Application of plant fracture mechanics to foraging strategies

Why are plant ecologists interested in the application of materials science theory in understanding bite mechanics? It is energetically profitable for animals to penetrate deep into the sward canopy [27,47], and yet empirical evidence has shown that ruminants forage using a stratum-orientated depletion style at the patch scale, where a stratum is defined as a depth of sward canopy confined between two distinct lines. Such a strategy implies that bites from one stratum are removed before penetration into a second stratum [48,49], with the depth of the stratum determined by the magnitude of structural complexity [50,51]. Understanding the conceptual basis of bite depth has been the subject of ongoing research over the past decade with a strong focus on the linkage with bite force. The hypotheses of Summit Force, Balancing Reward against the cost of bite procurement and Marginal Revenue have all been tested, but evidence to support any of the hypotheses is weak [33]. The original Summit Force theory implied that once a maximum force was attained, the bite dimensions, primarily bite area, would be moderated to maintain a constant bite force. Evidence does suggest that animals moderate the bite area when faced with increased strength of plant components or increased tiller density — and thus bite resistance — but the adjustment is much smaller than the magnitude of the increase in bite resistance [2,47,52,53]. Further, several studies have shown no constancy in the force per bite relative to the reward attained [27,52,54]. A study by Illius et al. [27] found that goats offered a group of broad-leaved grasses grazed to different residual sward heights — and so contrasting bite depths — and exerted variation in bite force to sever the bites, with bite depth being equated to common marginal revenue. However, in comparison with a group of fine-leaved grasses, the value for common Marginal Revenue differed, leaving insufficient evidence for the acceptance of the Marginal Revenue hypothesis.

In understanding the mechanical interactions and grazing strategies of ruminants, much of the interest lies in defining the force that animals exert in severing a bite. Griffiths and Gordon [33] contended that there are two important animal-based terms when assessing the magnitude of force animals exert in procuring forage:

• Biting effort

Peak bite force represents the maximum force that an animal exerts in three-dimensional space to sever a bite of herbage and has been referred to in studies with other vertebrates as the "maximal bite capacity" [55]. Care is required to differentiate between bite force and the force generated by the masseter muscle during the mechanical action of clamping forage between incisors and dental pad. Likewise, bite force should be differentiated from the force applied during food comminution, where substantial forces are required during occlusal motions of crushing and grinding the food bolus against the molars. The peak force exerted in severance of forage material is thought of as a response to muscle moving against a fixed anchor of body mass, and to generate the cyclic patterns depicted on force-time curves from bio-mechanical force plates, animals must move the mass of the head in rhythm. Biting effort, as defined by Griffiths and Gordon [33], is primarily determined by bite force but is regulated by the components of plant resistance and animal resistance, e.g., head resistance. It is conceivable that other animal anatomical characteristics will shape biting effort. These authors argued that biting effort represents a holistic approach to understanding the dynamic nature of the animal's response to severance constraints arising from forage complexity.

How can biting effort be measured in real terms? The area under a force-time curve, as output from a biomechanical force plate, can be used to represent the work done. Since the forces that a grazing animal exerts in severing a bite of herbage are related to time, an accurate assessment of energy is somewhat difficult to derive in the absence of a measure of displacement. A conservative assumption on displacement from force-displacement curves could be used [47], but in the absence of more evidence, the validity of the approach could be questioned. However, we would expect that bites of a longer duration would utilize more muscular effort than those bites that result in rapid fracture. The integral of force over time can be used as a measure of bite effort. This appears valid on the assumption that muscles use energy for isometric contractions and that energy is approximately proportional to force [56], so the term "bite effort" carries a physical meaning in a mechanical sense, even if it is somewhat subjective.

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