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.