Organisms use different modes of sensing to mediate behaviors in population and community interactions. Each of these senses faces selective pressures that enable survival and reproduction in these interactions, within the environments in which they occur. Although each sensory modality may be given situational priority, multiple modalities are used in conjunction to meet an individual's informational needs in many situations. The ecological and evolutionary influences that pressure species to depend on multiple types of senses has been a subject of growing research.The threespine stickleback (Gasterosteus aculeatus), a species of small fish found throughout the northern hemisphere, is a subject fit for many such investigations. Sticklebacks radiated into a multitude of freshwater habitats from ancestrally marine environments and have thus evolved to face diverse selection pressures from biotic and abiotic sources. By examining differences within and among populations from different environments, I examine how the behavioral ecology of a model species has shaped the interactions of different sensory systems.First, I use anatomical measures of visual, olfactory, and mechanosensory systems to compare how animals invest in different modalities depending on where they evolved or how they were reared. I find evidence of population differences that show populations either invest more heavily in eyes or in olfactory tissues, at the expense of the other sense. This work thus quantifies the interactions of different sensory modalities across populations.I then study the use of visual and olfactory systems both independently and in combination as sensory modes that facilitate joining social aggregations. Though the interpretations of the full data are in progress, preliminary findings indicate that sticklebacks rely on visual over olfactory cues, but do not combine the two in a social affiliation task. However, populations differ in the magnitude of these preferences, and interactions may be of significance among fish with different habitats. This work reveals the ways in which the visual system dominates olfaction in social affiliation.Next, I study the use of olfactory information to discriminate potential mates within and across benthic and limnetic ecotypes. Our evidence suggests that parallel evolution has resulted in the use of olfactory information as a component of mate decisions in benthic, but not limnetic, ecotypes. However, at least some limnetics show evidence of being sensitive to chemical stimuli. This work demonstrates roles of olfaction in behaviors that maintain reproductive barriers.Finally, the use of visual and olfactory senses alone and in combination are investigated in responding to a food stimulus when in the presence of humic acids, an environmental agent potentially disruptive to both modalities. I find fish are most responsive to olfactory stimuli, and this modality is most affected by humic acids. I also find evidence that fish reduce chemical sampling for food cues in the presence of humic acids, when relying on either sensory modality alone.In carrying out these studies, I find varying levels of support for hypotheses about the role of environment on the evolution and ecology of a model species. Further investigation of these areas will deepen understanding about how sensory systems are used in and across different species and habitats.
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