Author: Jonathan Losos Page 129 of 130

Anolis aequatorialis from Ecuador

Caribbean anoles are renowned for the repeated evolution of ecomorphs, the same set of habitat specialists evolving independently on each island in the Greater Antilles.  But what about in mainland Central or South America, where the majority of anole species occur?  Mainland anoles have received relatively little research attention, particularly with regard to questions of ecomorphology (but see Alan Pounds’ fabulous paper from 1988), primarily because mainland species are both less abundant than Caribbean taxa as well as more cryptic, making data collection much more difficult.  Mainland anoles are, as a first approximation, as diverse ecologically and morphologically as Caribbean anoles, and a preliminary study found that most mainland anoles do not fit neatly into any of the Caribbean ecomorph classes.  Now, in a much broader study, Schaad and Poe compared the morphology of 255 species for seven morphological characters: snout-vent length, sexual size dimorphism, femur length, head length, lamella number, snout scale number, and the ratio of tail to snout-vent length.  They found that very few species are, on morphological grounds, similar to Caribbean ecomorphs.  Most of the species that do have similarity are, surprisingly enough, categorized as grass-bush anoles, although ecologically most of these species do not seem to use grass-bush habitats, from what we know of their ecology.  This paper represents a good step forward in our understanding of the evolutionary diversification of mainland anoles, and how it compares to what has occurred in the Caribbean.  The next step will not be so simple, however—getting habitat data for all these species.

The just concluded SICB meeting in Salt Lake City featured 19 talks on anoles.  You can see the titles and authors here, or go to here to check out the abstracts.  Anyone who attended want to tell us about the meetings?

Anolis takes its rightful place on the cover of PNAS. Photo of A. distichus vinosus by R. Glor.

            Larger islands have more species.  Why?  MacArthur and Wilson’s theory explains island species richness as an equilibrium between the input of new species (a function of island isolation) and extinction (inversely related to island area).  Although certainly one of the most influential ideas in biology in the 20^th century, the theory had its limitations, most specifically, that it relied solely on ecological phenomena—colonization and extinction—to explain species richness.  Yet, that can’t be the whole story, because islands are renowned for their evolutionary exuberance—witness the adaptive flowering of lemurs on Madagascar, finches in the Galápagos, honeycreepers on Hawaii and so on.  MacArthur and Wilson were, of course, well aware of the evolutionary component of island diversity and discussed the need to incorporate evolutionary issues into their theory at the end of their monograph.

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Anolis grahami and A. lineatopus

Research on the escape behavior of lizards has become somewhat of a cottage industry in the last two decades, with scores, if not hundreds, of papers examining the effect of factors such as temperature, concealment, and crypticity.  Probably the most important early paper in this area (and perhaps the first period) was Stan Rand’s study of the effect of body temperature on flight initiation distance of Anolis lineatopus.  This work—conducted on the grounds of the University of the West Indies in Mona (a suburb of Kingston), Jamaica—reported that lizards with lower body temperatures fled at greater distances from an approaching predator.  Rand speculated that this pattern resulted because warmer lizards could run faster, setting the stage for the pioneering work on the effect of temperature on sprint locomotion by Ray Huey, Al Bennett, and others.

More than four decades later, Bill Cooper returned to the scene of Rand’s work to further study the escape behavior of A. lineatopus and its relative A. grahami.  Following the method used by Rand and many since, Cooper walked directly toward lizards at a constant pace and noted how far away he was when they fled, as well as the manner in which they escaped.  Although the two species differ in habitat use, A. grahami being more arboreal, escape behavior was very similar.  In both species, lizards tended to escape by running up trees, often by moving to the far side of the tree (termed “squirreling” by many anole aficionados); lizards initially perched lower in the vegetation tended to initiate escape at greater distances; and lizards in areas with greater human activity appeared to be habituated to the presence of people and delayed escape until the faux predator was relatively close.

None of these results is surprising; rather, they agree quite closely with work on other anoles and other types of lizards.  Cooper makes an interesting observation that anoles that flee to the ground, such as grass-bush anoles, show an opposite pattern, fleeing at greater distances when they are perched higher in the vegetation.  This, of course, makes sense because the higher they are, the further they are from safety, the opposite of the relationship that occurs in species that flee upward.  As Cooper notes, more comparative work on other species, both more types of ecomorphs and species from other islands, could prove instructive.  In addition, studies using non-human predators would also be welcome to establish the extent to which behavior elicited in response to approaching humans is representative of how anoles respond to their natural predators.  Other studies have used snake or bird models to study anole escape behavior.  In this paper, Cooper explains why he and others use humans for these tests—ease and repeatability of methods are certainly major advantages.  Nonetheless, research on other types of predators would be an interesting avenue for future work. 

Finally, Anole Annals awards a booby prize to the copy editor of this journal for the unique distinction of having a typo in the first line of the abstract (“fight” instead of “flight”) and what appears to be a sentence fragment that was supposed to have been deleted as the first words of the article itself.

Darwin’s finches are the iconic example of adaptive radiation.  Some researchers, including me, have had the temerity to suggest that the diversification of Caribbean anoles may join the finches as an exemplar case study.  But just how similar are these two radiations, in terms of evolutionary pattern and process?  And can we learn anything from a two-clade comparison?  I explore these questions in a chapter in a recently released book that resulted from a symposium held two years ago to honor Peter and Rosemary Grant.  My chapter concludes:

“Overall, adaptive radiation in Darwin’s finches and Greater Antillean anoles has occurred in very much the same way. Interspecific competition appears to have been the driving force leading to resource partitioning and subsequently adaptation to different niches, and speciation is probably primarily allopatric and may be promoted as an incidental consequence of adaptation to different environments. Differences exist as well, such as the extent of hybridization and of independent evolution on different islands; many of these differences probably result because the radiations differ in age and aspects of natural history.”

If you want to read the whole thing, it’s available here. 

Incidentally, the book, In Search of the Causes of Evolution: From Field Observations to Mechanisms, presents a nice overview of the breadth of evolutionary biology, with chapters by workers as diverse as Dolph Schluter, Andy Knoll, Cliff Tabin, David Jablonski, Scott Edwards, David Wake and Hopi Hoekstra, among others.