Attempted predation of a cave olm Proteus anguinus by a dice water snake Natrix tessellata, in Bosnia and Herzegovina

Abstract

T blind European cave salamander or olm Proteus anguinus is a large and slender neotenous salamander, only found naturally in the western Balkans (Speybroeck et al., 2016). Their maximum length varies according to which river basin they inhabit but is typically 25–55 cm. They are characterised by their pinkish-white colouration, paddle-like tail, and reduced number of digits on all four limbs. The species inhabits both flowing and stagnant subterranean waterbodies in karstic limestone formations throughout their range, as long as these are chemically unpolluted waters, with a stable temperature range of 8–15 °C. Proteus anguinus are almost always observed in caves, for which they have a number of adaptations to allow them to survive such a challenging environment (Hervant et al., 2001; Issartel et al., 2009; Balázs et al., 2020). Due to their specialised adaptations to subterranean life, P. anguinus are particularly vulnerable to changes to the karst ecosystems they inhabit, such as contamination from anthropogenic sources (Kolar, 2019). The European cave salamander also has no known predators within its natural underground habitats. The dice snake Natrix tessellata is a semi-aquatic species that may grow to over a metre in length and is found throughout most of central and south-eastern Europe (Speybroeck et al., 2016). Natrix tessellata is typically olivegreen or brown in colour with four rows of square-like black blotches along the body, although not all individuals are marked in this way. The species feeds primarily on fish and amphibians (Hutinec & Mebert, 2011; Weiperth et al., 2014), and inhabits both freshwater and saltwater environments such as lakes and shorelines (Speybroeck et al., 2016). On 4 July 2021, a P. anguinus was observed near the town of Trebinje (Bosnia and Herzegovina) swimming in a small stream after inadvertently straying out of a nearby cave system (Fig. 1). While seeking shelter under nearby boulders, the P. anguinus was dislodged by a juvenile N. tessellata, which latched onto the tail (BHS video, 2023). The N. tessellata was far too small to be able to consume the P. anguinus, and may have mistaken the tail for a smaller prey item. Unfortunately, the P. anguinus later died as a likely consequence of being flushed into sub-optimal conditions in the strong flow of the adjacent Trebišnjica River. The individual was some 36 cm in length and so almost certainly a fully mature adult P. anguinus. The N. tessellata is gauged to be approximately 54 cm long. These body-length measurements were estimated against those of the easily recognisable rocks in the video recording, and were accurately measured at a later date. A few minutes after the start of the confrontation, both animals were washed out into the strong flow of the surface river, and not observed again. In Bosnia and Herzegovina, this iconic amphibian is endemic to certain parts of only five river basins, where they live in many cave ecosystems of the Dinaric Karst. During periods of exceptionally high rainfall, P. anguinus can be washed out of their underground aquatic habitats into the adjacent surface waterways such as the observation we report here, although such observations have rarely been reported in the local media. The species has exceptionally good survival strategies including that of naturally swimming upstream (positive rheotaxis) (Durand & Parzefall, 1987; The Herpetological Bulletin 165, 2023: 34–35