What goes in does not come out: different non-lethal dietary methods give contradictory interpretation of prey selectivity in amphibians

Abstract

To date the most commonly used non-lethal methods in amphibian dietary studies are stomach flushing and faecal analysis. In this study, we compared the outcome of these two methods in interpreting prey selectivity when the available prey community in the environment is known. Stomach flushed contents and faeces were obtained from the same 27 individuals of the spectacled salamander (Salamandrina perspicillata) from a site in Central Italy. The interpretation of the population prey selection strategy varied in relation to the method used. Stomach content analysis suggested that salamanders were highly specialized on springtails, while faecal contents indicated a generalist trophic strategy. Prey selectivity indexes were also highly divergent: the analysis of stomach contents indicated a significant positive selection upon springtails, while exactly the opposite conclusion was obtained when faecal contents were analyzed. The results confirm that in amphibians, stomach analysis provides more reliable dietary data in comparison to faecal analysis. This is related to the fact that soft-bodied prey items tend to be more fully digested, disappearing in faeces while highly chitinized and less digestible prey taxa tend to increase their relative abundances in faeces.