EPOW - Ecology Picture of the Week

A Tale of Four Scales

Upper left: Western rattlesnake (Crotalus viridis), Oregon USA.
Upper right: Arizona black rattlesnake (Crotalus cereberus), Arizona USA.
Lower left: Carpet python (Morelia spilota), Queensland, Australia.
Lower right: Nile crocodile (Crocodylus niloticus), Zimbabwe, Africa.

Explanation:  This week we are exploring the landscape of scales -- three snakes and one crocodile.  

The top two photos are of species of rattlesnake found in the U.S.  Rattlesnakes belong to the family Viperidae which sport scales having keels or mid-center ridges.  What are the ridges used for?  In an earlier EPOW we briefly discussed this, and suggested that the keels could provide structural strength.  Some have suggested that they might also scatter light and make the body form of these stealthy predators more difficult to detect.  Perhaps the keels also make the snake more difficult to eat by other predators.  

Another use of keeled scales seems to be made by a set of eight Viperid species collectively called saw scaled vipers (Echis spp.) which are found in arid parts of south Asia, the Middle East, and Africa.  Saw scaled vipers have a threat display by which they rub sections of their body together to produce a hissing sound.  The keels on the scales might contribute to the production of this sound.  (Interestingly, too, some other snakes such as the common and harmless bullsnake actually do hiss as a warning signal.)

Also notice how the Arizona black rattlesnake in the upper left photo has not just a keel on each scale but also a series of ridges.  Again, this might serve for structural strength but also -- just speculating here -- might play some kind of role in trapping water, as this species is found in very arid areas of southwest U.S.   Whether such secondary uses were the primary advantages that led to selection of such traits is unknown; sometimes an anatomical (or behavioral) feature that has one advantage also has other uses.  

The photo in the lower left is a carpet python found in Australia, New Guinea, and Indonesia, belonging to the serpent family Pythonidae.  These snakes do not have keels on their scales, but instead their scales are quite smooth; compare with the corrugated scales of the Arizona black rattlesnake.  Why?  It might have to do with pythons' feeding habits; they are constrictors and literally squeeze their prey to death.  Having slick scale surfaces might aid being able to more quickly seize and squeeze their prey.  

And in final comparison is the Nile crocodile in the bottom right photo.  Of course, not a serpent but a crocodilian, the Nile crocodile's scales are quite different than those of snakes; they don't overlap (forming an imbricate pattern) and vary more considerably in size and shape ... but they probably play some similar roles of protection and reducing body water loss, and perhaps also some degree of waterproofing, as Nile crocodiles are quite aquatic creatures.

So there you have it ... superficially similar, the scales of these four species seem uniquely attuned to each species' habitats and life habits.  A tale of four scales.