Polarized light in communication and behavior of two fish species
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Many animals can see polarization of light (a property to which humans are visually insensitive) and use polarization for a variety of behavioral tasks such as navigation and foraging. The polarized light environment is spatially and temporally complex, presenting a unique challenge for signaling or crypsis in animals with polarization vision. Some invertebrates have polarization body patterning that may be used in communication, but only in one species has polarization body patterning been shown to affect receiver behavior, and polarization communication has never been investigated in vertebrates. Many species of fish see polarized light and the aquatic environment is highly polarized; body patterning in visual communication is also common in fish. We measured polarization patterning in the northern swordtail (Xiphophorus nigrensis) and used behavioral assays to measure response to polarization cues of social stimuli in the swordtail and in the rockhind (Epinephelus adscensionis). We found that swordtails have sexually dimorphic polarization patterning. By manipulating the light environment of stimulus males in a two-choice female preference test, we presented females a highly-polarized male and a male with reduced polarization patterning. Females preferred the polarized male, indicating that polarization patterning functions as a sexual signal in swordtails. We measured polarization patterning of swordtails alone and in social contexts, and did not find evidence that swordtails modulate their polarization patterning according to social condition. Rockhinds use color patterning in social dominance interactions and live in highly polarized environments in the Gulf of Mexico. We presented rockhinds with social stimulus images (e.g. images of displaying males, females) and measured behavioral response in two assays. In one assay, the images were not manipulated and thus composed of color, luminance and polarization contrast (as is typical of images displayed with LCD monitors). In the other assay, we manipulated the monitor to remove color and luminance contrast, leaving images of only polarization contrast (invisible to humans and other viewers without polarization vision). Rockhind behavior differed between control conditions (no image displayed) and treatment (social images displayed) for both the complete visual information assay and the polarization-only assay, indicating that they can respond to social stimuli when only polarization cues are present. For most behaviors, response did not differ between the two assay types. Rockhinds responded differently to the different social images for both assays. We find evidence that both swordtails and rockhinds use polarization cues in social behavior, and that polarization patterning functions as a sexual signal in swordtails.