La coloration (Coloration)

I generally don’t give much thought to lizard color, outside of making very scientific observations like “Oh boy, this is a pretty one!” or “Look at this handsome fellow!” when we find a particularly striking lizard in the field. (In our most recent experiment, Mahaut was so taken with one of the lizards that she gave him the nickname T.G.O., or The Gorgeous One, but I’ll save that story for another post.) We know, however, that coloration and color polymorphisms are extremely important in thermal ecology, mating systems, and predator avoidance. For example, my good buddy from graduate school, Andy Kraemer, studied the ecology and evolution of color morphology in a common, non-toxic salamander (Plethodon cinereus) that sometimes mimics a toxic newt (Notophthalmus viridescens). For example, see a couple of his very nice papers: Kraemer & Adams 2013 Evolution and Kraemer et al. 2015 J Evol Biol.

Color variation in a few species of small lizards has been well-characterized and has served as a textbook model for how discrete polymorphisms are maintained within populations. The most famous example is the three color morphs of the side-blotched lizard (Uta stansburiana), which are commonly described with the rock-paper-scissors game: each morph has an advantage over one other and a disadvantage against the other. This work comes largely from the Sinervo Lab at UC Santa Cruz and has received much attention in the popular scientific press.

Graphic from the Sinervo Lab Website:
http://bio.research.ucsc.edu/~barrylab/Lisa/Resources/ScienceRPSillustr-large.gif

Here in Europe, the common lizard (Zootoca vivipara; formerly Lacerta vivipara) displays some similarities in its color variation (for example, see Fitze et al. 2014 Front Ecol Evol). Much of this work has been conducted by Jean Clobert, who happens to be head of the Station d’Ecologie Théorique et Expérimentale here in Moulis.

A couple weeks ago, I gave a talk as part of the weekly seminar series at the station, which Jean attended. During the question session, he asked a couple of great questions related to variation in performance and physiology among the color morphs. At that point, I hadn’t given these color morphotypes much explicit though, so I wasn’t able to address the questions very well. That same day, I was fortunate to have Luis M. San-Jose visiting from Toulouse, who has done some really interesting work on the molecular basis of lizard (and owl) coloration and why it is important (for example, see San-Jose et al. 2013 Am Nat and San-Jose et al. 2015 Mol Ecol). Given Jean’s questions and my discussion with Luis, I was prompted to think a bit more explicitly about color variation. Having my curiosity piqued, I went to the literature to see what is known about color polymorphisms in P. muralis from this region and how it might relate to our current experiment. This geographic region is in fact unique in that we have a seemingly stable mix of color varieties, while populations in other parts of the species range exhibit little variation. Lizards can be broadly divided into three discrete color types, as well as their intermediaries (working from the simple and untested assumption that these colors are a single-locus, three-allele traits):

Figure from Calsbeek et al. 2010 J Evol Biol showing discrete color morphs in P. muralis.

A couple studies have been conducted to examine the drivers of how these apparently stable polymorphisms are maintained, both coming from work here at SETE. Because the color differences are part of a suite of correlated traits, including body size and immune function, it is thought that multivariate selection has resulted in these discrete polymorphisms, of which color is the most conspicuous trait (see Calsbeek et al. 2010 J Evol Biol). Other work demonstrates that the color morphs do not differ in their sprint speed, which is important for escaping predators, catching prey, and finding mates (see Zajitschek et al. 2012 Biol J Linn Soc). With this in mind, I was interested to see what our data showed about differences in performance or physiology related to coloration. When we initially processed our lizards, we did record the ventral color pattern and found striking differences.

Some of the variation in ventral coloration in the lizards from our experiment.

This past week I re-analyzed some of our data considering color morphology… and the results are interesting. It turns out that lizards with a solid orange belly run much slower than either white or yellow lizards, even though there is no difference in size. This result is in contrast to some of the previous work mentioned above, which found no difference in sprint speed despite that orange lizards were bigger.

Top sprint velocity achieved by lizards of different color morphs. Despite being the same size, orange lizards are significantly slower than white or yellow-bellied lizards.

Color did not seem to affect any other aspect of lizard performance or physiology – only sprint speed. At least biology is never dull and predictable…