Rapid development of tool use as a strategy to predate invasive land snails

A male of T. major was observed to catch juvenile snails, bring them repeatedly to the same stone (roughly oval shape, ≈20 × 10 cm), and break the shell by repeatedly striking against the “anvil.” This same individual was observed engaging in this activity four times over several days. Two other large piles of broken shells (estimated fragments from >10 individual snails) were also observed: one next to a stone and the other next to the iron base of a broken plant pot in an open area between the farm residence and the forest. The breaking of the shell in these two anvils was heard simultaneously, indicating that this behavior was not restricted to a single individual. There was no evidence of shell-breaking behavior in adjacent forest sites, possibly due to the lower density of A. fulica.

Great antshrikes are strongly omnivorous, feeding on a wide range of taxa that include seeds, crustaceans, insects, arachnids, mollusks, and small mammals (Beltzer 1987). However, these observations are the first documented account of tool use in this species. Given the ecological context, it is highly likely that this behavior is an example of flexible tool use, which developed in the absence of three key barriers (as defined by Hunt et al. 2014). First, there was a need to learn that an object can be used to facilitate solving a problem (extracting large snails from their shells) that would be difficult to solve without the tool. The smaller sizes of the majority of Brazilian snail species and their lower relative abundances may have reduced the selective benefits of this behavioral innovation in other sites. Second, there was an adequate working memory for adopting tool use. As an omnivore, T. major already had a broad repertoire of prey-capture behaviors and was thus a good candidate for tool-using behavior. Third, the practical difficulty of manipulating the object in a controlled way was low. As seen in other anvil-using birds, the physical act of breaking shells on rocks is relatively straightforward in terms of physical manipulation.

Giant African land snails were accidentally introduced into Brazil in the 1980s (Thiengo et al. 2007), and the snail-capture behavior documented here is likely to have developed at some time after this point. There is a small possibility that this behavior was already widespread in this species as a means to extract native mollusks. However, this is unlikely since mollusks typically make up a small fraction of the diet of T. major, with most populations specializing on arthropod prey (Beltzer 1987; Ridgely and Tudor 1994). Thus, the most parsimonious explanation is that tool use in this particular population is a flexible behavioral trait that arose due to the high density of exotic snails in this locality. Such a rapid development of tool use as a strategy to optimally exploit this abundant food source provides a potentially interesting case study of the origins of tool use. Given the huge damage being caused by A. fulica in South America, predation at high levels by an abundant native species may also have practical implications in relation to controlling this highly problematic mollusk (Carlsson et al. 2009).