Teeth and the gastrointestinal tract in mammals: when 1 + 1 = 3

Abstract

Both teeth and the digestive tract show adaptations that are commonly interpreted in the context of trophic guilds—faunivory, herbivory and omnivory. Teeth prepare food for the digestive tract, and dental evolution focuses on increasing durability and functionality; in particular, size reduction of plant particles is an important preparation for microbial fermentative digestion. In narratives of digestive adaptations, microbes are typically considered as service providers, facilitating digestion. That the majority of ‘herbivorous’ (and possibly ‘omnivorous’) mammals display adaptations to maximize microbes' use as prey—by harvesting the microbes multiplying in their guts—is less emphasized and not reflected in trophic labels. Harvesting of microbes occurs either via coprophagy after separation from indigestible material by a separation mechanism in the hindgut, or from a forestomach by a ‘washing mechanism’ that selectively removes fines, including microbes, to the lower digestive tract. The evolution of this washing mechanism as part of the microbe farming niche opened the opportunity for the evolution of another mechanism that links teeth and guts in an innovative way—the sorting and cleaning of not-yet-sufficiently-size-reduced food that is then re-submitted to repeated mastication (rumination), leading to unprecedented chewing and digestive efficiency. This article is part of the theme issue ‘Food processing and nutritional assimilation in animals’.

They chew by night? Night‐time behaviour in a ‘ruminating’ primate, the proboscis monkey (Nasalis larvatus)

“…Thus, while our study again confirms the occurrence of R/R in proboscis monkeys, a comparison with ruminants—in which time spent in rumination represents an equivalent of 50 up to more than 100% of ingestion time (Baumont et al ., 2006), in which rumination occurs particularly at night (Gordon, 1968), and in which it is linked to a particle sorting mechanism in the forestomach (Matsuda et al ., 2015)—clearly indicates that the two processes cannot be considered equivalent (Clauss et al ., 2023).…”

“…When a similar behaviour of regurgitating, re‐masticating and re‐swallowing is observed in non‐ruminant species, it is usually referred to as ‘merycism’, the Greek word for ‘rumination’ (Barker et al ., 1963), or simply as ‘R/R’ (regurgitation and re‐mastication). R/R has been described in a variety of mammalian herbivores (Clauss et al ., 2023), including koalas (Logan, 2001) and kangaroos (Vendl et al ., 2017), and also proboscis monkeys ( Nasalis larvatus ) (Matsuda et al ., 2011a).…”

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

“…The hypothesis that salamanders incorporate intraoral processing in addition to food transport was mainly motivated by the observation that chewing occurs in vertebrate taxa both basal and derived with respect to caudates [5,19,36], and further motivated by the crucial role of processing for food breakdown to aid nutritional assimilation in the gastrointestinal tract [58]. Moreover, several recent studies have drawn the conclusion of Lauder and co-workers [24][25][26][27][28], that salamanders do not chew, into question [30][31][32][33][34][35].…”

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

“…While one key aspect of processing is always the mechanical deterioration and potential reduction of the food, the food may also be coated with mucus, triturated, immobilized or killed. Thus, intraoral food processing reduces the risk of being injured during feeding (figure 3b) and facilitates swallowing and digestion [71,159,160,161]. Intraoral processing behaviours require that the food fits within the oral cavity (i.e.…”