How Gradient Affects the Foraging Decisions of Leaf Cutting Ants at the Food Source

Abstract

The eusocial insects have long held the fascination of scientists for their co-operative behaviour, which can range from a small group of workers, to millions strong colonies, such as those found in the leaf cutting ant, Atta cephalotes. Though decades of research have allowed us some insight into their organisation and methods, there are many things that have gone unexplained. One of these is the mystery of why Atta consistently takes leaf loads back to the nest that are significantly smaller than would be optimal, when they should in theory optimise leaf transport rate. While compelling evidence has been presented to suggest that it is at least in part to do with how the leaves are processed inside the nest, here I present evidence to suggest that gradient is another key factor. This is a factor which has been explored only very cursorily up until now in leaf cutting ants, with experiments investigating it being extremely limited in scope, suffer from potential methodological errors or deal with grass cutting ants, which share many traits with leaf cutting ants, but have adapted to face different challenges. Upon a thorough examination of the effects of gradient, it was discovered that A. cephalotes favour a cautious, but more reliable method of transport. At almost every point, their behaviour shows the importance of maintaining grip on steep and vertical gradients to the point where it is prioritised over everything else, including speed and load size. While it may seem paradoxical to suggest that smaller loads, carried slower might result in a higher overall rate of leaf collection, a fast, but reckless approach might result in a high proportion of unsuccessful foraging trips, each of which costs energy and time. As a result, by increasing their success, rather than speed, they minimise wasted effort, loss of workers and potentially, have a higher rate of leaf collection over time. This aspect of leaf cutting ant behaviour shows that leaf cutting ants can change their priorities at the feeding site to best maximise transport success at an individual level, which demonstrates previously unappreciated plasticity and a new lens through which to view future investigations into ant foraging behaviour.