By Sudhakar Deeti

Sudhakar’s study, examining waste dumping behaviour in Australian desert ants can be found here. They found that ants choose their dumping distance based on how spoilable the experimental materials are.

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Do you take your garbage out to be collected? Or do you just leave it lying around in the house? Well, a good number of ant species dispose of their waste outside too, but they, of course, do not have a garbage-collection service provided by the municipality. They have to drop their unwanted items on the ground somewhere. We have been studying the red honey ant, Melophorus bagoti, in their natural habitat in Central Australia for quite a while now and can’t help but notice their waste-dispsoal behaviour, which we dubbed dumping. We finally decided to look more systematically at their dumping behaviour.

If you had to dispose of household garbage yourself on the ground outside, how far would you take the refuse? Would it depend on the stuff being tossed out, whether it was some blades of grass and some grains of sand or some past-use-by-date meat? Our study asked red honey ants such questions experimentally, by introducing different materials into their nest or placing scraps near their nest. The ants’ workforce inevitably took hold of such experimental materials in their mandibles and either dropped the item somewhere outside or else tossed it with a stereotypical lunge looking a bit like a two-handed pass in basketball. We have yet to characterise the kinematics of such dumping acts (which could form a future study). In this study, we measured how far from the nest the ants dropped the experimental waste. But first, let us briefly introduce the red honey ant.

Australian red honey pot ant colony members

The red honey ant, Melophorus bagoti, is found across a wide swath of central Australia, where the land is decribed as semi-desert. We have been working in the convenient town of Alice Springs on a private property. The ant is the most heat-loving and heat-tolerant on the continent and is one of the most thermophilic ants world wide. This red ant, colour-coordinated with the soil of Australias Red Centre, stores nectar in the abdomen of repletes, a practice that gave rise to their name. The species is well known throughout history to the Indigenous people of Australia, who have given them nicknames such as ituny ituny, meaning sun sun. We only have the southern summer to study their activities outside. They hunker down in their nest for much of the cooler part of the year. Occupying a thermophilic niche, these timid centimetre-long ants share the physical habitat with other, less heat-tolerant ants, but red honey ants still mostly have the space to themselves in an ecological hot-desking arrangement. “Hot-grounding” is probably the better description, because red honey ants forage in the heat of the day, when it is too hot for other ants to walk around outside, and too hot for many other kinds of desert animals too. We sometimes call them nine-to-five ants. With long legs to lift their bodies a bit off from the burning hot ground that can crack 70°C, they run fast, often more like sprinting than jogging. We know quite a bit now about how they navigate: as a capsule summary, they rely much on their surroundings visual panorama. After opening up for the day and before starting foraging, the workers often dump sand and other materials around their nest.

With regard to the experimental garbage, we predicted that animal matter would be dumped far from the nest, while plant and mineral matter would be dumped close to the nest. We reasoned that animal remains have the potential to spoil, and ants would sense the potential for something to harbour pathogens, if not sense the contamination itself. We thus put dead larvae and ants, bits of a dead moth, the exoskeleton of a cicada, their own nestmates cocoon shells, and foraged food (mostly arthropod pieces) in the predicted-far category and sand, buffel grass (invasive grass common in the area), and cookie crumbs in the predicted-close category. The cookies contained a small amount of milk solids, but were mostly composed of plant materials; these delicacies can sit unrefrigerated on supermarket shelves for weeks. We measured the distance from the nest at which the dumpers dumped these experimental materials.

We were correct in our predictions in all materials but the ant cocoon shells, which were dumped on average less than 1 m from the nest entrance. The cicada shells, moth bits, foraged food, and dead ants were dumped on average about an order of magnitude farther away, the cookie crumbs were dumped about 1 m away, and the buffel grass and sand about 1/10 of a metre on average.

Why were we wrong on the cocoon shells? Digging further into research literature, cocoon shells are made of fine silk, which does not make good breeding ground for pathogens. Besides, the carer cohort of workers lick the cocoon coverings to disinfect them. Thinking back with the proverbial non-myopic hindsight, disinfected silk is not like rotting meat. We think that we overestimated the potential pathogenicity of cocoon shells.

Besides the kinematics of the dumping act, much else about dumping still needs examination. How do the ants determine the distance that they have travelled? Ants possess an internal odometer based largely on counting steps, and they can also learn a location based on the views around that spot. In dumping, do the ants as a group spread out randomly in all directions? Does each individual dumper head off in roughly the same direction on each trip, showing a kind of sector fidelity that characterises the trips of foragers? And can they differentiate actually contaminated materials from uncontaminated versions of the same stuff? This house-cleaning job that serves the health of eusocial societies so well can use further research.