Keep the nest clean: survival advantages of corpse removal in ants.

Sociality increases exposure to pathogens. Therefore, social insects have developed a wide range of behavioural defences, known as 'social immunity'. However, the benefits of these behaviours in terms of colony survival have been scarcely investigated. We tested the survival advantage of prophylaxis, i.e. corpse removal, in ants. Over 50 days, we compared the survival of ants in colonies that were free to remove corpses with those that were restricted in their corpse removal. From Day 8 onwards, the survival of adult workers was significantly higher in colonies that were allowed to remove corpses normally. Overall, larvae survived better than adults, but were slightly affected by the presence of corpses in the nest. When removal was restricted, ants removed as many corpses as they could and moved the remaining corpses away from brood, typically to the nest corners. These results show the importance of nest maintenance and prophylactic behaviour in social insects.

Post-mortem changes in chemical profile and their influence on corpse removal in ants.

In order to decrease the risk of pathogen transmission, ants remove corpses from the vicinity of nests, but little is known about the underlying mechanisms. In particular, it is unclear how the odor profile of corpses changes with time since death and how any changes might relate to behavior. We have addressed these questions in the red ant Myrmica rubra, where we asked how the time since death determines the ability of workers to discriminate a dead individual from a live one, and whether dead workers are removed in a similar way when they originate from the same or an alien colony. We found that ants could discriminate alien from nestmate corpses up to 2 days after death, since the former continued to elicit aggressive behavior over that period. For dead nestmates, only 15 % of corpses were removed when freshly killed but this rises to 80 % for corpses between 1 to 6 days post mortem. Using gas chromatography, we found that oleic and linoleic acids, which are absent on freshly killed corpses, appeared post mortem and were in higher quantities on those corpses that were ejected from the nest vicinity. When added to fresh corpses, linoleic and oleic acids, alone or blended, enhanced removal to levels observed for corpses of 2-6 days post-mortem. Thus, oleic and linoleic acids appear to be important cues involved in corpse recognition and necrophoresis over a long timeframe, and we advance the hypothesis that these fatty acids in combination with other cues may also trigger other behaviors such as prey retrieval.

Social prophylaxis through distant corpse removal in ants.

Living in groups raises important issues concerning waste management and related sanitary risks. Social insects such as ants live at high densities with genetically related individuals within confined and humid nests, all these factors being highly favorable for the spread of pathogens. Therefore, in addition to individual immunity, a social prophylaxis takes place, namely, by the removal of risky items such as corpses and their rejection at a distance from the ant nest. In this study, we investigate how Myrmica rubra workers manage to reduce encounters between potentially hazardous corpses and nestmates. Using both field and laboratory experiments, we describe how the spatial distribution and the removal distance of waste items vary as a function of their associated sanitary risks (inert item vs. corpse). In the field, corpse-carrying ants walked in a rather linear way away from the nest entrance and had an equal probability of choosing any direction. Therefore, they did not aggregate corpses in dedicated areas but scattered them in the environment. In both field and laboratory experiments, ants carrying corpses dropped their load in more remote-and less frequented-areas than workers carrying inert items. However, for equidistant areas, ants did not avoid dropping corpses at a location where they perceived area marking as a cue of high occupancy level by nestmates. Our results suggest that ants use distance to the nest rather than other occupancy cues to limit sanitary risks associated with dead nestmates.

Emergency measures: Adaptive response to pathogen intrusion in the ant nest.

Ants have developed prophylactic and hygienic behaviours in order to limit risks of pathogenic outbreaks inside their nest, which are often called social immunity. Here, we test whether ants can adapt the "social immune response" to the level of pathogenic risk in the colony. We challenged Myrmica rubra colonies with dead nestmates that had either died from being frozen or from infection by the fungus Metarhizium anisopliae. Ant survival was compromised by the presence of the fungus-bearing corpses: workers died faster with a significantly lower survival from the 4th day compared to workers challenged with freeze-killed corpses. When faced with fungus-bearing corpses, workers responded quickly by increasing hygienic behaviours: they spent more time cleaning the nest, moving the corpses, and self-grooming. Ants in fungus-threatened colonies also decreased contact rates with other workers, and moved corpses further in the corners of the nest than in colonies in contact with non-infected corpses. These results show that ant colonies are able to assess the risk level associated with the presence of corpses in the nest, and adjust their investment in terms of hygienic behaviour.

Aphid alarm pheromone as a cue for ants to locate aphid partners.

The mutualistic relationships that occur between myrmecophilous aphids and ants are based on the rich food supply that honeydew represents for ants and on the protection they provide against aphid natural enemies. While aphid predators and parasitoids actively forage for oviposition sites by using aphid semiochemicals, scouts of aphid-tending ant species would also benefit from locating honeydew resources by orienting toward aphid pheromone sources. The present study aims to provide additional information on the use of Aphis fabae alarm pheromone, i.e. (E)-ß-farnesene (EßF), by ant scouts. The perception and behavioral impact of EßF on Lasius niger were investigated using electroantennography and two bio-assays measuring their attraction and orientation towards aphid semiochemicals. Pronounced electrical depolarizations were observed from L. niger scout antennae to stimulations of A. fabae alarm pheromone, while other sesquiterpenes elicited weak or no responses. L. niger scouts were significantly attracted toward EßF in a four-arm olfactometer, as well as in an two-choice bioassay. These laboratory results suggest for the first time that low amounts of aphid alarm pheromone can be used by L. niger scouts as a cue indicating the presence of aphid colonies and could therefore mediate the aphid-ant partnership in the field.