Waste management by ants: the enhancing role of larvae

Exploring immune memory traits in the social immunity of a fungus-growing ant

The immune system is crucial for organisms to defend against pathogens. Likewise, analogous immune features evolved against similar pressures at the superorganism scale. Upregulating hygiene to the same fungus pathogen is one assumption for convergent immune mechanisms in social insects, although more evidence of immune memory features remains to be confirmed. Here, we assess immune memory traits at the colony level in the leaf-cutting ant Atta sexdens. We exposed their fungus cultivar to both homologous and heterologous challenges with the entomopathogenic fungi Metarhizium anisopliae and Beauveria bassiana, as well as the mycoantagonistic fungi Fusarium oxysporum and Trichoderma spirale. By measuring ants' behaviours, we evaluated the capacity of A. sexdens: (i) to enhance their collective hygiene, (ii) speed their hygiene in further infections, (iii) how long this capacity lasts in the colonies and (iv) the degree of specificity to increase hygienic responses. Fungus grooming behaviour was enhanced mostly against entomopathogenic fungi, with a trend of faster reactions during homologous challenges. In general, the capacity to elicit such upregulated actions lasted for up to 30 days, but no longer than 60 days. Overall, colonies exhibited a degree of immune specificity, enhancing hygiene only in response to homologous exposures but decreasing it when infected secondarily with a different fungus, indicating flexible social immunity of A. sexdens after immune challenges.

Desert Ant (Melophorus bagoti) Dumpers Learn from Experience to Improve Waste Disposal and Show Spatial Fidelity

The Central Australian red honey-pot ant Melophorus bagoti maintains non-cryptic ground-nesting colonies in the semi-desert habitat, performing all the activities outside the nest during the hottest periods of summer days. These ants rely on path integration and view-based cues for navigation. They manage waste by taking out unwanted food, dead nestmates, and some other wastes, typically depositing such items at distances > 5 m from the nest entrance, a process called dumping. We found that over multiple runs, dumpers headed in the same general direction, showing sector fidelity. Experienced ants dumped waste more efficiently than naive ants. Naive individuals, lacking prior exposure to the outdoor environment around the nest, exhibited much scanning and meandering during waste disposal. In contrast, experienced ants dumped waste with straighter paths and a notable absence of scanning behaviour. Furthermore, experienced dumpers deposited waste at a greater distance from the nest compared to their naive counterparts. We also investigated the navigational knowledge of naive and experienced dumpers by displacing them 2 m away from the nest. Naive dumpers were not oriented towards the nest in their initial trajectory at any of the 2 m test locations, whereas experienced dumpers were oriented towards the nest at all test locations. Naive dumpers were nest-oriented as a group, however, at the test location nearest to where they dumped their waste. These differences suggest that in red honey ants, learning supports waste disposal, with dumping being refined through experience. Dumpers gain greater spatial knowledge through repeated runs outside the nest, contributing to successful homing behaviour.

Spatial distancing by fungus-exposed Myrmica ants is prompted by sickness rather than contagiousness

The ecological success of ants relies on their high level of sociality and cooperation between genetically related nestmates. However, these group-living insects suffer from elevated risks of disease outbreak in the whole nest. To face this sanitary challenge, social and spatial distancing of pathogen-exposed individuals from susceptible nestmates appear to be simple, although efficient, ways to limit the propagation of contact-transmitted pathogens. Here we question whether spatial distancing in Myrmica rubra ants is an active response of diseased individuals that correlates with their level of infectiousness. We contaminated foragers with spores of Metarhizium brunneum entomopathogenic fungus. We daily tracked the location of these pathogen-exposed individuals and we analyzed their movement patterns until their death on the 5th day post-contamination. Quite unexpectedly, we found that contagious individuals, whose body was covered with infectious spores, did not reduce their mobility nor stayed far away from larvae in order to limit pathogen transmission to healthy nestmates. Spatial distancing occurred later when diseased individuals were no longer contagious because spores had penetrated their body. These sick ants mainly stayed outside the nest, were less mobile and showed a shift from a superdiffusive to subdiffusive walking pattern. Furthermore, these diseased ants did not actively head towards directions that were opposite to the nest entrance. This study found no evidence for early spatial distancing by contaminated M.rubra workers that would fit to the actual risk of colony-wide contagion. Coupled to a lower mobility and area-reduced walking patterns, the late distancing of moribund individuals appears to be a symptom of sickness resulting from fungus-induced physical and physiological dysfunctions. Besides questioning the truly altruistic nature of death in isolation in this system (and potentially others), we discuss about the ecological and physiological constraints that explain the absence of early distancing when some ant species are exposed to pathogens.

A fungus infected environment does not alter the behaviour of foraging ants

Eusocial insects are exposed to a wide range of pathogens while foraging outside their nest. We know that opportunistic scavenging ants are able to assess the sanitary state of food and to discriminate a prey which died from infection by the entomopathogenic fungus Metarhizium brunneum. Here, we investigate whether a contamination of the environment can also influence the behaviour of foragers, both at the individual and collective level. In a Y-maze, Myrmica rubra ants had the choice to forage on two prey patches, one of which containing sporulating items. Unexpectedly, the nearby presence of sporulating bodies did not deter foragers nor prevent them from retrieving palatable prey. Ant colonies exploited both prey patches equally, without further mortality resulting from foraging on the contaminated area. Thus, a contamination of the environment did not prompt an active avoidance by foragers of which the activity depended primarily on the food characteristics. Generalist entomopathogenic fungi such as M. brunneum in the area around the nest appear more to be of a nuisance to ant foragers than a major selective force driving them to adopt avoidance strategies. We discuss the cost-benefit balance derived from the fine-tuning of strategies of pathogen avoidance in ants.