I think that many very different parameters may involve the development of these structures that herpetologists have named "keeled scales". This morphological concept actually may reflect very distinct structures, adaptations to different ecological parameters/functions or may be involved by very different developmental pathways (in other words, we do not know if the keeled scales of a Vipera are homologuous to the keeled scales of a skink, nor if they are playing the same role, if any ? (I am actually a bit skeptical about it)). This is something that has never properly been studied... and certainly a fascinating topic of research.

Hey Michael - Great to get this fascinating question - and from you :-).  I have thought about this as well, since we had a visiting student looking into morphological characteristics associated with arboreality.  Sure enough, keeled scales are highly associated with off-the-ground locomotion - in snakes in the humid Asian tropics at least.  As far as your question about the rainfall issue - there appears to be no correspondence in the snakes we have looked at - but again, this is a limited study in tropical Asia where the most variation across the rainfall axis is high seasonality.  I reckon this is mostly about the arboreal aspect with many associated and interrelated ecological autocorrelates (like everything else!).  If you are interested in following up, ask Alex Figueroa or I can put you in touch with him.  Cheers, David

Hi Michael, I believe that in Anolis cybotes, keeled ventral scales appear within a species in xeric environments.

Hi David and Katharina,

Thanks for these answers. Weird having keeled ventrals in the Anole, I think in Sceloporus it's the dorsals that change.

David, the places I know best...Sn Africa and Australia, are short on arboreal snakes but those that exist tend to have keeled scales. I was thinking more of the terrestrial species (keelbacks, rough-scaled snakes, prickly skinks), especially those with smooth scaled relatives.

Chamaesaura have keeled and elongate scales, which helps them move through grassy vegetation. Try pulling them out of the vegetation, their scales hook onto everything. Perhaps these keels adds rigidity to the scales, so they do not bend when force is applied.  

http://www.ispotnature.org/sites/default/files/images/26896/a19d687728e9798ec72c61aa4b2ba87d.jpg

There is also a suggestion that keeled scales may help with crypsis by disrupting outline, giving disruptive texture and reduced reflection from scales...but I can't find a reference for that either.

Keeled scales are probably useful for locomotion and escape from predators in Chamaesaura but most Cordylids have keeled scales of some sort irrespective of life-style (excepting Platysaurus). A better comparison might be in Tetradactylus where some species, such as the long-bodied T. tetradactylus has strong keels but others like T. seps have minimal scale ornamentation.

Michael - the keeled scaled Anolis cybotes populations sit in xeric forest and on rocks/ limestone cliffs. The others more on trees/ broadleaf forest, so in this case it doesn't seem to have anything to do with arboreality. 

I think that many very different parameters may involve the development of these structures that herpetologists have named "keeled scales". This morphological concept actually may reflect very distinct structures, adaptations to different ecological parameters/functions or may be involved by very different developmental pathways (in other words, we do not know if the keeled scales of a Vipera are homologuous to the keeled scales of a skink, nor if they are playing the same role, if any ? (I am actually a bit skeptical about it)). This is something that has never properly been studied... and certainly a fascinating topic of research.

Miralles, scale ornamentation may have arisen through different processes across squamates, and it may be "a trait" with a fair amount of phylogenetic inertia, such that whole families exhibit "keeled" or smooth scales, across diverse environments. But, irrespective of structural homology, there may be some level of convergence - improved crypsis could benefit both skinks and vipers. It seems to be a varying trait with many proposed explanations but few phylogenetic, ecological or physiological comparative tests.

I had a similar question about keeled scales. Although not studied yet, the keeled scale was strongly observed in the case of diurnal reptiles, and weak or no in the case of nocturnal reptiles. Snakes living in the water (or near water) or in the forest have little or no keeled scale. I think it is related to the function of reducing light reflection. Also keeled scales seems to be related to the activity time and the environment of habitat. I want to proceed with the research related to this personally.