Detection of prey odours underpins dietary specialization in a Neotropical top‐predator: How army ants find their ant prey

Abstract

1. Deciphering the mechanisms that underpin dietary specialization and niche partitioning is crucial to understanding the maintenance of biodiversity. New world army ants live in species-rich assemblages throughout the Neotropics and are voracious predators of other arthropods. They are therefore an important and potentially informative group for addressing how diverse predator assemblages partition available prey resources.2. New World army ants are largely specialist predators of other ants, with each species specializing on different ant genera. However, the mechanisms of prey choice are unknown. In this study, we addressed whether the army ant Eciton hamatum: (a) can detect potential prey odours, (b) can distinguish between odours of prey and non-prey and (c) can differentiate between different types of odours associated with its prey. 3.Using field experiments, we tested the response of army ants to the following four odour treatments: alarm odours, dead ants, live ants and nest material. Each treatment had a unique combination of odour sources and included some movement in two of the treatments (alarm and live ants). Odour treatments were tested for both prey and non-prey ants. These data were used to determine the degree to which E. hamatum are using specific prey stimuli to detect potential prey and direct their foraging. 4. Army ants responded strongly to odours derived from prey ants, which triggered both increased localized recruitment and slowed advancement of the raid as they targeted the odour source. Odours from non-prey ants were largely ignored.Additionally, the army ants had the strongest response to the nest material of their preferred prey, with progressively weaker responses across the live ant, dead ant and alarm odours treatments respectively.5. This study reveals that the detection of prey odours, and especially the most persistent odours related to the prey's nest, provides a mechanism for dietary specialization in army ants. If ubiquitous across the Neotropical army ants, then this olfaction-based ecological specialization may facilitate patterns of resource partitioning and coexistence in these diverse predator communities. K E Y W O R D S

Eciton Army Ants—Umbrella Species for Conservation in Neotropical Forests

“…E. hamatum predominantly preys on the brood of Acromyrmex leaf-cutting ants, E. dulcium on Pachycondyla and Odontomachus ants, and E. burchellii on Camponotus ants [14][15][16][17][18][19][20]. Although the diet of E. burchellii is more varied and includes a diverse range of non-ant litter arthropods, in particular the brood of other social insects [56][57][58][59][60][61], this non-ant component of the prey might be comparatively low [55]. The large colony size of Eciton species, with estimates ranging from 150,000 to 500,000 workers in E. hamatum and 500,000 to 2 million workers in E. burchellii [38], and the relatively large body size of of some of the workers imply high colony-energy demands [62].…”

Diurnal and nocturnal foraging specialisation in Neotropical army ants

“…Despite their shared behavioural ecology, a dozen or more species of army ants typically co‐occur at forested Neotropical sites (O'Donnell et al ., 2007, 2009). Many army ant species target different genera or even species of ants as prey, but there is some diet overlap among sympatric species, suggesting army ants could experience interspecific exploitation competition for prey resources (Rettenmeyer et al ., 1983; Otis et al ., 1986; Kaspari et al ., 2011; Hoenle et al ., 2019; Manubay & Powell, 2020).…”

Temporary prey storage along swarm columns of army ants: an adaptive strategy for successful raiding?

“…Traffic partitioning and caches emerge across relatively distant ant taxa (i.e Atta, Camponotus and Eciton [4,20,21,[29][30][31][32][33]), possibly as a result of convergent evolution in large ant societies with thousands of individuals transporting large amounts of food through long distances [16,34,35]. Such huge societies must maximize foraging efficiency and compensate for the energy spent in collecting and transporting food [36][37][38].…”

“…How did Eciton caches evolve? Eciton workers are strongly attracted by prey nest material, prey workers or recruiting pheromones [30], which may become especially concentrated at trail junctions or other places that hinder workers' flow. We hypothesize that, originally, prey caches exclusively emerged as by-products of intense traffic at such bottleneck sites.…”