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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Sunday Night Movie: How Ants Communicate

From the recent documentary Ants: Nature’s Secret Power, a glimpse of how researchers study ant behavior in the lab:

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New Study: Older Ants are Better Workers

Pheidole dentata, older worker with larva.

A study out in pre-print by Muscedere, Willey, and Traniello in the journal Animal Behaviour finds little support for a long-held idea that worker ants change specializations to perform different types of work as they age.  By creating colonies out of different age classes in the ant Pheidole dentata, the researchers showed that older workers were good at pretty much everything, while younger ants performed only a few tasks, but did those less efficiently.  Here is the abstract:

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If it quacks like a queen…

As if butterflies weren’t flamboyant enough already, it seems that some of them actively impersonate queens.

Queen ants, that is.  A report by Francesca Barbero et al in today’s issue of Science documents a clever strategy employed by a European butterfly, the Mountain Alcon Blue  Maculinea rebeli, to infiltrate nests of Myrmica schencki.  The immature stages of the butterfly are parasites of ant colonies, and it seems the secret to their success is acoustic mimicry.  The larvae and pupae squeak like queens, eliciting preferential treatment from the workers. Here’s the abstract:

Ants dominate terrestrial ecosystems through living in complex societies whose organization is maintained via sophisticated communication systems. The role of acoustics in information exchange may be underestimated. We show that Myrmica schencki queens generate distinctive sounds that elicit increased benevolent responses from workers, reinforcing their supreme social status. Although fiercely defended by workers, ant societies are infiltrated by specialist insects that exploit their resources. Sounds produced by pupae and larvae of the parasitic butterfly Maculinea rebeli mimic those of queen ants more closely than those of workers, enabling them to achieve high status within ant societies. We conclude that acoustical mimicry provides another route for infiltration for ~10,000 species of social parasites that cheat ant societies.

Sounds of the ants and the butterfly are posted at Science News.   See also commetary by Discover and Not Exactly Rocket Science.

source:  Barbero, F. and J.A. Thomas, S. Bonelli, E. Balletto, K. Schönrogge. 2009. Queen ants make distinctive sounds that are mimicked by a butterfly social parasite. Science 323(Feburary 6): 782-785.

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Grooming or Aggression?

Forelius mccooki (small ants) & Pogonomyrmex desertorum
Tucson, Arizona

In last August’s National Geographic, photographer Mark Moffett has a controversial photo essay depicting a large, motionless harvester ant being worked over by smaller Dorymyrmex workers. Moffett’s interpretation of the behavior is this:

While observing seed-harvester ants on the desert flats west of Portal, Arizona, I noticed workers would approach a nest of a tiny, unnamed species of the genus Dorymyrmex. A harvester would rise up on her legs with abdomen lifted and jaws agape, seemingly frozen in place. Soon one or more of the little Dorymyrmex would climb aboard, licking the harvester here and there. This odd ant cleaning behavior brings to mind the interaction between some reef fish and typically smaller “cleaner fish”.

Cleaner ants are an amazing hypothesis, if true.  However, most myrmecologists with whom I’ve discussed the behavior are skeptical. For starters, no one has done the experimental work required to test the hypothesis.  To have an idea, attractive though it may be, splashed about on the pages of a major magazine without first undergoing any sort of rigorous evaluation rubs scientists the wrong way. After all, National Geographic has been burned by fraud in the past.  And then, the “cleaner ants” in some of the photos look like they are biting pretty hard, more in line with ordinary defensive behavior.

Regardless of what it means, these harvester ant/dolichoderine interactions are striking.  I’ve seen it a few times myself, with Pogonomyrmex barbatus and Dorymyrmex near Portal, and here in Tucson with P. desertorum and Forelius mccooki (shown in the photograph at the top). The harvester ants freeze up when they bump into a foraging trail or nest entrance of the smaller dolichoderines, and the smaller ants swarm up over them until the large ant eventually wanders away.  Someone really ought to give the behavior a proper study.

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