Snakes that polish their scales, and why they do it

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Psammophis schokari eating a lizard, Phrynocephalus
mystaceus, in Kazakhstan

I really like these snakes, and they have about them a pretty interesting mystery. In the tribe Psammophiini (in family Lamprophiidae), there are at least 50 species of snake in 8 genera native to Africa, the Mediterranean, the Middle East, and central Asia. They are united by several unusual synapomorphies, the most unique of which is the presence of a morphological feature called the external narial valve. This structure, located in the loreal region between the eye and the nostril, is the outlet of a special nasal gland that secretes fluid containing long-chain fatty acids. The function of this secretion is enigmatic. Some experiments show that it can serve to retard evaporative water loss, and some evidence suggests that some of these molecules could be pheromones used in marking territories. Territoriality is only slightly less non-existent among snakes than herbivory, but according to some it is apparently present among certain psammophines, few of which have been well-studied in the wild. Aside from a very interesting study suggesting that releasing small mammals from competition with large herbivores can indirectly increase the abundance of their snake predators (including Psammophis mossambicus), we don't know much about their ecology, but careful observations have revealed a little about the lives of these intriguing snakes.

Subadult Montpellier snake, Malpolon monspessulanus

The external narial valve was described in 1956 by renowned Russian herpetologist Ilya Darevsky, the second person ever to earn a PhD in the Soviet Union and the discoverer of parthenogenesis and polyploidy in reptiles. Darevsky described the gland from a specimen of the Montpellier snake (Malpolon monspessulanus), and such glands have now been reported from all eight genera in the Psammophini. In addition to the gland, psammophines also share peculiar hemipene morphology - that is, the male reproductive organs are unusually thin and smooth for an advanced snake, most of which possess thick, spiny hemipenes that enable prolonged copulation. Sexual dimorphism is also quite pronounced in many of these snakes, although not of tail length (typically, the tails of male snakes are longer and thicker than those of females). For example, male M. monspessulanus are stout, uniformly colored, and up to 2.3 m long, whereas females are slender, spotted, and reach only 1.4 meters.

Beginning in 1898, the earliest observations of these snakes mention their peculiar behavior. Psammophines press the outlet of their narial valve to their skin and thoroughly apply a coating of colorless, fast-drying valve secretion all over their body. Watch this Malpolon insignitus to get an idea, because it's hard to describe.

This behavior has been variously called self-rubbing, self-polishing, or  grooming, and seems to be present in all species of psammophine observed. Several keepers in Europe have made extensive efforts to acquire and videotape species of psammophines, and self-rubbing has now been documented in seven of the eight genera. More intriguing, conspecific psammophines housed together occasionally rub one another, presumably anointing the other snake with secretion from their narial valve. What could this mean?

Psammophis leightoni from Namibia

Several hypotheses have been put forth to explain this unique and intriguing behavior. To date, none have been sufficiently tested to unequivocality, nor are they mutually exclusive. Prior to the 1970s, the prevailing thought was that, since psammophines generally inhabit arid regions, the gland might aid in salt excretion, evaporative cooling, or water retention. In 1978, William Dunson and colleagues published their work on the histology of the gland, and concluded that it did not contain the specialized cytological features associated with salt secretion in the salt glands of reptiles such as sea snakes and marine iguanas. Dunson also characterized the chemical composition of the secretion for the first time, and suggested that the long-chain fatty acids he found might help retard water loss through the skin.

Dunson tested five Malpolon to see if their dermal water loss was unusually low, and indeed it was, approximately ten times lower than that of Kingsnakes (Lampropeltis getula), although water loss rate varied depending on where in the shedding cycle the snakes were. Malpolon also lost proportionally more water via the mouth and cloaca (and less via the skin) than did other reptiles. Dunson also kept Malpolon without giving them access to water, and they did not lose weight, indicating that they were capable of obtaining all the water they needed from their prey. In another experiment, he showed that dehydrated Malpolon did not secrete salt from their narial valve. He made the interesting observation that several frog species of the genus Phyllomedusa decrease their dermal water loss by wiping lipid secretions from skin glands over the surface of their skin:

Could psammophids be accomplishing the same thing with their narial valve secretions? Dunson did not test whether snakes that had just applied the secretion lost less water than those that had not. The snakes polish themselves frequently, especially after ecdyisis and feeding, so water loss rate could be tracked over time. 

Other mysterious pits have been described from the head scales of psammophines: parietal pits on the top of the head and infralabial pits on the lower jaw, both of which seem to be sporadically occurring. Series of shed skins from the very same snake sometimes show these features and sometimes do not. Because histology is lacking for these features, it is difficult to say what they might represent.

Dipsina multimaculata

Because of the remote areas inhabited by many of these snakes, most studies to date are insufficiently replicated to permit concrete conclusions about the function of the polishing behavior. Furthermore, determining the sex of living psammophines is quite difficult on account of their small hemipenes, so behavioral studies are often hampered by inadequate knowledge of the sex of the animals involved. Observations of captive psammophines suggest that these snakes have complex social behaviors, not the least of which is their tendency to polish one anothers' scales. Could this behavior represent mate guarding? A nuptial gift from males to females of fatty acids to help them avoid water loss during pregnancy? Do these snakes mark their territories? Only replicated, scientific studies will tell; until then, competing hypotheses will continue to wax on and wax off.

ACKNOWLEDGMENTS

Thanks to Heather Heinz for drawing my attention to this fascinating system, to Jane Bugaeva for translating Darevsky's 1956 article from Russian, and to photographers Bernard Dupont, Altyn Emel, Michael & Patricia Fogden, and Jeremy Holden, and videographer Ton Steehouder.

REFERENCES

Microdermatoglypic SEM photograph of Dipsina scale.
The lipid layer covering the scale is visible.
From de Pury 2010

Darevsky IS (1956) O stroyenni i funktsionirovani nosovoy zhelezy u yashtsheritsnoy zmei Malpolon monspessulanus Herm. (Reptilia, Serpentes). [On the structure and function of the nasal gland in the lizard snake Malpolon monspessulanus Herm. (Reptilia, Serpentes)] Zoologicheskij Zhurnal-Moskva 35:312-314

Dunson WA, Dunson MK, Keith AD (1978) The nasal gland of the Montpellier snake Malpolon monspessulanus: fine structure, secretion composition, and a possible role in reduction of dermal water loss. Journal of Experimental Zoology 203:461-473

de Grijs P (1898) Beobachtungen an reptilien in der gefangenschaff. Zoologischer Garten 39:233-247

de Haan CC, Aymerich M (2012) Des comportements frotteur et marqueur, pour la chasse et la vie sociale. In: Aymerich M (ed) A la Découverte de la Faune du Maroc Oriental

de Haan CC, A Cluchier (2006). Chemical marking behaviour in the psammophiine snakes Malpolon monspessulanus and Psammophis phillipsi. Proceedings of the 13th Congress of the Societas Europaea Herpetologica, 211-212. <link>

Mimophis mahfalensis killing a chameleon in Madagascar

de Haan CC (2003) Extrabuccal infralabial secretion outlets in Dromophis, Mimophis and Psammophis species (Serpentes, Colubridae, Psammophiini). A probable substitute for ‘self-rubbing’ and cloacal scent gland functions, and a cue for a taxonomic account. Comptes Rendus Biologies 326:275-286. <link>

de Pury S (2010) Analysis of the Rubbing Behaviour of Psammophiids: A Methodological Approach. PhD dissertation, Rheinischen Friedrich-Wilhelms-Universität Bonn, Bonn, Switzerland.

McCauley, D. J., Keesing, F., Young, T. P., Allan, B. F. & Pringle, R. M. 2006: Indirect effects of large herbivores on snakes in an African savanna. Ecology 87, 2657-2663. <link>

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.