Anole Annals

Bat eating frog. from http://www.impactlab.com/wp-content/uploads/2008/04/bat-eating-frog.jpg

Micronycteris microtis. Photo from http://www.chiroping.org/images/bats/microtis2.jpg

A question that comes up from time to time is whether bats are among the panoply of species that munch on anoles, particularly in the mainland neotropics. As we all know, some bats are renowned for catching and eating frogs, but will they also sup on our little friends? As far as I’m aware, there are no records in the literature of anolivory in bats, but perhaps a reader can correct me on this point. One can imagine two scenarios: first, bats active at dusk or dawn might nab anoles while still active. Alternatively, second, perhaps bats can use their sonar to locate sleeping anoles on leaves. This latter point has generally been considered unlikely because the acoustic clutter in a thick vegetational matrix has been thought to be make it difficult for bats to identify and locate non-moving objects in the vegetation.

A recent study shows that this is not so. Studying the insectivorous bat Micronycteris microtis from Panama, Geipel et al. have just shown that bats can use echolocation to find and capture non-moving prey, in this case dragonflies. More details are provided in the abstract pasted below. It would seem to follow, then, that bats may, indeed, prey on sleeping anoles, but in a critical oversight, the authors fail to comment on this pressing issue.

Abstract: “Gleaning insectivorous bats that forage by using echolocation within dense forest vegetation face the sensorial challenge of acoustic masking effects. Active perception of silent and motionless prey in acoustically cluttered environments by echolocation alone has thus been regarded impossible. The gleaning insectivorous bat Micronycteris microtis however, forages in dense understory vegetation and preys on insects, including dragonflies, which rest silent and motionless on vegetation. From behavioural experiments, we show that M. microtis uses echolocation as the sole sensorial modality for successful prey perception within a complex acoustic environment. All individuals performed a stereotypical three-dimensional hovering flight in front of prey items, while continuously emitting short, multiharmonic, broadband echolocation calls. We observed a high precision in target localization which suggests that M. microtis perceives a detailed acoustic image of the prey based on shape, surface structure and material. Our experiments provide, to our knowledge, the first evidence that a gleaning bat uses echolocation alone for successful detection, classification and precise localization of silent and motionless prey in acoustic clutter. Overall, we conclude that the three-dimensional hovering flight of M. microtisin combination with a frequent emission of short, high-frequency echolocation calls is the key for active prey perception in acoustically highly cluttered environments.”

Mentally put an anole in there, and you can see we’ve got trouble!

Large prey taken by brown anoles (top two photos) and Swinhoe’s tree lizard (bottom two).

Starting in the 1970s, Caribbean anoles became a model system for studying community ecology, especially interspecific competition. Such studies generally focused only on anole species. Though seemingly chauvinistic, this anolocentrism is reasonable in many localities, where resource competition probably is primarily between anole species (although there was a boisterous debate in the 1980s on the extent to which anoles and insectivorous birds might compete).

However, this is not always the case. In Central and South America, for example, the much greater non-anole saurifauna than on Caribbean islands makes it likely that anoles may experience much greater resource competition with non-anole lizards, as well as other taxa. And the same may be true for anoles introduced to far-flung regions.

Take, for example, the brown anole in Taiwan, which occurs with the native Swinhoe’s tree lizard. Like brown anoles, the agamid is found on the ground and low on tree trunks, and thus might be considered a trunk-ground anole. Being only slightly larger than brown anoles, the tree lizard probably eats much the same food. Gerrut Norval posted a while back on the amazingly large prey that brown anoles and tree lizards eat in Taiwan, and now he and colleagues have published a paper documenting the extensive diet overlap between the species (Gerrut previously provided a post on the background to this study, including some interesting information and photographs on the research methods). Very likely they are strong competitors, although Norval et al. argue that the size discrepancy means that the effect is asymmetric. However, at least in some areas, brown anoles have much higher densities, meaning that their aggregate effect on tree lizards may be just as great as the reverse.

Brown anoles are most dense in hot, open areas, whereas the tree lizards reign supreme in shaded habitats, suggesting that environmental effects mediate the outcome of interspecific interactions between the two species. In addition, this difference indicates that reforestation efforts would be a good conservation move to stem the effect of the brown anole invasion.

Do events unfold in a predictable sequence when organisms undergo adaptive radiation? Anoles have diversified in many ecologically important characteristics as they have radiated both in the Caribbean and on the mainland. As one of our best-understood cases of extraordinary evolutionary diversification, they make a great system in which to ask how ecological diversity builds up during adaptive radiation.

The idea that anoles radiated in stages dates to at least 1972, when Ernest Williams derived some hypotheses from his observations of Puerto Rican Anolis in particular, drawing upon earlier work by Stanley Rand and Rodolfo Ruibal. Williams noticed that the most closely related species on Puerto Rico tend to belong to the same ecomorph class and occupy similar structural habitats (e.g. branches, trunks or twigs), but occur in different thermal habitats (e.g. closed forests or hot open areas). He proposed that anoles on Puerto Rico diversified first in structural habitat, and later in thermal habitat, a pattern that might scale up to the entire adaptive radiation of Anolis. While this idea has been discussed many times, and helped to inspire more general hypotheses about stages of radiation (e.g. Streelman and Danly 2003), until now it had not been tested using modern analytic techniques that incorporate phylogenetic information for many species.

Figure 11 from Williams (1972), modified to show only the 8 species in the main Puerto Rico radiation.

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