Non-alphabetically arranged, the ABC islands lie in a row 20-50 miles north of Venezuela with Curaçao sitting in between Aruba 50 miles to the west and Bonaire 30 to the east. Like many small Caribbean islands, each of the islands harbors but a single species of anole. Previous posts have discussed the inhabitant of Aruba and Curaçao, Anolis lineatus. Bonaire’s anole, however, is a beast of an entirely different stripe.
What is curious is not the fact that a small island, not too far from other islands, has its own endemic species. Such anole species occur routinely in the Caribbean, such as A. lividus on Montserrat, A. nubilus on redonda, and A. sabanus on Saba. Rather, the oddity of A. bonairensis is its evolutionary heritage. Unlike A. lineatus, sensibly related to anoles of neighboring northern South America, Bonaire’s pride and joy is phylogenetically closest to A. luciae from distant St. Lucia, fully 500 miles away (check out map above). Given that A. luciae is a member of the roquet clade distributed ubiquitously throughout the southern Lesser Antilles, we must assume that the direction of colonization was from St. Lucia to Bonaire. But how could that have happened? Why didn’t A. lineatus get there first? Or some other anole from a more geographically proximate part of the Lesser Antilles? This discrepancy perhaps can be explained by the geological antiquity of Bonaire and the fact that it is on a different continental plate than the Lesser Antilles; as a result, Bonaire and St. Lucia may have been in much closer proximity in the distant past, facilitating a saurian hop between the two.
Phylogenetic affinities, however, are not the only difference between A. bonairensis and A. lineatus. I have previously noted that A. lineatus is unusual for a solitary anole (i.e., a species which is the only anole on an island) in that it is very similar, both morphologically and ecologically, to Greater Antillean trunk-ground anoles, whereas most solitary anoles in the Lesser Antilles are trunk-crown anoles. In this regard, A. bonairensis is quite normal: it looks and acts just like a trunk-crown anole, exhibiting relatively short legs and being found regularly up in the trees.
One might postulate that this discrepancy reflects environmental differences among the islands. Truth be told, Bonaire is a cactus-patch of an island, very xeric and for all the world looking like southern Arizona. But Aruba and Curaçao aren’t that different. Moreover, if anything, one might expect more desert-like settings to work against the evolution of arboreality, rather than promoting it.
No, I think the answer lies in evolutionary heritage. Anolis luciae, very likely the Bonaire anole’s ancestor, is also a trunk-crown anole, whereas A. lineatus hails from a clade of anoles as terrestrial as any in existence. I would put my money that the differences between the two species primarily reflect their evolutionary starting point rather than their present-day environs.
That leads to a subsidiary question: if the environments are reasonably similar, why haven’t they converged? The bonairistas have apparently been there long enough that lack of time isn’t a viable answer and, in any case, evidence from the Lesser Antilles suggests that being a trunk-crown anole is the norm. So the focus reverts to A. lineatus. Has it not been in existence long enough to evolve to become a trunk-crown anole? Or are circumstances on Aruba and Curaçao different enough to favor trunk-ground instead of trunk-crown anoles? Or another possibility: maybe either anole lifestyle works, two adaptive peaks (if you will), and species simply evolve to the nearest peak, depending on their starting condition. Plenty of fodder for a research project, if you can just figure out how to test these questions.
But as my mind reels, another thought pops up. If A. bonairensis is old and A. lineatus is young, then why didn’t the Bonaire boys colonize Curaçao and then Aruba? Maybe the ocean currents are the wrong way (I think that’s correct). Or some other species, no longer extant, was there? Or A. bonairensis was there, but then evicted when A. lineatus arrived? Now we’re really getting in to the realm of the difficult-to-test speculation.
But one observation is pertinent: even though the two species are ecologically and morphologically different—similar to two different ecomorphs—they have not been able to colonize each other’s islands, even though trunk-ground and trunk-crown anoles coexist merrily in the Greater Antilles, even on islands much, much smaller than the likes of the ABCs. Yet another mystery to be plumbed.