Avid readers of this blog might have noticed that Jonathan Losos likes trying to get readers to confuse the white-fanned variant of the South Asian agamid lizard Sitana ponticeriana with the only toepad-less anole, Anolis onca. Indeed, the two lizards look quite similar:
Sitana ponticeriana, white-fanned variant, in Saswad, India
Anolis onca in Maracaibo, Venezuela. Photo by Jonathan Losos
Posts [1, 2, 3] detailing the habits and habitats of these two lizards point to a number of similarities between the species. Both occur in incredibly hot and windy environments. Both are primarily terrestrial, where they are very well-camouflaged, but are also observed perching on vegetation. The causes for the loss of A. onca’s toepads remain a mystery, and here I lay out some observations of S. ponticeriana’s behaviour that lead to a hypothesis for why A. onca might have lost it’s toepads.
Perch use in S. ponticeriana follows a predictable temporal pattern from about 8:00 a.m., when the lizards first emerge, to about 12:00 noon, the hottest part of the morning. Lizards perch primarily on rocks, if available, in the early morning while basking. During the middle of the morning, male lizards are found perching on and displaying from a variety of perches, including the ground, mud piles, rocks, twigs, and shrubs. By noon, however, lizards are often found resting in shrubs. Here are a couple of lizards resting:
Male Sitana ponticeriana, white-fanned variant, resting in the shrubs. Saswad, India.
Female Sitana ponticeriana, white-fanned variant, resting in the shrubs. Kutch, India.
But what happens when you chase lizards out of the shrubs in the heat of the day? Peak air temperatures at the site I was at this summer hovered around 40 degrees Celsius, and soil or rock surface temperatures were likely higher (they certainly felt so). Lizards that we chased out of shrubs onto the sand would run rapidly as usual, but when they paused, were often observed lifting the toes of their hind feet off the ground. Here are two photos–compare the toe positions on the hind foot to get a sense of the behaviour I’m referring to:
If the highest risk of heat exposure comes from the ground, any adaptation that reduces the transfer of heat from the ground to the lizard will be favoured. Such adaptation would explain S. ponticeriana’s behaviour of resting in vegetation during the hottest part of the day and lifting toes off the ground when forced onto hot terrestrial perches. Like most agamids, Sitana have very skinny toes, leaving a small surface area for transfer of heat from the ground. But what about A. onca? If the ancestral A. onca had typically anole-like toepads on moving to the beaches, they might have been at high risk of heat transfer from the sand when forced onto terrestrial perches in the heat of the day. This would lead to the evolution of reduced toepads to avoid such heat transfer. A temporal pattern of perch use in A. onca, similar to that of S. ponticeriana, would be the first piece of evidence useful for establishing what might be an exciting example of trans-continental convergence.
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Peter Mudde
Heat conduct might be a reason. I have another hypothesis though. First, despite the referred observations, my late friend and early anole observer Leo Wijffels observed A.onca on Isla Marguerita on sand, seeking shelter between the low vegetation and hoping to be camouflaged by remaining motionless. ( In Leo’s words:” You dont actually ‘catch Anolis onca, you just pick them up..”) ( Wijffels, L. 1998. Een opmerkelijke zandhagedis: Anolis onca. Lacerta 57(1): 12-18. short english summary, but mail me if you want a scan). Now ‘loss of toepads’ isn’t a Anolis unicum. You find it with African ‘Pachydactyloid’ Geckoes as well… when they live in sandy habitats.
On sand, as you state, toepads have little use as they do not add much to grasping/adhereing abilities while walking on sand…
Several gecko species when forced to walk over sand or dust, tend to pull up their pads, whatever the surface-temperature. I have seen this often with captive Phelsuma’s (especially because I have seen much more Phelsuma’s then other gecko species) and Tarentola species. You can try this at home.. force them to walk over either sand or torf and they will raise their pads under an almost 90 deg to avoid contaminating them.
The mechanism for adhering and releasing the pads may be a bit more sophisticated in geckoes ( see Russell 1972 for details) but I expect it is basically the same.. If you walk over sand or dust, being an anole, your pads get spoiled and loose their function until the next shredding.
Now if, like Anolis onca, but also being Pteropus, or Palmatogecko, and you live on sand/dust permanently, clogged up pads would be of no use at all and a hindrance.. Heat or no heat.. ( Sand-Geckoes tend to be nocturnal.. Leo Wijffels said he couldn’t go out at night, since it was a bit dangerous for tourists to walk on the beach at night and no one dared to accompany him. he does however state that diurnal activity in Anolis onca is high and being nocturnal as well would draw a lot of energy from them. But he could not proove that by not finding them at night)
There is however no reason why there should only be a single reason to do away with toepads.
James (Skip) Lazell
LOTS of species of all sorts of lizards lift up their toes on hot surfaces, not just a few.
Ambika Kamath
More hypotheses the better! I agree that it’s very likely something to do with interacting with a sandy substrate, but thought I’d throw this possibility into the ring as well. A diurnal gecko (or anole) living on a hot substrate e.g. rocks might be a good comparison, but the benefits of having toepads to cling to rocks might outweigh the costs of heat transfer, which can be avoided behaviourally.
Peter Mudde
Skip is right, off course. We have mostly lacertids here in Europe and these tend to lift their feet of the hot surface. A bit counterproductive I would think, as lifting their belly would be more effective in avoiding heat.
Lacertids however shine another light on the matter. In hot sandy regions some genera (Acanthodactylus and Meroles f.i.) have an expansion in the form of a fringe on their toes. To get a grip on the sand, but that does increase the heat-conducting area of the toes.
The South African day geckoes, Rhopthropus, do have expanded disks and hunt on rocks by day (from shaded hideouts no doubt. I must admit I only know these geckoes pickled).
Ambika Kamath
Thanks for all the information! Sounds like a cool review paper/comparative museum project in the making…
James (Skip) Lazell
Cogitating, I have never seen a skink do it…. Skip