Mandibular morphology, task specialization and bite mechanics in Pheidole ants (Hymenoptera: Formicidae)

Abstract

Ants show remarkable ecological and evolutionary success due to their social life history and division of labour among colony members. In some lineages, the worker force became subdivided into morphologically distinct individuals (i.e. minor versus major workers), allowing for the differential performance of particular roles in the colony. However, the functional and ecological significance of these morphological differences are not well understood. Here, we applied finite element analysis (FEA) to explore the biomechanical differences between major and minor ant worker mandibles. Analyses were carried out on mandibles of two Pheidole species, a dimorphic ant genus. We tested whether major mandibles evolved to minimize stress when compared to minors using combinations of the apical tooth and masticatory margin bites under strike and pressure conditions. Majors performed better in pressure conditions yet, contrary to our expectations, minors performed better in strike bite scenarios. Moreover, we demonstrated that even small morphological differences in ant mandibles might lead to substantial differences in biomechanical responses to bite loading. These results also underscore the potential of FEA to uncover biomechanical consequences of morphological differences within and between ant workers.

Morphological determinants of bite force capacity in insects: a biomechanical analysis of polymorphic leaf-cutter ants

“…In all insects with chewing mouthparts, bite forces are generated by large muscles located in the head capsule, and transmitted to the cutting edge of the mandible via an apodeme and a mandibular joint [20,21]. Because this musculoskeletal bite system is of behavioural, ecological and evolutionary relevance and can be analysed with first principles, it has received increasing attention from biomechanists [22][23][24][25], evolutionary biologists [26][27][28][29][30], functional morphologists [24,[31][32][33][34][35][36][37][38] and (behavioural) ecologists alike [21,[39][40][41][42][43][44]. Concretely, for an isometric contraction at zero fibre stretch, the force exerted at any point of the mandible, F b , may be written as the product between the ratio of muscle volume V m and the average fibre length L f (the physiological cross-sectional area of the muscle, A phys = V m /L f ), the muscle stress σ m , the cosine of the average pennation angle w, and the mechanical advantage MA [21,31,39,41]:…”

The First Reconstruction of the Head Anatomy of a Cretaceous Insect, †Gerontoformica gracilis(Hymenoptera: Formicidae), and the Early Evolution of Ants

“…Unusual mandibular shapes have also been a special point of interest in Cretaceous ants, especially in the †Haidomyrmecinae (Barden et al 2020) and †Zigrasimeciinae (Cao et al 2020). The biomechanics of the long, saber-shaped mandibles of Harpegnathos have been studied in several publications (Zhang et al 2020a(Zhang et al , b, 2021, whereas no ant species with 'normal' mandibles has received similar attention, with the arguable exception of leaf cutting ants (Püffel et al 2021) and a single analysis of Pheidole mandibles (Klunk et al 2021).…”

Mechanical demands of bite in plane head shapes of ant (Hymenoptera: Formicidae) workers

“…Although recent attempts suggest that the morphological variation between Pheidole species in head shape and size seems not to be related to differences in diet (Casadei-Ferreira et al, 2021;Holley et al, 2016), little is known about how the head shape is associated with the mechanical demands of biting, which differ between worker subcastes. Recent works suggest that even slight morphological modifications in Pheidole worker mandibles could lead to differences in bite performance between worker subcastes and species, according to the primary roles of each worker subcaste in the colony (Huang, 2012;Klunk et al, 2021), as also observed in other ants (Larabee et al, 2018).…”