Animals use a diversity of signals for communication (e.g. songs, colors), many of which are honest indicators of their worth as a mate or rival. However, the ways that animals perceive the world and receive transmitted signals may also be linked to their quality (e.g. health, condition), and such control of signal reception could impact the evolutionary pace and directions of animal communication systems. This research will involve investigations of the degree to which the color-visual sensory system of a songbird species (the house finch, Carpodacus mexicanus) is impacted by environmental factors, including diet, disease, and light exposure, and affects critical behavioral tasks like food selection and mate selection, both of which involves color preferences. Condition-dependence of color vision will be explored by tracking accumulation of the types and amounts of carotenoid pigments (e.g. the orange colorants of carrots) dedicated to the retina, where they are thought to filter light, protect the eye from photodamage, and tune color visual sensitivity. This approach provides a key molecular link between color vision and color production, which in house finches and many other animals occurs in the form of red, orange, and yellow carotenoid pigments whose supplies in feathers are also limited and condition-dependent. The biochemical, nutritional, and behavioral approaches taken in this research will enrich understandings of (a) the suite of benefits conferred by antioxidant nutrients in animals, (b) resource allocations and trade-offs in the context of animal communication, and (c) the coevolutionary mechanisms governing sexual traits and preferences in animals, tracked in a common chemical currency. The urban nature of the field research also affords the opportunity to involve local schoolteachers and children in data collection and science training, so that they may test first-hand the impacts of human encroachment on the communication system and viability of a desert-adapted organism.
We acquire many carotenoid pigments from the fruits and vegetables we consume in our diet, and these have important effects in our body as antioxidants, stimulants of the immune system, and on eye health. Carotenoids serve as key vitamin-A precursors for maintaining healthy eyes, and also are thought to shape eye function by acting as screening pigments to protect from harmful UV and to shape color vision. Little is known of the mechanisms or function of carotenoids in the eyes of wild animals. In a common wild species of bird - the house finch (Haemorhous mexicanus) - we tested (a) how environmental and physiological factors shape accumulation of carotenoids in the retina, and (b) the possible visual role of carotenoid variation in the eye as it relates to foraging for colorful foods, selecting colorful, high-quality mates, and protecting the eye from solar damage. Our first field study of wild finches revealed that carotenoid variation in the eye did not differ between the sexes but was linked to plumage coloration and body condition. Redder birds in better condition had higher more carotenoid in retina. We followed this initial work with a series of lab experiments, in which we manipulated dietary access to carotenoids and found that retinal carotenoid deposition is sensitive to nutrition, but only over a long period of time (ca. 2 months on a manipulated diet); this is different from carotenoid circulation and incorporation into other body tissues, which is much more sensitive to current nutrition (i.e. over a few days). We also found that perturbations to the immune system lowered levels in retina, perhaps to combat the immunological challenge. These results support the idea that eye carotenoid deposition is a condition-dependent trait, but that carotenoid stability or turnover rates are high in eyes, perhaps due to the high priority for maintaining daily high-quality eye function. We proceeded to test if carotenoid manipulations through diet could have lasting effects on food color selection or mate color selection in adult house finches. We discovered that high carotenoid levels impair achromatic discrimination of food, likely because high levels of light-absorbing molecules in the eye affect ability to perceive light-dark variations in a scene. We failed to find any effect of carotenoid status in retina on mate selection based on plumage color, though we did find that circulating carotenoid levels were coupled to mate choosiness. In a separate study where we manipulated how much light birds were exposed to during the day, we found that retinal carotenoid accumulation did not differ as a function of ambient lighting conditions. Taken together, these results suggest that variation in retinal carotenoid levels may function most in a foraging context and less so for mate selection or eye protection. In completing this extensive set of field and lab studies, we trained over 25 undergraduates (including many females and ethnic minorities), 2 graduate students, 1 post-doc, and one high-school teacher in our behavioral, nutritional, morphological, immunological, or biochemical techniques. We have published 11 peer-reviewed articles from our work on this grant since 2009 and delivered many lectures and posters on these research projects at national and international conferences in our disciplines. We developed an urbanization component to this work and thus cooperated with many local and city park and wildlife managers, to better understand carotenoid variation across the different habitat types and land uses in urban, suburban, and rural areas. We found that carotenoid levels in tissue (including plumage) are lower in urban areas, but that natural areas (including zoos and farms) nearer to the city did not experience such effects. We used these data to inform officials on proper land management for maintaining vibrant, healthy birds.
