Several years ago I was involved in a study showing that the dewlaps of individual male green anoles change size over the course of a breeding season, increasing in area from winter to spring and then shrinking from spring to winter. This result was first noted in the field and verified in the lab, and is not a statistical artefact – individual dewlaps really do change size!
Shortly after that study appeared I found myself in Australia doing postdoc work on crickets. During that time I gained an appreciation for life-history and the battery of approaches, ranging from artificial diets to mating schedule manipulations, which researchers use to expose resource allocation priorities in animals. (On a related note, I also gained an allergy to crickets). When I returned to the lizard world I started thinking about dewlaps and resource allocation, and I wondered if it might be possible to apply some of these life-history techniques to anoles to figure out the mechanisms underlying the incredible growing/shrinking dewlaps.
It turns out that not only is it possible, it’s actually pretty easy, and my research group was recently able to conduct a simple dietary restriction experiment that yielded some unexpected results. We wanted to test whether dewlap size is affected by resource availability, as it may be that male green anoles put more energetic resources towards growing bigger dewlaps during the spring and summer. Courting, mating and defending territories are energetically expensive though, so if males spend lots of energy breeding, then by the time the fall rolls around they may have few resources left with which to maintain those large dewlaps – hence the winter shrinkage.
We went about testing this idea by feeding one group of field-caught juvenile male green anoles lots of crickets, and another group very few. We then tracked the development of morphology, dewlap size and bite force in the lab over the next 3 months to see how lizards were allocating their acquired energetic resources during ontogeny. Unsurprisingly, by the end of the experiment lizards on the restricted diet were smaller, lighter and exhibited poorer bite forces (both absolutely and corrected for body size) than lizards raised on the ad libitum diet, most likely because restricted diet lizards were unable to grow the large jaw adductor muscles required for generating high bite forces. The results for dewlaps, however, were strikingly different – lizards raised on the restricted diet were able to grow dewlaps that were proportionately the same size as those on the unrestricted diet! So dewlap size does not appear to be affected by resource availability at all.
There are a few caveats to make about these findings, though. First, our dietary restriction regime was pretty extreme, and it’s unlikely that anoles ever experience that degree of resource limitation in nature. Still, if dewlaps aren’t affected by such a draconian treatment, they likely won’t respond to more moderate resource limitation either. Second, our experiment doesn’t give us any insight into why dewlap size is not condition-dependent. To my mind, there are two possible explanations: 1) since dewlaps are used in several different selective contexts, they may be important enough to be “protected” against resource allocation trade-offs; 2) dewlaps are just really cheap to produce!
I’ve yet to come up with a cunning way to empirically discriminate between these two explanations. In the meantime, we are conducting further experiments to investigate why dewlaps change size now that life-history trade-offs are seemingly off the table (my money is on skin elasticity). Given that anoles seem to be so amenable to these kinds of techniques, I hope to have more to say about this subject soon.
- Eye of the
/TigerGreen Anole - October 24, 2014 - Are You Displaying At Me? There’s No One Else Here… - November 15, 2012
- Sizing Up Green Anole Dewlaps - April 13, 2012
James Lazell
Very interesting: thanks! Skip
Thomas Sanger
Cool stuff. Two questions: First, were their any proportional changes in skeletal morphology anywhere in the nutritional experiment? I tend to think of “body size” as reflecting a combination of nutritional and genetic factors while the “relative size” of traits as something regulated at the genetic level. Finding changes in proportion would imply that particular skeletal elements respond uniquely to nutritional stimuli. With this in mind, maybe it isn’t surprising to find that the dewlap didn’t change in its relative size.
My second question likely pertains more to the first study. How does an adult anole change the size of their dewlap? Are they changing the length of the cartilage, increasing the stretch of their skin, or using some other mechanism? Growth of the cartilage would not all that surprising as cartilage length tends to scale with positive allometry in many species between hatching and adulthood (my personal obs.). In other words, a mechanism is already in place to elongate the dewlap cartilage. However, observing cartilage regress would be surprising as it would require seasonal fluctuations in chondrocyte proliferation and degradation. That would really be something worth further study!
Simon Lailvaux
Thom,
We have those morphology data, but I haven’t analyzed them properly (yet). Based on some quick-and-dirty preliminary analyses, restricted diet animals are shaped quite differently to the ad lib males. They’re actually more female-shaped, although they don’t quite cluster with females in morphospace. So they’re not just smaller males.
As for your second question, I can’t answer it just yet. In that original study, we found some evidence that the ceratobranchial was growing in the spring, but I’m not 100% convinced. However, Michele Johnson and I have an experiment underway right now that will hopefully tell us whether we’re dealing with elasticity or growth/regress (or both!).
jerryhusak
Some relevant info: We tried to enlarge dewlaps with testosterone in green anoles. They didn’t respond with a change in area or in ceratobranchial length, but more importantly here, ceratobranchial length was not correlated to dewlap area. This is surprising since the cartilage has androgen receptors, but that’s a whole other story…
Thomas Sanger
Thanks Simon. Quite intriguing.
Jerry, I am trying something similar with my studies of skull shape dimorphism right now. I wonder whether the receptors are creating a bottleneck – no matter how much hormone you pump in the tissue can only respond up to a point. Did you have a negative control? If androgens were driving growth I presume that a lack of hormone would stunt growth or turn it off completely. (sorry, I don’t know what paper this is)
Cristian
Hi, i have a question.
This dimorphism also occurs according to the habitat of the species?
Martha Munoz
I remember an interesting talk at SICB 2012 by Bieke Vanhooydonck looking at relative dewlap size in crown-giant anoles. Basically, as anoles get bigger they tend to have relatively smaller dewlaps. As referenced above, the authors thought that skin elasticity had something to do with this. Perhaps this also pertains to your species.