Are There Solitary Mainland Anolis Lizards?

One of the most important questions in ecology and evolution is about the role of biotic interactions in driving phenotypic and behavioral changes across species. The insular Anolis species are a good model to address this kind of question due to their high abundance and pervasive ecological interactions across islands. Some insular species, however, live in isolation on small islands across the Pacific and Caribbean islands (21 species). These species have evolved similar morphologies across islands. For instance, Poe et al (1) found that body size evolved by exaptation (remember the classic Gould and Vrba 1982 paper) to colonize these small (and depauperate) islands successfully. By contrast, Poe et al. (1) showed that sexual size dimorphism (SSD) evolved by adaptation likely after island colonization to minimize intraspecific competition.

In brief, these solitary insular anoles evolved phenotypic (body size and SSD) traits by two different processes. Cool! But, what happens in mainland areas? Much work has been devoted to Caribbean species, but the mainland offers many more species and very little research has been conducted there to understand ecological and evolutionary processes. So, we decided to establish whether solitary ecology can be extended to mainland species or whether it is an island ecological phenomena.

The first problem that we had to resolve was trying to establish whether mainland species tend to live in geographical/ecological isolation as insular species. We adopted a novel concept in macroecology (the diversity field concept) developed by Mexican macroecologists (Hector Arita and Fabricio Villalobos see 2, 3) implemented here using extensive distributional information for almost all known Anolis species (377 spp), which I generated during my Ph.D. thesis (see 4 for an example using these maps). The diversity field concept allows us to establish how many species co-occur with a given species across its geographic range.

We calculated how many congeners can co-occur within the distributional area of each Anolis species using the range maps (see figure below). We divided mainland species into two groups: those co-occurring with few congeners (i.e., “solitary-like”, I had to say that his term did not like to reviewers, so we used a “species-poor” forms in the paper). Then, we test whether these “solitary-like” mainland species are different from other mainland species using a randomization approach. Our results revealed that “solitary-like” mainland species exhibit different traits from random mainland assemblages. These unique traits (i.e., uniform body size and greater SSD) suggest that solitary ecology from insular anoles can be extended to mainland settings.

Figure. Diversity fields for some Anolis species. Note that the diversity field is the set of richness values of co-occurring anoles inside each distributional area.

The next question was focused to establish whether similar (ecological and evolutionary) processes affected body size and SSD patterns in a similar way. We found that the phylogenetic position of body size and SSD shifts did not coincide and also with the evolutionary transitions to solitariness (i.e., reduced level of sympatry). We suggested that both traits are decoupled across the entire Anolis radiation and likely that both traits evolved exaptatively. In other words, it is possible to think that “solitary-like” species retained body size and SSD from their most recent common ancestors to facilitates the lonely life.

The paper is very short (less than 2500 words) and was published in the May number of Biology Letters(5).