The role of individual variation in auditory and visual processing: Implications for mate choice
Intersexual interactions often include a dynamic exchange of courtship signals in different sensory modalities. My work focuses on understanding the factors that contribute to variation in male sexual signals and variation in female sensory perception and behavioral responses to these signals. I addressed several specific research questions using the brown headed cowbird (Molothrus ater) as a model species: (i) how does male signaling behavior differ with the sex of the receiver? (ii) how do components within a signal combine to generate a response by the female, (iii) how is female sensory capacity in one modality related to her sensory capacity in a secondary modality, and (iv) does female multimodal sensory biology help us to understand which properties of male mating signals she prefers? I used a cross-correlation analysis of males’ song-types and found that pairs of songs were more dissimilar if they were directed to receivers that differed in sex compared to songs directed to receivers of the same sex. These data support the Motivational Structural Rules hypothesis because males sang songs with significantly higher entropy to males than to females. In addition to singing, many songbirds couple their songs with a visual display. I tested how preference functions (i.e., the function describing female preference across variation in male traits) and subsequent selection on male traits can be altered under four scenarios of varying multimodal signal content. To do this, I assessed female preferences to different levels of male song attractiveness and visual display intensity in an audiovisual playback study. This study was the first to show a switch in the direction of female preferences for a signal component (i.e., visual display) in one modality depending on the attractiveness of the other modality (i.e., song). I then measured female auditory sensory capacity (via auditory evoked potentials) and visual sensory capacity (via visual evoked potentials and cone photoreceptor density counts) to determine if (1) females were sensory generalists and showed correlated sensory capabilities across auditory and visual modalities (i.e., a positive relationship) or (2) females were sensory specialists that showed a trade-off in their sensory capabilities (i.e., a negative relationship). Our data generally show that birds seem to be either (1) sensory generalists in that they are superior at processing both modalities simultaneously, or (2) the sensory capacities are related in unexpected, non-linear way. Finally, I assessed how female mating preferences may be affected by the female multimodal sensory biology. From this study, we have the first evidence suggesting that female sensory capacity (i.e., auditory temporal resolution and visual temporal resolution) affected her mate-preferences. Females with relatively better auditory temporal resolution preferred songs that ended with higher frequency, lower entropy, and shorter notes while females with relatively poorer auditory temporal resolution preferred the opposite. Similarly, females with relatively better visual temporal resolution preferred visual displays that were less intense than those preferred by females with relatively poorer visual temporal resolution. Taken together, our findings suggest that females vary in their sensory capacities across multiple modalities and that this can affect their preferences for male mating signals; male signals, therefore, may be designed to not necessarily reach a single female model but rather a population of females that will differ in how they will process and respond to male signals.^
Jeffrey R. Lucas, Purdue University, Esteban Fernandez-Juricic, Purdue University.
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