The stench of the living

It has long been known that ants recognize their deceased nestmates using the smell of fatty acids that accumulate as the body decomposes.  The chemical signature of deadness helps ants remove the corpses from their midst, keeping a clean and sanitary nest. Indeed, this classic tale of ants and oleic acid is one of E. O. Wilson’s favorite stories.

But it turns out that the story is even richer than previously supposed.  A study by Dong-Hwan Choe et al published in yesterday’s PNAS note that Argentine ants (Linepithema humile) carry away the dead even before the fatty acids appear.  It seems that the ants not only recognize the scent of death, they also pick the scent of life.  (Apparently, life smells of “dolichodial” and “iridomyrmecin”.)  Here’s the abstract:

Abstract: One of the most conspicuous and stereotyped activities of social insects such as ants and honey bees is necrophoresis, the removal of dead colony members from the nest. Previous researchers suggested that decomposition products such as fatty acids trigger necrophoric behavior by ant workers. However, fatty acids elicit both foraging and necrophoric responses, depending on the current nest activities (e.g., feeding or nest maintenance). Furthermore, workers often carry even freshly killed workers (dead for <1 h) to refuse piles before significant decomposition has a chance to occur. Here, we show that the cuticular chemistry of Argentine ant workers, Linepithema humile, undergoes rapid changes after death. When the workers are alive or freshly killed, relatively large amounts of 2 characteristic ant-produced compounds, dolichodial and iridomyrmecin, are present on the ants’ cuticle. However, these compounds disappear from the cuticle within about 1 h after death. We demonstrate how this phenomenon supports an alternative mechanism of ant necrophoresis in which the precise recognition and rapid removal of dead nestmates are elicited by the disappearance of these chemical signals associated with life.

Source: Dong-Hwan Choe, Jocelyn G. Millar, and Michael K. Rust. 2009. Chemical signals associated with life inhibit necrophoresis in Argentine ants. PNAS 2009 : 0901270106v1-pnas.0901270106.

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Cheating ants smell funny….

Aphaenogaster cockerelli, Arizona

Here’s a new study in Current Biology from Adrian Smith, Bert Hoelldobler, and Juergen Liebig:

Abstract: Cheaters are a threat to every society and therefore societies have established rules to punish these individuals in order to stabilize their social system [1–3]. Recent models and observations suggest that enforcement of reproductive altruism (policing) in hymenopteran insect societies is a major force in maintaining high levels of cooperation [4–6]. In order to be able to enforce altruism, reproductive cheaters need to be reliably identified. Strong correlational evidence indicates that cuticular hydrocarbons are the means of identifying cheaters [7–14], but direct proof is still missing. In the ant Aphaenogaster cockerelli, we mimicked reproductive cheaters by applying a synthetic compound typical of fertile individuals on nonreproductive workers. This treatment induced nestmate aggression in colonies where a queen was present. As expected, it failed to do so in colonies without a queen where workers had begun to reproduce. This provides the first direct evidence that cuticular hydrocarbons are the informational basis of policing behaviors, serving a major function in the regulation of reproduction in social insects. We suggest that even though cheaters would gain from suppressing these profiles, they are prevented from doing so through the mechanisms of hydrocarbon biosynthesis and its relation to reproductive physiology. Cheaters are identified through information that is inherently reliable.

In less technical language, fertile ants smell different than infertile ants, and when researchers painted the fertility odor on non-reproducing workers, in colonies that already had active queens, the hapless ant was attacked by her sisters.  So colonies have a way to maintain social order that does not rest entirely on the altruism of individuals to forgo reproduction.  They, like us, have police.

Of course, we already knew that ants, bees, and wasps engage in this sort of policing.  What is novel in the Smith et al study is an experimental test of how the worker police recognize the cheaters.  As in much of ant communication, the cue is chemical.

source: Smith, A. A., Hoelldobler, B., Liebig, J. 2009. Cuticular Hydrocarbons Reliably Identify Cheaters and Allow Enforcement of Altruism in a Social Insect. Current Biology 19: 78-81. doi: 10.1016/j.cub.2008.11.059.

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