There’s been no shortage of enthusiasm when it comes to thinking about the anoline dewlap. From the recent findings that dewlaps are highly functional in low light environments to large scale characterization of dewlap diversity, these charismatic ornaments are captivating to biologists from many fields. However, they are also proving to be amazingly dynamic and difficult to functionally characterize universally. What we know definitively is that dewlaps in anoles are used during territorial and mating interactions. There is some evidence to support a correlation between dewlap morphology and a lizard’s physiological characteristics or body condition. Dewlaps are highly diverse in both size and color; some species having enormous dewlaps that extend from snout to vent and some having virtually no extendable dewlap at all. Dewlaps also differ between the sexes. In most, but not all, species, males have a larger dewlap than females, but the degree of sexual dimorphism is highly variable across the genus. Strikingly, dewlaps are not unique to anoles! Within iguanid lizards, dewlaps have evolved at least twice, in anoles and Polychrus. Ambika Kamath has done extensive work on the Indian lizard genus Sitana, a dewlap sporting agamid genus. Going a step further, lizards such as Pogona, bearded dragons, and Chlamydosaurus, the frilled lizards, also have elaborate throat ornamentation that develops from similar throat structures as the dewlap (the hyoid apparatus).
While the ecology and diversity of the “dewlap” have been studied in a variety of contexts, the evolution of its underlying skeletal structure is a black box. Questions ranging from, “How many times has the dewlap evolved among lizards?”, “Do the same skeletal structures support an extensible throat in different groups?”, to “What selective forces are driving dewlap diversification?” have yet to be answered systematically. Recently, Ord et al. examined the factors influencing throat morphology diversification and systematically describe throat morphology evolution across iguanid and agamid lizards. They conducted a comparative phylogenetic study, first asking if the diversity in elaborate throat ornamentation across lizards can be explained either by the influence of male-male competition or by the need to signal in visually complex environments. They found that there are more species with colorful dewlaps or throat appendages in forested environments, suggesting selection for conspicuousness in visually complex environments.
Looking at the evolution of colorful and elaborate throat morphologies more broadly, the authors found a single origin of a colorful throat ≥ 110 MYA for agamid lizard. From there, throat morphologies took one of two evolutionary routes, either being modified to include a large appendage or reverting to a non-ornamented/non-colorful state. Following from their previous result, they hypothesize that the loss of a throat ornamentation is due to historical transitions into more open habitats where displays are less constrained.
Anolis is characterized by their large moving dewlaps because the skeletal elements that support the dewlap are highly specialized for rapid extension. However, both iguanids and agamids have a hyoid apparatus, and distinct skeletal elements support the extensible throat skin in each group. The dewlap of anoles is supported by the second ceratobranchial cartilage in the hyoid apparatus and is extended by the contraction of muscles attached to the front end of the hyoid system, causing the second ceratobranchials to rotate downward from the throat of the lizard. Most iguanid lizards exhibit a similar, conserved hyoid morphology, but typically with a smaller second ceratobranchials and no dewlap compared to anoles. Alternatively, across agamids, the authors found amazing diversity in hyoid morphology. While some agamids have a strikingly iguanid-like hyoid, others demonstrate extreme reduction in the length of the second ceratobranchials and extension of other elements of the hyoid system. For example, the beard of a bearded dragon results from the loss of second ceratobranchials altogether coupled with an extension of the first ceratobranchials to support the charismatic lateral frills. When mapped on a phylogeny, hyoid morphological diversity supports a single origin of a movable dewlap with extended second ceratobranchials in iguanid lizards (in Anolis) and two independent origins of a moveable dewlap with extended second ceratobranchials in agamids. Including throat fans that are supported by alternative hyoid modifications, as seen in the bearded dragon, there are another two origins of moveable throat morphologies in agamids (see Figure 3 from Ord et al below).
There is much more work to be done to further understand the specific environmental factors influencing the evolution of elaborate throat morphologies in lizards. However, this study strongly suggests that there are many factors at play. Also, the extensive characterization of hyoid morphology across agamid and iguanid lizards in this study sets up many questions about the physiological processes driving the diversification. What processes cause hyoid morphology to vary so dramatically across a group of closely related groups?
