper's perspective. Specifically, I ask if a signaler would face fitness consequences if a potential eavesdropper could detect its signal more clearly. By integrating studies on private communication with those on the evolution of vision, I suggest that published studies find few taxon-based constraints that could keep potential eavesdroppers from detecting most hypothesized forms of visual private communication. However, I find that private signals may persist over evolutionary time if the benefits of detecting a particular signal do not outweigh the functional costs a potential eavesdropper would suffer from evolving the ability to detect it.