Eye of the dragon : visual specializations of the Jacky dragon, Amphibolurus muricatus

Abstract

The Jacky dragon (Amphibolurus muricatus), a semi-arboreal agamid lizard native to south{u00AD}eastern Australia, competes for territories using a rapid sequence of distinct body movements initiated by tail flicks and followed by foreleg waves, head bobs and push-ups. Detection of these signals is particularly challenging against a background of wind-blown plants, and it has been shown that lizards overcome 'motion noise' by adjusting the duration and intensity of the tail flick. Clearly signal efficacy is limited by detection problems, but the visual capabilities of the Jacky dragon, or other lizards for that matter, remain largely unknown. In order to better our understanding of the relationship between visual system design, ecology and phylogeny, and to identify how sensory capabilities relate to signal design, I examine the Eye of the dragon. Jacky dragons are small, fast-moving insectivorous predators which are typically found basking motionless on elevated perches. A broad visual field provides almost complete coverage of their surrounds irrespective of body posture or orientation of the head, and allows lizards to remain vigilant to predators, and to detect potential prey or conspecific intruders entering their territory. Using a custom-built light chamber I show that that they have a dynamic pupil which responds quickly to shifts in ambient light intensity, presumably allowing lizards to maintain high spatial and temporal resolution for accurate control of daily movements through their dynamic environment. I develop a schematic model of the Jacky dragon eye and map the topography of the retina, revealing a centrally located region of elevated photoreceptor and ganglion cell density and a deeply incised foveal specialization. Although the Jacky dragon retina comprises ganglion cell somata that are characteristically small in size, a subset of disproportionately large ganglion cells occupy the peripheral retina. If these giant ganglion cells share functional homologies with the giant ganglion cells reported among other animals, their high concentration in the retina's periphery would increase the likelihood of detecting salient visual motion in the peripheral field of vision, integral in the detection of prey and potential predators. I hypothesize that the introductory tail flick signal is designed to stimulate these giant ganglion cells, capturing the intended receiver's attention and orienting their gaze. This visual grasp reflex allows later signal components to be viewed though a centrally placed fovea, where high photoreceptor and ganglion cell densities provide superior visual acuity. Show more