The eye of the dragon: retina differences in dragon lizards

Vision is incredibly important for spotting predators and prey, especially for animals that live in open and flat environments. Dragon lizards, a genus of 29 different lizard species endemic to Australia, are one such group. Despite a lot of similarities among species within this genus, dragon lizards live in different microhabitats; some take shelter in burrows dug in sand ridges, while others find shelter among rocks. Both of these microhabitat types leave lizards exposed in flat environments for at least part of their day, so vision is important for detecting predators, but there may be microhabitat specific selective pressures on vision that could lead to differences in the retinas between species.

To look at this question, Nicolas Nagoo and colleagues mapped the density of neurons in the retina as well as examining broader morphological retinal traits in six different dragon lizard species. Three of the species were burrow-dwelling and three were rock-dwelling, which allowed them to compare between the two microhabitats and look for potential differences.

They found that species who had an ancestral trait of using burrows for shelter have larger eyes, more total retinal cells, and higher spatial resolving power (SRP) than rock-dwelling species. Larger eyes and more retinal cells may be related to higher sensitivity while SRP is related to the size of objects that an animal can see. Living in burrows may require dragon lizards to have more developed visual systems than rock-dwellers in order to detect predators, since they may have less protection or ability to hide than rock-dwelling lizards.

The ancestral burrow-dwelling group only includes two of the three current burrow-dwelling lizards because the use of burrows only recently re-evolved in the third species (painted dragons). Among the current burrow-dwelling lizards, the painted dragon has the smallest eye, lowest number of retinal neurons, and lowest SRP, meaning that it more closely resembles rock-dwelling lizards than fellow burrow-dwellers. This may indicate that there has not been significant selective pressure to reacquire these traits since the re-emergence of burrow use. Vision has a high energetic cost, so if there aren’t strong selective pressures, it would make sense that the more developed visual system has not re-evolved in the painted dragon species.

Overall, they found that the eyes of dragon lizards appear specialized for high visual acuity when compared to other lizards that have been studied and that shelter type is a strong predictor of retinal traits such as eye size and retinal neuron density, but the shelter type of the direct ancestor is better than current shelter type. These findings help us to understand what types of selective pressures might influence visual systems and how these differences have evolved over time.

Reference:

Nagloo, N., Coimbra, J. P., Hoops, D., Hart, N. S., Collin, S. P., & Hemmi, J. M. (2019). Retinal topography and microhabitat diversity in a group of dragon lizards. Journal of Comparative Neurology, 1–17.