Evolution and Function of Multimodal Communication in <i>Sceloporus</i> Lizards

Despite numerous energetic and fitness costs associated with signaling and signal perception, most animals produce and use multiple sensory modalities (e.g., olfactory, visual, auditory, etc.) to communicate with each other. Selection for or against multimodal communication may arise from a variety of factors, including but not limited to, signaling context and phylogenetic or mechanistic constraints. Here, I took an integrative approach that considers morphology, behavior, and evolutionary history of Sceloporus lizards to examine whether signaling context is linked to chemical and visual signal utility and whether these two modalities are evolutionarily and mechanistically linked. First, in a field study, I measured S. jarrovii responses to territorial intrusions of isolated conspecific visual and chemical signals, asking whether one of two competing hypotheses (Modality Appropriateness or Sensory Specialization) better explains signal utility. I found support for the Modality Appropriateness hypothesis, as S. jarrovii individuals adjusted space use in response to chemical signals but not visual signals, despite evidence that suggests this species to be a visual specialist. Next, in a separate field study, I investigated whether chemical and visual signals function for species discrimination when perceived simultaneously. Despite being a visually oriented species, I found that both visual and chemical signals are used by S. graciosus to discriminate between conspecific and heterospecific individuals, and that the combination of the two signals is likely interactive (i.e., signal dominance was not observed). Lastly, combining evolutionary history and morphology for 11 Sceloporus species, I examined whether evolutionary loss of a visual signal coincided with morphological shifts in a chemosensory organ, as expected if the two modalities are evolutionarily linked. I found that the two modalities are instead dissociated, and that chemosensory morphology is more closely related to body size. In conclusion, the results suggest that Sceloporus visual and chemical modalities are not evolutionarily constrained by one another, which may explain flexibility in signal function, such that visual and chemical signals are functionally integrated in some contexts (e.g., interspecific interactions) and not others (e.g., intraspecific interactions).