Do mandibles matter? Towards explaining the economy of mandible functional morphology in 'chewing' herbivores using the spiny leaf insect, Extatosoma tiaratum, as an exemplar.

2016-11-29T02:17:30Z (GMT) by Caarels , Sherrie-Anne
The complexity of oral structures among ‘chewing’ herbivores implies that plant food is difficult or costly to process. In addition, oral processing is constrained by preoral food choice and post-ingestive digestive mechanisms. While these contexts have been researched among mammalian herbivores, they are rarely integrated in studies of mandible functional morphology in chewing insects. Hence, the relative importance of the mandibles on digestive evolution and subsequent life strategies are unknown. This research therefore aims to progress our understanding of the economy of mandible functional morphology in chewing insect herbivores by using the adult female spiny leaf insect (Extatosoma tiaratum, Macleay) as an exemplar. These insects were chosen because they are large in size, easy to maintain and, in captivity, consume the foliage from a wide variety of eucalypt species. In addition, they persist on relatively tough and nutritionally poor diets, and are also cryptic and inactive except when feeding. Thus, there are probably strong selective advantages for rapid and efficient feeding mechanisms. To address the research aim, four conceptually foundational levels of enquiry were conducted. First, it was necessary to establish the digestive strategy of adult female leaf insects by quantifying relative digestibilities of the two fundamental components constituting plant foods, the cell wall (difficult to digest) and the cell contents (easy to digest if accessible), as well as parameters of gut passage using indigestible markers. The significance of these findings were further elucidated by examining the morphology and physiochemistry of the post-oral gut. Although digestion of the cell content fraction in natural diet leaves was low (less than 35%), the cell contents represented their primary nutritional resource (at least 66%). In addition, these data suggest little, if any, digestion of cellulose and lignin/cutin, a moderate digestion of hemicelluloses and pectins (at least 30%) and relatively long gut transits (14 ± 3 h, first marker in frass). It is proposed that while digestion of hemicelluloses and pectins may contribute to overall nutritional budgets, it may also act to weaken or disrupt cell walls, thereby enhancing access to the otherwise entrapped cell contents. Second, the features of the mandibles and how they interact with food, as well as the implication of time and scale on these dynamics were determined. Occlusion is demonstrated to be a relatively simple process, with the right mandible moving inside the left to produce a cutting action on food caught between them. Particles were regular in size and shape and closely corresponded to the functional parts (working surfaces)of the mandibles. Accordingly, damage was primarily located on the outside edge where the ‘molar’ and ‘incisal’ ridges traverse, and where the large molar ‘cusps’ cross. Consistent with an ‘energy-use minimising’ strategy characteristic of consumers of tough diets, these mandibular features enable large forces to be efficiently directed into the continued propagation of cracks. Allometric scaling associations revealed an isometric and an hypometric increase in the size of the mandibles and the head, respectively, relative to body length. It is suggested that proportionally larger head sizes in smaller instars enable them to house mandibular muscles of sufficient size to exert the forces required to fracture relatively tough leaves; whereas, proportionally larger body sizes in adult females enable them to house eggs of sufficient size to accommodate the relatively large heads of smaller instars. In addition, in contrast to younger counterparts, adults with moderately worn mandibles are able to produce two smaller particles (as opposed to a single large one) with each occlusal stroke. This is facilitated by wear-induced ‘activation’ of the large molar cusp on each mandible. In doing so, reductions to digestive ability that would otherwise be limited by the scaling-up of mandible size with development is potentially counteracted or ameliorated. It is further postulated that relatively long lifespans, facilitated by mandibular features that curtail the effects of wear, enables adults to further invest in egg quality or to increase egg output, which would otherwise come at the expense of large egg sizes. Third, the action and limitations of the post-oral gut in the extraction of cell contents from the obstructive cell wall was investigated by examining changes to the physiochemical integrity of ingested leaf particles as they progressed through the gut. As leaf food transitioned from the voluminous and acid crop (pH 4.5 ± 0.1) to the anterior midgut, there was an increased proportion of particles that were rifted along their central axis between adaxial (upper) and abaxial (lower) leaf halves, and the subsequent disappearance of cell contents proximal to these regions. Consistent with this action, access to cell contents by the post-oral gut appeared to be limited by leaf attributes that impaired the penetration of digestive enzymes and/or bifacial rift, such as particles reinforced by more than one vascular bundle with sheath extensions linking epidermal layers. It is concluded that these findings collectively imply strong selective pressures for mandible structures that enable the production of small particle sizes. Finally, the compensatory plasticity of adult leaf insects exposed to leaves with different physical (fracture) properties and/or subjected to moderate mandible wear were examined by conducting experiments that measured parameters of feeding effort. For all individuals, feeding on mature and tougher leaves was associated with a significant decrease in crop (excision) rates (c. 40%), and increases to the number of bites per crop (c. 80%) and rates of oxygen consumption (c. 85%). While those with artificially worn mandibles appeared to compensate for reduced ‘chewing effectiveness’ by having lower basal rates of oxygen consumption, relatively more bites per crop when feeding on mature and tougher leaves imply unsustainable consequences when such foods must be handled. The principles underlying the economy of mandible functional morphology in the leaf insect are synthesised and discussed. It is contended that traditional studies seeking explanations that invoke the gross fracture properties of the diet only provide a superficial ‘snap-shot’ of form-function dynamics. However, by examining the mandibles as part of a functionally integrated system, mandible morphology was found to be moulded by two fundamental requirements: first, to facilitate the ingestion of relatively tough leaves in a way that minimises energy use associated with fracture; and second, to reduce these leaves into particle sizes that are sufficiently small to optimise digestion within the post-oral gut. While the relative importance of each of these requirements appeared to change with scale, the positioning of the oral gut with respect to the post-oral gut means that the first requirement poses an overriding influence on mandible morphology. However, despite having clear adaptations, the mechanical challenges associated with handling older and tougher leaves continue to impair chewing effectiveness in adults and presumably reduce survival prospects. It is concluded that mandible morphology, moulded by their positioning within the digestive system and the requirement to fracture and ingest a relatively tough diet, presents as a major driving force in the evolution of, and interaction between, digestive system, body size and overall life-strategy dynamics in these chewing herbivores.