Allometric scaling of foraging rate with trail dimensions in leaf-cutting ants

Abstract

Leaf-cutting ants (Atta spp.) create physical pathways to support the transport of resources on which colony growth and reproduction depend. We determined the scaling relationship between the rate of resource acquisition and the size of the trail system and foraging workforce for 18 colonies of Atta colombica and Atta cephalotes. We examined conventional power-law scaling patterns, but did so in a multivariate analysis that reveals the simultaneous effects of forager number, trail length and trail width. Foraging rate (number of resource-laden ants returning to the nest per unit time) scaled at the 0.93 power of worker numbers, the -1.02 power of total trail length and the 0.65 power of trail width. These scaling exponents indicate that individual performance declines only slightly as more foragers are recruited to the workforce, but that trail length imposes a severe penalty on the foraging rate. A model of mass traffic flow predicts the allometric patterns for workforce and trail length, although the effect of trail width is unexpected and points to the importance of the little-known mechanisms that regulate a colony's investment in trail clearance. These results provide a point of comparison for the role that resource flows may play in allometric scaling patterns in other transport-dependent entities, such as human cities.

A locally-blazed ant trail achieves efficient collective navigation despite limited information

“…Scent marks deposited during the transport of small food items assist traffic between two stationary locations (e.g. the pile of seeds and the nest) (Franks et al, 1991; Bruce and Burd, 2012; Plowes et al., 2013; Bottinelli et al, 2015; Buhl et al, 2009). Conversely, during cooperative transport, scent marks bridge between a fixed location (the nest) and a dynamically moving object (the carried load).…”

“…Second, classical ant trails are typically clearly bounded and narrow. This is a result of the relatively strict manner in which ants follow the scent marks (Bruce and Burd, 2012; Eidmann, 1927; Carthy, 1951) which dominates over a secondary stochastic process where some ants distributively and individually wander off the trail (Beekman and Dussutour, 2007; Deneubourg et al, 1983; Perna et al, 2012; Gordon, 1995; Goss et al, 1989; Reid et al, 2011). The situation is very different for the locally-blazed trail which does not have well defined boundaries.…”

Optimal construction of army ant living bridges

“…More importantly, while (34) is similar in form to equations (12) and (20) of section 2; it is only locally equivalent due to the presence of the square root in (11) and the absolute value in (24). Nevertheless, an analysis of (34) still provides 340 valuable insight into the results we obtain from equations (12) and (20), namely it aids in the explanation for the symmetry of the results derived from (20).…”

“…As a consequence of this nomadic lifestyle, these ants -unlike most other ants -face severe time constraints when generating new foraging routes each day. While ants living in a permanent nest site may thoroughly explore 30 their environment [20,21] or clear trails of vegetation [22,23,24,25] in order to create relatively straight and efficient foraging paths, army ant trails often weave tortuously through the complex tropical forest floor [26,19].…”

Branching angles reflect a trade‐off between reducing trail maintenance costs or travel distances in leaf‐cutting ants

“…These ants cut huge amounts of plant biomass from the surrounding vegetation and carry it back to the nest, where the plant fragments are used as a substrate for a symbiotic fungus that they cultivate inside underground chambers (Wirth et al 2003). The construction of this essential transport network for leaf-cutting ants clearly represents a benefit for the colony, but also entails costs (Burd et al 2002, Bruce andBurd 2012). Because the way in which organisms explore space in order to discover and retrieve food is Manuscript received 31 January 2014; revised 14 May 2014; accepted 24 June 2014.…”