There are certain evolutionary constraints in carnivorans that have restricted the ways in which they can acquire herbivorousness. We should seek to overcome those constraints opening up different and complementary pathways for the transition to herbivory. Two of these key constraints are related to microbial digestion and jaw movement. Herbivorous carnivorans have more so compensated for these constraints rather than overcome them, but it would be far better to reduce any limitations for herbivorizing.
“In this study, we have shown that skull shape does correlate not only with feeding behaviour but also with phylogenetic legacy. As stated earlier, all the adaptations of the carnivoran skull towards herbivory have been interpreted as functional traits that allow exerting high bite forces and such traits are similar to those of the hypercarnivorous species that subdue large vertebrate prey (Christiansen & Wroe, 2007). At first sight, this fact seems to be counterintuitive if we do not pay attention to historical contingency.
All the herbivorous carnivorans evolved from a generalized carnivorous mammal (Van Valkenburgh, 2007), with a body plan early established in the phylogeny of Carnivora. Therefore, the remarkable morphological resemblance of the skull among the herbivorous carnivorans was most probably driven by extrinsic factors (e.g. natural selection) as well as by intrinsic ones (e.g. a shared developmental pathway) and the latter may have posed constraints (sensu Alberch, 1982) on the direction of skull shape evolution towards herbivory. Thus, the adaptations of herbivorous species for exerting high bite forces are constrained functional solutions that were reached in a process driven by natural selection within the set of possibilities allowed by phylogenetic legacy.
The herbivorous carnivorans retain the simple stomach (i.e. without a caecum) and short gastrointestinal tract typical of carnivores, in contrast to the chambered stomachs and complex intestinal morphologies of ruminants and other mammalian herbivores (Roberts & Gittleman, 1984; Schaller et al., 1985; Johnson et al., 1988; Bleijenberg & Nijboer, 1989; Reid et al., 1991).
Microbial digestion only plays a minor role in the digestive strategy of carnivorans, resulting in a low digestibility rate for cellulose and other complex carbohydrates (Wei et al., 1999, 2007; Senshu et al., 2007). Furthermore, all carnivores have a fixed temporomandibular joint (Ewer, 1973) which, together with their enlarged canine teeth, limits jaw movements to the vertical plane, with some lateral movements for grinding (Davis, 1964). This prevents the teeth from being occluded simultaneously for grinding tough foods, as in ungulates. As a result of these limitations, herbivorous carnivorans must eat huge quantities of food, which forces the development of the main muscles involved in mastication (i.e. masseter and temporalis), hence the capacity for exerting similar bite forces than those of carnivorous mammals that usually hunt prey much larger than themselves. The need in a carnivoran for high input moment arms for the temporalis and masseter muscles translates in the development of a long coronoid and an enlarged angular process, which precludes the development of a wide gape.
Constraints on digestive tract and craniomandibular morphology as a result of a basic body plan adapted to carnivory have resulted in a very different set of solutions in herbivorous carnivorans, compared to ungulates. Skull shape in herbivorous carnivorans evolved towards the capacity of exerting a high bite forces in a process that paralleled the evolution of hypercarnivores. The same adaptive solution (i.e. an increase in bite force) was taken for reaching two opposite feeding strategies (i.e. herbivory and hypercarnivory) through carnivoran evolution. This strongly suggests that not all possible adaptive solutions are under the domain of natural selection, which operates on the variability available by the phylogenetic legacy of each particular group."
Figueirido, B., F. J. Serrano-Alarcón, G. J. Slater, and P. Palmqvist. 2010. Shape at the cross-roads: homoplasy and history in the evolution of the carnivoran skull towards herbivory. J. Evol. Biol. 23:2579–2594.