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).
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| 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.
