There are many chilling realities associated with global warming. One of the major lines of research in climate change is to understand how organisms will respond to increasing temperatures. Ectotherms such as reptiles are excellent model systems for learning how organisms will be affected by climate warming as their performance (running, jumping, etc.) is so tightly linked to temperature. Research by Ray Huey and colleagues, for example, has shown that increasing temperatures is pushing some lizards to their thermal limits, leading scientists to suggest that some lizards might not be able to take the increase in heat that is expected over the next few decades.
But spending three years working at high elevation in the Dominican Republic has made me wonder a different question – Can lizards take the cold? Beginning around 1,700 meters or so in the DR you begin to enter a strange habitat. At these high elevations the habitat is composed of pine forests that are reminiscent of New Hampshire, and require that you remind yourself that you are, indeed, still in the Caribbean. It is cold up there – near Valle Nuevo in the Eastern Mountains and near Loma de Toro in the Western Mountains the mean winter temperature hovers just above freezing. Even in summer the nights are cold and the crepuscular hours tangibly chilly.
Like so many unexpected and seemingly inhospitable places in the New World, anoles abound. Anolis shrevei and A. armouri are found at high elevations in the Eastern and Western Mountains, respectively. Both are phylogenetically nested within A. cybotes, the widespread Hispaniolan trunk-ground anole. What is remarkable about these taxa is that closely related populations can be found in such different environments. While some A. cybotes can lounge on palm trees in balmy Punta Cana, A. shrevei and A. armouri have to brave the elements in chilly pine forests.
Last year I found a strange A. shrevei carcass on the road connecting Valle Nuevo to Piramide, which I described in an AA post. By the by, casually ogling the macabre milieu of anoledom is not unusual on this blog (see 1, 2, 3, 4, and 5). At the time, I figured that this female had perhaps gone onto the road to lay an egg and met an unfortunate end under the back tire of a rambling moped. But then this summer I saw another stiff A. shrevei, this time far removed from the road.
As you can see in Figure 1, this lizard is dried out and doesn’t appear to be crushed. I found it in an open clearing at least 15 meters from the road. For comparison, I did find an A. shrevei that I believe was crushed by a car or a moped (Fig. 2). I found it right on the road and, as you can see, its head appears quite squashed. It is also possible, however, that something caught it and ate half of its face off. I find this unlikely. I flipped the lizard onto its back, and it appears intact, suggesting that the damage seen on the dorsal side is from a tire flattening its skull.
Not long after seeing this lizard, I found another dead A. shrevei in a state similar to that of the first dessicated lizard. These two dried out lizards were found within 5 meters of each other! I put these side by side to show that they are, indeed, two different individuals (Fig. 4). What is puzzling is that I generally don’t see many dead lizards, at least not so intact if dessicated, elsewhere in the DR. And because these hapless lizards were quite far from the road (and bear no obvious signs of having been squashed) I think they might have frozen to death.
It gets quite cold in Valle Nuevo, even in summer. Mornings are frosty and lizards have to deal with temperatures than can hover just above freezing. In winter, those temperatures dip below freezing with surprising regularity, and a cover of frost is not unusual. Is it possible that these lizards froze? The only other guess is that perhaps there was a bad rainstorm and that they drowned. But if that happens here, then why not elsewhere?
I discovered that this does happen elsewhere – at high elevation in the Sierra de Baoruco! As stated above, there are two high elevation taxa, A. shrevei and A. armouri. The former is found in the Cordillera Central (Eastern Hispaniola) and the latter in the Sierra de Baoruco (Western Hispaniola). These two taxa are surprisingly similar in morphology, ecology, and behavior. In fact, they were originally placed into their own genus (Audantia), which I discussed in a previous AA post. Apparently they also share the habit of leaving the most puzzling carcasses.
While we were collecting lizards for a common garden experiment, Miguel Landestoy came upon this A. armouri wedged between two rocks in a meadow near Loma de Toro (Fig. 5). It is remarkably similar to the dead A. shrevei I had seen only a few weeks earlier in the Cordillera Central. I must confess that I was particularly intrigued with this specimen – that wide open gape triggers my Rue Morgue and Fangoria search image. As it was found stuck between two rocks, it clearly was not hit by a vehicle. How did these anoles die? After several years of freezing my tail off in these high elevation forests, my intuition is that these diminutive ectotherms couldn’t take the cold and froze to death.
In his “Anole Murder Mystery” post Miguel Landestoy shared a picture of a dead A. whitemani, also a Hispaniolan trunk-ground anole, from Salinas. The consensus was that this lizard had likely overheated while crossing a sand dune between forest patches. Perhaps it started crossing while the conditions were cool and simply got stuck in the sun halfway through its journey. If lizards can “make mistakes” with the heat, can they also make them with the cold? We know that A. shrevei and A. armouri often (though not always) retreat under rocks during conditions unsuitable for basking. We also know that site selection is important – a lizard that picks a thermally unsuitable perch can run the risk of overexposing itself. If a lizard remains exposed long enough for its temperature to drop below its Critical Thermal Minimum (i.e., the temperature at which locomotor function ceases), then it can get stuck and potentially freeze to death.
But how else might this have happened? The high elevation forests are often overcast and rainy – could these lizards have drowned? Dehydrated? Died of old age? Has anyone seen something like this in populations at lower elevation?
How these tropical lizards manage to live in such extreme conditions has filled an entire thesis for me, but I am continually outmatched as I find that they are still full of surprises. What do you think happened to these anoles?