The energetics of running stability: costs of transport in grass-cutting ants depend on fragment shape

Abstract

SUMMARYGrass-cutting ants (Atta vollenweideri) carry fragments that can be many times heavier and longer than the ants themselves and it is important for them to avoid falling over during load transport. To investigate whether the energetic costs of transport are affected by the need to maintain stability, the rate of CO 2 production was measured in both unladen workers and workers carrying standardized paper fragments of different size and shape. We tested: (1) the effect of mass by comparing workers carrying either light or heavy fragments of the same size, and (2) the effect of shape by comparing short and long fragments of the same mass. Consistent with previous studies, metabolic rate increased but running speed remained constant when ants carried heavier fragments. The net cost of transport (normalized to the total mass of ant and fragment) was the same for heavy and light fragments, and did not differ from the costs of carrying a unit body mass. Ants carrying long fragments showed similar metabolic rates but ran significantly slower than ants carrying short fragments. As a consequence, net cost of transport was significantly higher for long fragments than for short ones, and higher than the costs of carrying a unit body mass. The observed reduction in running speed is likely a result of the antsʼ need to maintain stability. When the absolute costs of transport were compared, smaller ants required more energy to carry heavier and longer fragments than larger workers, but the opposite was found for lighter and shorter fragments. The absolute costs of transport per unit fragment mass suggest that it is energetically advantageous for a colony to allocate smaller workers for the transport of small fragments and larger workers for large fragments. The present results underline the importance of biomechanical factors for the understanding of leaf-cutting ant foraging strategies.

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).…”

“…Factors such as distance to food sources, properties of the terrain and quality of available resources may affect the demands imposed by these tasks and the need for inter-individual interactions (LOPES et al, 2003;RÖSCHARD & ROCES, 2003a, 2003bCLARK, 2006;LEWIS et al, 2008;MOLL et al, 2012). Accordingly, task partitioning may be better interpreted as a flexible process that is influenced by context-specific nuances.…”

“…Accordingly, task partitioning may be better interpreted as a flexible process that is influenced by context-specific nuances. For example, Acromyrmex ants may transfer leaf fragments from one ant to another in the middle of the trail, thus apparently enhancing foraging efficiency (LOPES et al, 2003).…”

Taylor's Treadmill Menagerie

“…Finally, since 1982, the cost of terrestrial locomotion has been quantified in a diverse assortment of other vertebrate and invertebrate taxa, extended down in size to ants (e.g. Moll et al, 2012) and up in size to elephants (Langman et al, 1995). Taylor et al's (Taylor et al, 1982) Size effect = primary allometric signal…”

Aspectos temporais da organização coletiva do forrageamento em formigas saúvas (Atta sexdens rubropilosa)

“…For example, Acromyrmex ants may transfer leaf 2 3 4 fragments from one ant to another in the middle of the trail, thus apparently enhancing foraging efficiency (Lopes et al 2003). In addition, factors such as distance to food sources, properties of the terrain and quality of the resources may affect the demands imposed by those tasks and the need for interindividual interactions Lewis et al 2008;Lopes et al 2003;Moll et al 2012;Röschard and Roces 2003a;Röschard and Roces 2003b;Roces 1990). Accordingly, task partitioning may be better interpreted as a flexible process that is influenced by context-specific nuances.…”