Digestive and ingestive adaptations of mammalian herbivores to low-qualityforage

For herbivores utilizing fermentation strategies, digestibility depends on the retention time of plant cell wall (fibre) particles in the digestive tr act. Clearance of indigestible plant components from the gastrointestinal t ract limits the ability of herbivores to process large quantities of food. This leads to a positive relationship between digestibility and intake rate of different diets. Allometric studies across species suggest that metabol ic requirements increase with body mass(0.75) whereas gut capacity scales w ith body mass(1.0) Larger animals should thus have a more favourable ratio of gut capacity and digesta retention time to mass specific requirements, a nd hence be able to tolerate lower quality forage. We review digestive strategies that alleviate the passage rate constraint. Some hindgut fermenting herbivores have a separation mechanism that acceler ates excretion of larger indigestible particles and selectively retains sma ller and soluble particles in the caecum. This facilitates a negative relat ionship between digestibility and intake. Foraging strategies of ruminants have long been classified along a continuu m from concentrate selectors (browsers) to bulk and roughage feeders, which eat mainly grasses (grazers). Recent studies have attributed several measu res of digestive function to body mass whilst finding a surprising lack of digestive differences between browsers and grazers. We compiled data from 2 78 digestion trials using 20 ruminant species, to test the prediction that fibre digestibility by grazers was greater than that by browsers. The negat ive effect of the concentration of the poorly digestible plant structural c omponent lignin, explained the largest proportion of variation in cell-wall digestibility, followed by the predicted grazer-browser difference and a w eak positive effect of body mass. Digestibility of cell wall by browsers wa s relatively lower at higher lignin concentrations indicating that they dig est poor-quality diets less well than grazers. Although these cell-wall dig estive effects are as predicted, the grazer-browser classification currentl y lacks a mechanistic digestive basis. We tested the plasticity of digestion within a species, using mountain hare s (Lep us timidus). These are typical of intermediate feeders that switch b etween a graminoid-eating (grazer) strategy in summer, to browsing in winte r. During a 2-week period of dietary changeover of captive hares from grass to natural woody browse forages, growth rate was reduced. Browse-adapted h ares digested woody browse significantly better than non-adapted hares. A d ietary switch involves costs and represents a nutritional bottleneck. The m arked digestive adaptation suggests that digestive constraints to foraging behaviour must be considered as flexible. Whilst herbivore body size and qualitative aspects of forage plants are imp ortant (relatively) fixed constraints, flexibility of ingestive behaviour ( chewing,a and ruminating) interacts with digestive processes to provide alt ernative strategies for meeting requirements. More intense chewing or diet selection increases diet digestibility, but results in a negative relations hip with intake rate because of the greater time required. This may lead to interactions between nutrition and antipredator behaviour. The intimate relationship between ingestion and digestion suggests an impor tant role of digestive limitation in the functional responses of mammalian herbivores and mechanisms underlying population and community processes.