Tail breakage and predatory pressure upon two invasive snakes (Serpentes: Colubridae) at two islands in the Western Mediterranean.
Tail breakage is an important anti-predator mechanism in snake populations, which can be used as a proxy for predation intensity as natural observations of predator-prey interactions are scarce. Frequency of tail breakage was calculated for two Iberian colubrids recently introduced in the Balearic Islands (Western Mediterranean, Spain): the Horseshoe Whip Snake (Hemorrhois hippocrepis (Linnaeus, 1758)) in Eivissa and the Ladder Snake (Zamenis scalaris (Schinz, 1812)) in Formentera. The effect of sex, life stage, dorsal coloration pattern, body length, and body condition on frequency of tail breakage and on remaining subcaudal scale pairs were analyzed and compared between the indigenous range and the invaded islands. An increase of the frequency of tail breakage with body size was found, supporting a size-related effect, which also occurs in the indigenous range. Frequency of tail breakage of H. hippocrepis was lower in Eivissa when compared with the original area, whereas in Formentera, Z. scalaris showed a higher frequency, which could be related to the different predator community on each island compared with the mainland. The study of the main ecological aspects of these recently introduced species may allow one to assess their potential impact on insular ecosystems and their indigenous biodiversity, as well as to promote future control actions in these areas previously free of snakes.