The behavioral ecology of amphibians remains largely understudied [1,2], although there is growing interest in the study of ecological processes and adaptive behaviors that shape the evolutionary dynamics of amphibian species and populations [3]. Amphibians represent a highly diverse group that include species with different habitus (from fossorial to exclusively aquatic), different reproductive strategies (oviparous and ovoviviparous) and even individuals that revolutionize their shape through life (larva vs. adult form). According to the intrinsic features of the species, but also to the local environmental conditions, the behavior of amphibians may change dramatically. For examples, in species adopting an r reproductive strategy, eggs are produced and laid by females in mass, which are often externally fertilized. On the other hand, in K-selected species, parents may spend substantial time taking care of their eggs and newborns, actively fighting against potential predators, driving them to a safer environment, or even feeding their brood [4]. The evolution of parental care in amphibians indeed seems to have played an important role for their colonization of different habitats. Looking at bi-phasic species, the behaviors of aquatic larvae can be completely different from those adopted by terrestrial adults. It is, therefore, quite complicated to standardize and compare amphibians’ behavior, unless specific groups with a similar ecology and life traits are considered.

Among Urodela, the family Plethodontidae (Gray, 1850) is the most speciose [4]. This family comprises more than 500 species out of 800 worldwide salamander and newt species, of which the overwhelming majority is found in Northern, Central and Southern America, while there are just 8 species in Europe and only 1 in Asia [4,5,6]. Plethodontids are characterized by the absence of lungs at the adult stage, and by the presence of naso-labial grooves that connect the upper lip to the external naris [4]. The main function of these peculiar structures is to canalize chemicals towards the olfactory structures [7,8].The wide diversity and the high local abundances that New World Plethodontids can reach make them relevant nodes within local ecological webs, where functional guilds are composed of many different interacting species [9,10,11,12]. Indeed, epigean plethodontids are predators which often occupy an intermediate position in the local food web, meaning that they hold the critical role of being prey and predators at the same time. Many species of New World plethodontids occur sympatrically, a condition that allowed several studies on species habitat selection and competition to be performed. Plethodontid salamanders have been even used as proxy for biodiversity assessments and monitoring in North American forests, cases that further highlight the importance of this group of amphibians and the need to deepen our knowledge on their biology and life traits [3].

On the other hand, the discovery of the Old World plethodontid species is relatively recent, and studies on their ecology and behavior lagged behind compared to their American relatives. Some not exclusive causes may contribute to this knowledge gap. Asian and European plethodontid species usually do not occur in sympatry, preventing the completion of studies on competitive behaviors of species living sympatrically. To this purpose, syntopy between two European plethodontid species has been artificially created, but such unnatural conditions do not provide any substantial information for advancing our knowledge on their behavior. Furthermore, although eight species of European plethodontids are currently known, they have been historically considered equivalent in their biology and ecology, meaning that single-species studies were often translated to the whole genus (see [13] and references therein). Indeed, only recently has more emphasis been given on evaluating interspecific (or even intraspecific) divergences among European plethodontid species, emphasis that strongly contributed in raising the number of studies performed. Furthermore, European plethodontids usually occur in subterranean environments, habitats where prolonged studies and monitoring activities are more challenging compared to surface ones.

American plethodontids became one of the main amphibian model systems used to better understand ecological questions in amphibians such as interspecific competition, predation and hybridization. Starting from field observations, classical ecological manipulative experiments were carried out, initially in natural woodland habitats [14,15,16]. However, in natural settings, there are many external uncontrollable factors that may influence the observed outcomes. For these reasons, plethodontids are often tested in laboratories to facilitate the avoidance of external confounding factors, while simultaneously controlling biological variables such as age, sex or reproductive status [17,18,19].

In this context, the current understanding of the behavioral ecology of European plethodontids, known as European cave salamanders, remains poorly developed in comparison to American plethodontids. The eight European species belong to the genus Speleomantes Dubois, 1984, although they are sometimes referred to as Hydromantes [5,20,21]. However, the use of Speleomantes better highlights the phylogenetic independence of European plethodontids, which form a well-defined monophyletic group with a large genetic distance from the five Californian species of Hydromantes [22,23,24]. All European cave salamanders are fully terrestrial and usually inhabit habitats where specific microclimatic conditions (e.g., high humidity and relatively low temperature variations) occur, such as humid crevices, forest floor environments and natural or artificial subterranean habitats [13,25,26]. Historically, behavioral studies on European plethodontids were sporadic because zoologists were focusing mainly on species description at the morphological and genetical level [25,27].

The monography dedicated to the taxonomy, biogeography and ecology of European cave salamanders written by Lanza et al. [13] constitutes the main reference concerning the natural history, ecology and behavior of the genus Speleomantes. This monograph lists under the chapter “Ethology” four sub-sections: “Feeding behaviour”, “Activity, habitat use and displacement”, “Antipredatory adaptations” and “Communication”. Concerning “Feeding behaviour”, most of the studies dealt with the anatomical structure of the eye [28] and of the tongue [29,30]. This sub-section summarizes the extraordinary tongue protrusion capability of Speleomantes and the ability to capture static prey in complete darkness [31,32]. Finally, the trophic strategy of Speleomantes is described as an “ambush strategy”, although this assertion seems based more on sporadic observations than on robust evidence [13]. The second sub-section, “Activity, habitat use and displacement”, relates exclusively to the species’ auto-ecological requirements, such as habitat features and seasonal activity, with little connection to behavior ecology sensu [3]. The third sub-section, “Antipredatory adaptations”, describes the presence of yellow-reddish or ochre dorsal colorations and the production of toxic skin secretion observed in many Speleomantes individuals and populations [13]. Finally, in the fourth sub-section, “Communication”, the presence of chemical intraspecific interactions was inferred from anatomical and histological studies on mating glands, but without providing any experimental evidence of their use during courtship and mating. In any case, the typical “nose tapping” behavior of plethodontids was reported in S. strinatii, suggesting that this specific behavior related to chemical communication is present [33]. Consequently, the relative paucity of the literature on European cave salamanders’ behavior does not allow robust comparative studies with American plethodontids [3,34,35].

2. Advances in the Behavioral Ecology of European Plethodontid Salamanders