Do salamanders chew? An X-ray reconstruction of moving morphology analysis of ambystomatid intraoral feeding behaviours

Using salamanders as model taxa to understand vertebrate feeding constraints during the late Devonian water-to-land transition

The vertebrate water-to-land transition and the rise of tetrapods brought about fundamental changes for the groups undergoing these evolutionary changes (i.e. stem and early tetrapods). These groups were forced to adapt to new conditions, including the distinct physical properties of water and air, requiring fundamental changes in anatomy. Nutrition (or feeding) was one of the prime physiological processes these vertebrates had to successfully adjust to change from aquatic to terrestrial life. The basal gnathostome feeding mode involves either jaw prehension or using water flows to aid in ingestion, transportation and food orientation. Meanwhile, processing was limited primarily to simple chewing bites. However, given their comparatively massive and relatively inflexible hyobranchial system (compared to the more muscular tongue of many tetrapods), it remains fraught with speculation how stem and early tetrapods managed to feed in both media. Here, we explore ontogenetic water-to-land transitions of salamanders as functional analogues to model potential changes in the feeding behaviour of stem and early tetrapods. Our data suggest two scenarios for terrestrial feeding in stem and early tetrapods as well as the presence of complex chewing behaviours, including excursions of the jaw in more than one dimension during early developmental stages. Our results demonstrate that terrestrial feeding may have been possible before flexible tongues evolved. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Rhythmic chew cycles with distinct fast and slow phases are ancestral to gnathostomes

Intra-oral food processing, including chewing, is important for safe swallowing and efficient nutrient assimilation across tetrapods. Gape cycles in tetrapod chewing consist of four phases (fast open and -close, and slow open and -close), with processing mainly occurring during slow close. Basal aquatic-feeding vertebrates also process food intraorally, but whether their chew cycles are partitioned into distinct phases, and how rhythmic their chewing is, remains unknown. Here, we show that chew cycles from sharks to salamanders are as rhythmic as those of mammals, and consist of at least three, and often four phases, with phase distinction occasionally lacking during jaw opening. In fishes and aquatic-feeding salamanders, fast open has the most variable duration, more closely resembling mammals than basal amniotes (lepidosaurs). Across ontogenetically or behaviourally mediated terrestrialization, salamanders show a distinct pattern of the second closing phase (near-contact) being faster than the first, with no clear pattern in partitioning of variability across phases. Our results suggest that distinct fast and slow chew cycle phases are ancestral for jawed vertebrates, followed by a complicated evolutionary history of cycle phase durations and jaw velocities across fishes, basal tetrapods and mammals. These results raise new questions about the mechanical and sensorimotor underpinnings of vertebrate food processing. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Introduction: food processing and nutritional assimilation in animals

How animals process and absorb nutrients from their food is a fundamental question in biology. Despite the continuity and interaction between intraoral food processing and post-oesophageal nutritional extraction, these topics have largely been studied separately. At present, we lack a synthesis of how pre- and post-oesophageal mechanisms of food processing shape the ability of various taxa to effectively assimilate nutrients from their diet. The aim of this special issue is to catalyse a unification of these distinct approaches as a functional continuum. We highlight questions that derive from this synthesis, as well as technical advances to address these questions. At present, there is also a skew toward vertebrates in studies of feeding form-function mechanics; by including perspectives from researchers working on both vertebrates and invertebrates, we hope to stimulate integrative and comparative research on food processing and nutritional assimilation. Below, we discuss how the papers in this issue contribute to these goals in three areas: championing a functional-comparative approach, quantifying performance and emphasizing the effects of life history, and food substrate and extrinsic factors in current and future studies of oral food processing and nutritional assimilation. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.