Geometry explains the benefits of division of labour in a leafcutter ant

Abstract

Many ant species have morphologically distinct worker sub-castes. This presumably increases colony efficiency and is thought to be optimized by natural selection. Optimality arguments are, however, often lacking in detail. In ants, the benefits of having workers in a range of sizes have rarely been explained mechanistically. In Atta leafcutter ants, large workers specialize in defence and also cut fruit. Fruit is soft and can be cut by smaller workers. Why, therefore, are large workers involved? According to the geometry hypothesis, cutting large pieces from three-dimensional objects like fruit is enhanced by longer mandibles. By contrast, long mandibles are not needed to cut leaves that are effectively two-dimensional. Our results from Atta laevigata support three predictions from the geometry hypothesis. First, larger workers cut larger fruit pieces. Second, the effect of large size is greater in cutting fruit than leaves. Third, the size of fruit pieces cut increases approximately in proportion to the cube of mandible length. Our results are a novel mechanistic example of how size variation among worker ants enhances division of labour.

Variation in nestmate recognition ability among polymorphic leaf-cutting ant workers

“…Such neurophysiological adaptations could evolve when large rather than small workers defend the nest, possibly because majors may be stronger (cf. Helanterä and Ratnieks, 2008). In such a scenario, minors could save those neural resources that are necessary for maintaining a sensitive nestmate recognition system (Chittka and Niven, 2009).…”

Quality versus quantity: Foraging decisions in the honeybee (Apis mellifera scutellata) feeding on wildflower nectar and fruit juice

“…Indeed, fruit often serves a similar function to nectar in that it offers a reward to animal mutualists in plant reproduction. Many species of insect, including social insects, also forage on fruit (Evison & Ratnieks, ; Helanterä & Ratnieks, ; Jander, ; Noll, Zucchi, Jorge, & Mateus, ). In São Paulo State, Brazil, we have observed honeybees foraging on fallen Mangifera spp.…”

Two castes sizes of leafcutter ants in task partitioning in foraging activity

“…This behavioral plasticity among individuals, which enables task partitioning, may result from adaptations that improve the efficiency of foraging activities in the colony as a whole (OSTER & WILSON, 1978;BURD, 1995BURD, , 1996BURD, , 2000. This idea has been explored empirically (HELANTERA & RATNIEKS, 2008) in several contexts; for example, the distance from the nest, the hardness and thickness of leaves and the shape of the fragments (ROCES, 1990;RÖSCHARD & ROCES, 2003a;BURD, 2001;MOLL et al, 2012).…”

“…Some species of social insects, especially some ants, are adapted to their respective tasks through morphological differentiation within the colony, physical castes (OSTER & WILSON, 1978) with higher probability of a certain caste engaging in a certain task (WILSON, 1980a). Morphological differentiation may be a result of evolutionary optimization of colony performance, enhancing task efficiency and fitness (OSTER & WILSON, 1978;HELANTERA & RATNIEKS, 2008). However, morphology, behavior and task partitioning reinforce the behavior of the entire colony as an inseparable one-to-one relationship.…”