Amphibians in Eurasian otterLutra lutradiet: osteological identification unveils hidden prey richness and male‐biased predation on anurans

Abstract

Amphibians form a major component of the diet of the otter Lutra lutra in several areas of its wide geographic range. Yet, amphibian remains are rarely identified to species level and therefore information on the diversity of this food resource is generally scarce. The aims of this study were: 1) to assess the overall pattern and trends in the use of amphibians as a resource by otters at the range scale, and 2) to highlight current knowledge on the diversity of amphibians taken as prey by otters. Additionally, we carried out osteological identification of amphibian remains in otter spraints (faeces) from southern Italy, with the aim of demonstrating how this method may improve our knowledge on predator–prey relationships. The frequency of occurrence of amphibians in 64 dietary studies averaged 12%. Predation of amphibians by otters increased with longitude and was the highest in the Alpine biogeographical region. Predation by otters was reported on 28 amphibian species (35% of European species). Peaks in their frequency of use were reported for all seasons, mostly in winter and spring. In southern Italy, we identified 355 individuals belonging to at least seven amphibian taxa (64% of available species; Rana italica, Rana dalmatina/italica, Pelophylax kl. bergeri/hispanicus, Hyla intermedia, Bufo bufo, Bufotes balearicus, and Lissotriton italicus), and pointed out male‐biased predation within the Order Anura (frogs). We conclude that the contribution of amphibians to the richness of the otter's prey community is far higher than commonly perceived, and that osteological analyses allow the detailed investigation of the feeding behaviour of this top predator of freshwater habitats.

Tadpoles in the diet of otters – an overlooked prey item in the diet of a riparian predator?

“…Our study demonstrates the potential importance of tadpoles in the spring diet of otters, particularly during specific periods. To our knowledge, tadpoles have never been described previously in the diet of otters in Białowieża Forest or from other locations (Jędrzejewska et al 2001, Smiroldo et al 2019b. We are also not aware of any studies providing evidence of tadpole consumption by other species of aquatic carnivores, such as the American mink Neovison vison or polecat Mustela putorius (Jędrzejewska & Jędrzejewski 1998, Jędrzejewska et al 2001).…”

“…Reviews of otter diet have highlighted the importance of amphibians, mostly frogs and toads, but always in their (sub)adult phase; tadpole consumption has hitherto never been reported (Jędrzejewska et al 2001, Clavero et al 2003, Georgiev 2008, Krawczyk et al 2016, Smiroldo et al 2019b. Similarly, newts are rarely mentioned from otter diet analysis.…”

“…For example, in Doñana National Park small fish species, such as Gambusia (weight of about 1 g), are one of the main otter food items (Adrian & Delibes 1987). Otters also consume amphibian eggs and small juvenile and subadult frogs (Georgiev 2008, Smiroldo et al 2019b. The importance of small prey may have been overlooked in many studies (Carss & Elston 1996), despite being abundant during specific periods.…”

Reciprocal Role of Salamanders in Aquatic Energy Flow Pathways

“…The consumption of salamanders by mustelid species has also been reported widely e.g., [72,73,78,79]. Smiroldo et al [80] recently reviewed the amphibians in the diet of the Eurasian otter (Lutra lutra) and, in the case of salamanders, the list included eight species (see Table 2 in the work by Smiroldo et al). However, Jobe et al [68] only listed one salamander species (great crested newt Triturus cristatus) as prey for Eurasian otter (see Appendix A in the work by Jobe et al).…”

“…Pagacz and Witczuk [118] estimated that the consumption of amphibians by Eurasian otters can be equally important as fish, constituting 54% of the biomass of consumed prey. Smiroldo et al [80] revealed that the frequency of occurrence of amphibians in dietary studies in Eurasian otters averaged 12%. Considering that the frequency of occurrence only covers the total number of stomachs with prey and thus without taking the relative abundance of each prey type into account [119], it is doubtful that the use of literature sources only including frequency of occurrence can draw conclusions regarding the importance of salamanders as energy resources for otter species compared to papers reporting the abundance (numerical, biomass, or volume) of prey categories.…”