Cleaner Ants and Cleaner Fish

Among the previously unpublished observations described in the book concerns Dorymyrmex cone ants (probably of the species smithi) climbing on the bodies of the harvesting ant Pogonomyrmex maricopa, which I first described in brief in the August 2007 issue of National Geographic Magazine. I interpret the behavior of the cone ants as a kind of cleaning activity because they often seem to be grooming the larger ant, and I compare the interaction to that between a cleaner fish and the larger fish that visit it.

I saw this behavior more than a hundred times over several days on the flats below Portal, Arizona, not far from the Southwestern Research Station. First the harvester ant would walk within one to three inches of the cone ant nest entrance, and (even when there were no actual cone ant workers nearby), adopt a curious positure, high on her legs with her head and abdomen and her mandibles somewhat open. (Fish that are ready to be cleaned by a cleaner fish adopt a similar rigid posture after arriving at a "cleaning station").

Here's a series of pictures of one such interaction, after a cone ant climbs on board and walks all over the rigid harvester ant, probing, nibbling and licking the larger species' body surface for up to a minute. (The harvester ant lifted her forebody mid-visit, but otherwise was immobile.)

Sometimes more cone ants join in. This image shows a cone ant licking at eye of a harvester ant.

Here one of the cone ants walks from one rigidly standing harvester ant to another.

If, after adopting the rigid posture, a harvester ant didn't get satisfaction (that is, no cone ant arrived in a minute or two), the worker sometimes walked closer to the cone ant nest, even backing up to put her body inside the entrance, as in the next picture. Now a cone ant almost invariably showed up. Neat!

The harvester ant would not jab at, or otherwise attempt to remove, the cone ant, even when the cone ant examined between the jaws of the larger ant, in effect putting her head in the lion's mouth. Cleaner fish take the same risk. (fish image thanks to Noam Kortler).

However, in both the cleaner fish and cleaner ant situations the larger individual can eventually tire of the attention and knock away the smaller animal. Harvester ants do this after a few seconds to a minute or two, or when they get too many riders on them, or a rider gets too feasty and begin to nibble (as cleaner fish can sometime do, in which case they get chased away). In the first image below, for example, the harvester ant has begun to kick off the cone ants, one of which is nibbling at her leg. In the second image, the harvester rolls on her back to scare off riders, which she never bites.

Is this cleaning? In that the cone ant licks the larger worker, almost by definition it must be. But whether this activity offers some kind of advantage for the harvester ant is unclear.

Curiously, the same is true for the activities of most cleaner fish. Only in the last decade have some of these fish been definitively shown to increase the health of the cleaned fish, but the advantages, if any, of their probing remain uncertain for most "cleaner fish." Some species could in fact be "clever behavioral parasites" who do the larger fish little or no good; even species that can clean sometimes cheat by biting off the larger fish's flesh rather than removing parasites (On this question, I recommend particularly the research of Redouan Bshary and Alexandra Grutter).

As I suggest in the book, the bodies of harvester ants are likely to become flecked with sugar-rich bits from the seeds they harvest, one possible enticement to hungry cone ants; and it's possible decaying sugary bits can cause infections of the harvester ants, and therefore need removing from tight corners those ants might be unable to reach in each other. But this remains only a hypothesis.

This essay expands upon the discussion on pages 191-192 of AAA.