Biting effort

The quantification of biting effort is an under-researched area deserving further investigation, particularly in view of the mounting evidence on the concern over the validity of peak bite force from recent studies in ruminants. Biting effort is an index of efficiency that takes into account the activity budget for gathering as well as severance of forage.

One of the exciting challenges ahead lies in understanding the mechanistic basis for the foraging strategy contrasts between animals of different body mass. As the bite composition of a large-bodied ruminant generally contains more fiber relative to that of a small-bodied ruminant, larger-bodied ruminants must rely on their strength to harvest forage material to meet their requirements and in accordance with forage supply. Animal mechanics permit this since it is believed that bite force scales allometrically with body mass on the premise that force generated by muscle is proportional to the muscular cross-sectional area (W067). Consequently, the costs of severance for a given force are lower for animal species of greater body mass. This begs the question of how small-bodied ruminants cope when faced with the same constraints as large-bodied ruminants. When presented with identical swards, sheep and deer [28] have been observed to exhibit a very similar grazing strategy despite a threefold difference in body mass between the two species. While it appears that there is a fixed force exerted on a bite of forage directly from the momentum of the head, as supported by the observation that force does not increase linearly with increasing number of leaves procured at very low leaf density [26], there has been little quantification of the effort and costs incurred in upholding the tension in the muscles of the neck to maintain head momentum, although Illius et al. [27] commented that it must be considerable.

Allometric relationships between elements of feeding behavior in ruminants and body mass have been studied in detail [76-78], and it would be reasonable to think that an allometric relationship between bite force and body mass has evolved. Figure 5.4 depicts the allometric relationship between body mass and peak bite force for a range of animal species (goat, sheep, deer, cattle, and horse) from the limited data available in the literature [25,26,44,52,65]. The calculated exponent (0.77 ± 0.18) indicates that peak bite force scales allometrically with body mass. However, the high standard error reflects the greater variation among the smaller-bodied animals, and the reasons for such responses should be investigated. Further, there have been inconsistent reports on the inter- and intraspecific allometric relationships between body mass and bite depth [78,79]. In a horse-cattle comparison, Hongo and Akimoto [25] hypothesized that horses, which have anterior incisors on their upper jaw, would be more efficient grazers on complex swards consisting of perennial ryegrass leaves and reed canary grass culms, but instead they found that horses exhibited little competitive advantage over cattle.

A number of studies have shown that incisor breadth scales with body mass [9,78], and total masseter weight, which reflects masseter tissue size, is correlated with body mass [80]. However, unlike in some carnivores [81], there appears to be no compensation between mandibular length and masseter muscle mass in ruminants despite the fact that mandibular length scales with body mass. Given that the mandible acts as a moment, the absence of any correlation between mandibular length and masseter muscle mass implies a weaker gripping force at the incisors. Consequently, animals with a high mandibular length to body mass ratio (e.g., goats and moose) are, theoretically, mechanically disadvantaged when procuring material that is tall and/or of high tensile strength. However, little is known about the mechanical consequences of changes in mandibular length on the torque around the temporomandibular joint in ruminants. Given that torque is the force over distance,

Log body mass (kg)

FIGURE 5.4 Relationship between peak bite force (N) and body mass (kg) for a range of animal species. Solid circle: goat; open circle: sheep; solid square: deer; solid triangle: cattle; open triangle: horse. Solid line represents the fitted linear relationship (y = 0.08 + 0.77x ± 0.18, p = 0.002, R2 = 0.818).

Log body mass (kg)

FIGURE 5.4 Relationship between peak bite force (N) and body mass (kg) for a range of animal species. Solid circle: goat; open circle: sheep; solid square: deer; solid triangle: cattle; open triangle: horse. Solid line represents the fitted linear relationship (y = 0.08 + 0.77x ± 0.18, p = 0.002, R2 = 0.818).

it could well be that the balance between incisor breadth and the magnitude of protrusion that has evolved in ruminants not only caters for selectivity but also minimizes the competitive advantage of grazers over browsers of a similar body mass. Compensation mechanisms for body mass are regularly observed in other species, e.g., in flying insects [82], so it is more than likely that forms of mechanical design, aside from rates of acceleration, contribute to the competitiveness of smaller-bodied ruminants, and the challenge is to think of approaches to investigate those features.

Quantification of biting effort would be a step forward, but success will be determined by the accuracy of defining the endpoints of bites on force-time curves. Since electric equipment generates noise, the potential for the location of endpoints to become clouded can be significant, particularly when the ratio between exerted forces and noise is weak. There have only been a few studies that have examined biting effort [44,47,52], limiting any generalization across plant species and animal body mass. Hughes [52] found that the effort expended by sheep grazing prairie grass (Bromus willdenowii) and tall fescue (Festuca arundincea) swards was similar but lower than that on perennial ryegrass (Lolium perenne) swards. Unpublished preliminary data of Griffiths [44] showed that the across-bite contrast in biting effort from cattle grazing swards of Italian ryegrass (L. multiflorum) exceeded the contrast in peak bite force. Nevertheless, the data showed evidence of similarity in bite effort for individual bites where there were strong contrasts in peak bite force. Further research on biting effort would help to clarify the consistency of biting effort across animal species and sward complexity. The challenge is to investigate whether animal mechanics can assist with understanding the mechanisms for diet selection and the bite dimensions. It will only be after we have addressed questions concerning biting effort that we will be able to draw conclusions regarding the mechanistic basis for the contrast or similarity in grazing strategies across species of contrasting body mass and understand functionality of cospecies grazing in ecosystems.

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