Anole Annals

Variation in female A. sagrei patterns. Figure from Calsbeek et al. (2010)

Many species of anoles exhibit sexual dimorphism in back patterning, often with the male being relatively uniform and the female festooned with stripes, diamonds, speckles or other geometries (in a few cases, the situation is reversed and the males are the dandies). Surprisingly, there are no reviews documenting the extent of this phenomenon, much less comparative studies explaining its significance (adaptive or otherwise).

Perhaps even more interesting, in some species females exhibit multiple pattern phenotypes within a single population. Most study to date has focused on A. sagrei in the Bahamas. The seminal paper on this topic was Schoener and Schoener’s 1976 study in Evolution, which suggested that pattern variation was related to crypsis, with different patterns being more cryptic in different parts of the structural habitat (e.g., stripes are cryptic on narrow branches). In support of this hypothesis, the Schoeners showed that within a locality, females with different patterns occurred in different parts of the habitat, and among sites, the relative frequency of the types varied in relation to inter-site variation in vegetation.

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"The rediscovery of a missing anole"

Everyone’s favorite anole, A. proboscis, is featured in an article in the most recent issue of Lacerta. The article is chockful of beautiful pictures, such as the one above, but does have one shortcoming, at least for most of AA’s readers: it’s in Dutch! However, thanks to Harvard undergrad Jelle Zijlstra, we can provide a translation of at least part of the text.

Jelle writes:

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The Puerto Rican grass-bush anole, A. pulchellus, displaying. Recent research indicates that this and some other, but not all, anole species time their displays to occur when the wind isn't blowing. Photo @ Rich Glor.

Successful communication requires that a message be detected by the intended receiver. One trick animals have when they communicate is to use signals that stand out against the background, so that they are more easily detected, such as waving light colored structures against a dark background, or making high-pitched calls when surrounded by low-pitched sounds. But what happens when the background isn’t constant? Just as we tend to talk when conversation partners are quiet, animals would be expected to signal at those times when their signals contrast to the greatest extent with the background and thus are most detectable. Reasonable as this hypothesis is, it has only been tested once, in a study which showed that lab monkeys vocalized in silent periods between bursts of machine generated white noise.

Anoles signal primarily in two ways, by moving their head and body up-and-down and by extending their dewlaps. With regard to the former, research has shown that headbobs are effective at catching the attention of other lizards because the rapid and jerky movements contrast strongly with motion in the background. However, this is only true when, in fact, the background—that is, the vegetation and other stuff behind the lizard—isn’t moving very much. When the wind is blowing and leaves and branches are swaying back and forth, headbobs should be more difficult to detect. Consequently, on a windy day, a savvy anole should time its headbobs to occur when the wind is not blowing.

And that’s just what they do—at least some of them.

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Anolis chlorocyanus (photo @ Rich Glor). Anolis chlorocyanus occurs north of Mertens' Line, A. coelestinus to the south. Map on right (from Glor and Warren, 2011) illustrates that suitable conditions for both species occur in the range of the other species (the warmer the color, the more suitable the area).

Many sets of closely related species exhibit a geographic distribution in which species only come into contact at their range border, with one species replacing another across the geographic landscape. Such a “parapatric” distribution could be explained in many ways, such as:

1. The species are adapted to different environments, and their distributions reflect geographic differences in environmental conditions;

2. The environment does not change geographically, but the species are so ecologically similar that neither is able to displace the other from its current range;

3. The species are not reproductively isolated; when they come into contact, they interbreed, thus  preventing coexistence;

4. The species are newly-arisen, and have not yet expanded their ranges into sympatry, or one species has not yet displaced the other completely.

A case in point are the large green anoles of Hispaniola, Anolis chlorocyanus and A. coelestinus. Except for their dewlap, these two trunk-crown species are nearly identical in morphology, and they also occupy similar structural habitats. Yet, A. coelestinus occurs only in the southern peninsula, whereas A. chlorocyanus occurs throughout the rest of the island.

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