Target detection is enhanced by polarization vision in a fiddler crab

We are constantly surprised by the ability of relatively simple animals to perform precise visually guided movements within complex visual scenes, often using eyes with limited resolution. Exceptional examples include the capture of airborne prey by dragonflies [1], the learning flights of bees and wasps [2], and the tracking of conspecifics by crabs on intertidal mudflats [3-5]. Most studies have focused on how animals do this using sensitivity to intensity or color. However, it is increasingly evident that a third ability, polarization vision, may contribute to such tasks. In many insects, polarization-sensitive photoreceptors are confined within an area of the eye known as the dorsal rim [6], which detects the polarized sky pattern specifically for navigation [7]. However, some animals, including fiddler crabs, are sensitive to the polarization of light across the majority of their image-forming eyes [8, 9], potentially allowing them to use polarization information to increase perceived contrast for general visual tasks [10-13]. Investigations into the use of polarization image-parsing by animals have largely been confined to laboratory settings under artificial lighting [10, 13-18]. This approach can occasionally mislead if the lighting conditions are different from natural [19]. This study presents the first behavioral evidence from the natural context for a function of polarization image parsing. Using experimental manipulations in wild populations of the fiddler crab Uca stenodactylus, we provide evidence that these animals use their polarization vision to enhance contrast in their visual environment, thereby increasing their ability to detect and respond to objects on the mudflat surface.