The Anolis dewlap is a recurrent topic of discussion on Anole Annals. This is not surprising considering that it is commonly view as playing a role in many aspects of social interaction, including species recognition and even sexual selection. Although, I am unaware of empirical studies supporting sexual selection, in the context of female choice.
A recent post by (Ian Wang) asked the question, "Does This Dewlap Go With My Signalling Environment?” In order to answer this question I would encourage the readers of Anole Annals to have a discussion of what is really is an "anole's signaling environment."
The paper by Ng et al. (2012) presents some interesting results, and I would encourage everyone to read this paper. The amount of data presented in this paper is impressive, with the authors combining molecular, dewlap reflectance, and satellite data (i.e., GIS data), to evaluate if there is a relationship between dewlap traits and climatic variables across populations of A. distichus. As the precision of GIS data increases, the ability of exploring questions at a finer geographical scale is becoming more common. This paper nicely illustrates this approach. Additionally, A. distichus is a nice system for the study of dewlap variation. In fact, in my opinion, one of Al Schwartz (1968) best anole monographs describes all sorts of geographic variation in the distichus complex. This monograph is a must read for all Anole Annals fans, with beautiful plates and a lot of natural history data.
One of the main findings of Ng et al. is that geographic variation in dewlap coloration is correlated with the "habitat types" in which populations are found. Interestingly, habitat type seems to have a stronger signal than geographic or genetic distance between populations. I have to admit that I am biased, but this is music to my ears. However, before we jump into further conclusions, I feel that it is important to take a step back and evaluate the question I posed at the start of this post – namely, what is the "signaling environment"?
Trying to figure out the correct way to measure the signaling environment has kept me up for many, many nights. Based on what we know, the signaling environment is not simply the “forest type” or degree of vegetation cover, which at this stage is the type of information that can be extracted from GIS data. Those variables are too coarse to provide a proxy for the signaling environment. From the perspective of an anole, the signaling environment is much finer, on a scale that can account for variation in signaling environment within a single “habitat type”. For example, within a moist forest there can be multiple signaling environments with regard to light levels, as we have demonstrated for Puerto Rican anoles (Fleishman et al 2009). In a similar vein, both A. cooki and A. cristatellus inhabit xeric forest, and their signaling environments are different (Leal and Fleishman 2002).
|Anole Signaling Environment|
But, does this mean that if we measure light intensity (i.e., irradiance) alone, we are done characterizing the signaling environment? The short answer is no. There is also a chromatic component to dewlap detection, which means that we need to know not only the intensity of the light, but also the color shape of the background (i.e., radiance). I would like to raise a word of caution and suggest that for now GIS data might not be a feasible approach to addressing questions of dewlap evolution as it relates adaptation to "signaling environment".
Also, I have learned the hard way, what makes a dewlap brighter or darker is not its reflectance. Although the light reflected off a dewlap has an obvious contribution to overall brightness, the largest contribution typically comes from the amount of light that is transmitted through the dewlap.
Going back to the paper by Ng et al, this is an interesting first step and can serve as a springboard for further research. But for now, I believe that when using coarse measurements of habitat type, the jury is still out with regard to the effect of “signaling environment” on dewlap diversity. A logical next step for this type of approach should be to evaluate whether data collected at the micro-scale, which at the moment is our best proxy for what is relevant to the lizard's "signaling environment," offer similar results to those provided by the large-scale data used in their study.
Finally, the dewlap is one the coolest traits of anoles, and compared to all the work that has been done in anoles, it is mind-boggling that we know so little about its function. This includes a lack of experimental evidence to support the widely common view that the dewlap is used during the process of sexual selection. Unless sexual selection is used in a broad fashion (see Leal and Losos 2010 for discussion), it might be premature to discuss the role of sexual selection in dewlap evolution.