Analysis of the Rubbing Behaviour of Psammophiids

Abstract

Several snakes belonging to the eight genera of the family Psammophiidae are the sole known snakes to smear the secretion of their nasal gland on their own belly and flanks by little rapid head movements. The main objective of the present study is to determine behaviourally the function of this “rubbing behaviour”. The rubbing behaviour itself and its possible functions have been mentioned several times in literature, but most of them without specific experimental analysis. This is the first behavioural study focussing on the rubbing behaviour of psammophiids in order to examine its function with more clarity. Psammophiids belonging to the four genera <i>Malpolon, Psammophis, Psammophylax</i> and <i>Scutophis</i> were studied. <br /> Since it had been stated that snakes have pores on their scales allowing the passage of lipids through the skin to protect it against desiccation, it was important to verify this feature in psammophiids. The scanning electron microscopic analysis of the dorsal scale ultrastructure of several psammophiids and control species was undertaken: the results showed that neither psammophiids, nor control taxa have pores on their scale surface. <br /> Some years ago, the nasal gland secretion of <i>Malpolon monspessulanus</i>, a psammophiid known to do rubbing behaviour, had been analysed. This analysis showed the presence of lipids, known to protect the skin against desiccation. Since other species of psammophiids also perform rubbing behaviour, it was verified whether their nasal secretion also contains lipids. The presence of lipids in the nasal gland secretion of <i>Psammophis mossambicus</i> and <i>Scutophis moilensis</i> could only be presumed by gas chromatographic analysis. But it has to be assumed that these species also have a lipid-rich nasal secretion since they also perform rubbing behaviour -as <i>Malpolon monspessulanus</i>.<br /> One hypothesis on the function of rubbing behaviour is that it serves to mark an unknown environment. To verify this, the snakes were place into a terrarium unknown to the snakes: The frequency of its rubbing behaviour on the first day was compared to the mean frequency of rubbing of the following days. If the rubbing behaviour serves to mark, its frequency would be more elevated on the first day. This frequency was however not significantly higher, so that rubbing behaviour cannot be considered as marking behaviour. <br /> In a further experiment, it was tested whether nasal secretion is used to leave an informative trail to conspecifics. Two couples of <i>Psammophis mossambicus</i> and <i>Scutophis moilensis</i> were studied. The snakes were place one by one into a terrarium. No changes in behaviour (e.g. elevated tongue flicking) could be detected in the snakes placed at second in the terrarium, regardless of their sex of the sex of the snake introduced at first. <br /> Finally, in order to determine whether rubbing behaviour depends on external factors, temperature was varied in an experimental setup. There was a significant correlation between increasing temperature and the number of rubbing behaviour in all psammophiids studied. This states the hypothesis that rubbing behaviour is primarily dependent on external factors and that it is used to avoid the desiccation. This is done by thanks of the nasal secretion known to be lipid-rich as in <i>Malpolon monspessulanus</i> and it has then to be considered that this secretion is also lipid-rich in the other psammophiid species since their number of rubbing behaviour also increased with temperature. This allows these snakes to be active even in the hottest hours of the day, e.g. minimising concurrence with other predators. <br /> Additionally to the behavioural experiments, for the first time the rubbing behaviour of the genus <i>Rhamphiophis</i> could be verified in <i>R. rubropunctatus</i> and <i>R. rostratus. </i> This elevates the number of genera performing rubbing behaviour on seven, of the eight comprised in this family. The “last” genus in which rubbing behaviour has not been recorded yet is Dipsina. The rubbing behaviour could additionally be documented for a further <i>Psammophis</i> species, <i>P. elegans</i> and for a further <i>Psammophylax</i> species, <i>P. acutus acutus</i>.<br /> A further morphological analysis should confirm the affiliation of the species moilensis to its own genus. It had been affiliated long time to the genus <i>Malpolon</i>. Its attribution to its own genus, <i>Scutophis</i>, had been proposed but not unanimously adopted in the literature. The affiliation was primarily based on the ability of <i>moilensis</i> to spread the neck when threatened -<i>Malpolon</i> species are unable to do this. The comparative morphological analysis between the neck ribs of <i>Malpolon monspessulanu</i>s and <i>Scutophis moilensis</i> confirmed differences at the level of the length of these ribs, those in <i>S. moilensis</i> being obviously longer. This difference reinforces the affiliation of both species to different genera.