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Herbivory, and thus, the ability to acquire energy from consuming plant material, evolved numerous times independently within vertebrates. Several mechanical solutions for processing plant material have emerged along with the independent rise of herbivorous feeding strategies. Comparative studies of these mechanical solutions suggest that herbivory might generally relate to "improved" and hence comparatively effective food processing mechanisms, often involving the jaws. Such processing behaviours involving the jaws can be referred to as chewing sensu lato. While herbivory is relatively common among vertebrates, it is generally believed that salamanders exclusively feed on other animals (i.e., carnivory). Further, it had been hypothesized that salamander food processing is limited to "simple" and comparably ineffective puncture crushing movements (i.e., opening and closing movements of the jaws in combination with pointed teeth), which may not be efficient enough to break down plant material. However, sirenid salamanders (i.e., dwarf sirens and sirens, genera Pseudobranchus and Siren respectively) have long been reported to consume large amounts of plant material besides feeding on animal prey. Additionally, some sirenids have only recently been shown to apply complex three-dimensional chewing behaviours. These complex chewing behaviours could potentially enable the mechanical breakdown of plant material. Here we comparatively study sirens with particular regards to the form and function of their feeding apparatus and their feeding habits. We demonstrate that sirenids generally seem to apply complex chewing mechanisms and, in fact, consume both plant- and animal foods. Comparative gastrointestinal morphology, however, suggests that among sirenids, only sirens are apt to digest and extract energy from plant material, suggesting that dwarf sirens may not benefit from the energy of their "plant food", while sirens appear to be facultative herbivorous - or better, omnivorous. Consequently, our results indicate that some salamanders can eat, process, and acquire energy from plant material, hence, contradicting the commonly accepted idea that salamanders are generally carnivorous. [ABSTRACT FROM AUTHOR]