Ants and Seeds – Pogonomyrmex desertorum and Pheidole xerophila

Pheidole xerophila minor workers and Pogonomyrmex desortum. The Pogonomyrmex is holding one of the pilfered seeds in her mandibles. You can see other seeds scattered in the dirt.

One July day in 2004, I was out in the desert watching ants. Everywhere I looked, I saw something neat. We’d had monsoon rains a day or so previously and it had brought the desert to life.

I was sitting and watching a Pheidole xerophila nest. These are small seed harvesting ants. Their workers are of two castes – small, blackish minor workers, and larger mahogony-brown major workers. The majors have heads that look enormous in relation to their body size. It’s like watching bulldozers with legs. But the majors are he majors are surprisingly shy, so mostly the ants I was seeing were minor workers.

They were carrying seeds out of one of their nest hole. The seeds were, relative to the workers, enormous. Each seed was almost the size of a minor worker’s head. Every now and then a worker would take one of the seeds into the nest, but mostly they seemed to be taking them out. I wondered if the seeds were too large, or tough, for the Pheidole to eat?

As I watched, I saw a single red Pogonomyrmex Harester Ant come up to the Pheidole nest.

The Pogonomyrmex was walking with its gaster tucked under, so that it was curved into a “C” shape.

Pogonomyrmex desertortum worker pausing while looking for seeds. She would often move around in this hunched posture, even when no Pheidole nearby.

From my readings, I suspected that this ant was  Pogonomyrmex desertorum.

I’ve read that Pogonomyrmex ants will take this hunched posture when they are laying a trail. I’ve also read that the act of curving the gaster forward like this ant exposes the openings to some glands, perhaps trail markers. The notion of the ant exposing gland openings by flexing its body, thus moving its various plates of exoskeleton armour, is very pleasing to me. I’m not sure why, but I think it’s pretty neat.

Robert Johnson, in his 2000 paper on seed-harvesting ants, describes Pogonomyrmex desertorm as a “Solitary Forager with Limited Recruitment”. Cole, in his 1968 Harvester Ant book, mentions that P. desertorum workers:

…move slowly, forage on trails at a steady gait, and frequently have the gaster turned downward as though they were laying a chemical trail.

Both of those descriptions fit what I was seeing. I had not seen this species before, I was pretty excited.

The Pogonomyrmex walked closer and closer to the Pheidole xerophila nest that I’d been watching. But then something really cool happened. She started examining the seeds that the Pheidole workers had brought out of the nest, even as the Pheidole workers were all around her.

Pogonomyrmex desortum approaches Pheidole xerophila nest entrance. The seeds the Pogonomyrmex wants are scattered in the foreground. You can see the Pheidole minors carrying the seeds – some would carry the seeds into their nest, some would carry them out.
Pogonomyrmex desortum worker gathers seeds as Pheidole xerophila minor threatens her with gaping jaws

The Pheidole workers would gape their jaws at the much larger Pogonomyrmex. Some of the Pheidole would clamber over her.

Pogonomyrmex desertotum gathers seeds as Pheidole xerophila minor clambers on her. You can see the seeds in the foreground of the photo. One of the Pheidole nest entrances is in the background, with a couple of minors emerging.

The Pogonomyrmex worker did not seem to be troubled by this attention. She worked her way closer to one of the Pheidole nest entrances.

Pogonomyrmex desortorum worker with seed, as Pheidole xerophila minors guard nest entrance. The nest entrance is under the stick.

The Pheidole workers would react to the Pogonmyrmex when they got close to her, gaping and biting at her. But they were not very persistent, and did not seem frantic in their attacks. It was more as if they were saying “Hey! Get out of here!” than trying to do the Pogonomyrmex harm.

Then the Pogonomyrmex put her head into the Pheidole nest entrance.

Pogonomyrmex desertortum looking for seeds near a Pheidole xerophila nest entrance. She actually puts her head into the P. xerophila nest entrance at one point. On the left side of the photo, you can see a Pheidole minor with one of the seeds.

“Oh!”  I thought, “Well! That’s torn it! They’ll react to that!”

The Pogonomyrmex calmly took her head out of the nest, then started looking for more seeds. A couple of Pheidole major workers came out of the nest, presumably in response to the Pogonomyrmex putting her head into the entrance.

The Pheidole majors came out of the various nest entrances, but were very calm. They didn’t seem to be even looking for the Pogonomyrmex…just slowly walking around.

Pheidole xerophila major workers would emerge from the nest, but did not engage with the Pogonomyrmex.
Pheidole xerophila major worker emerges from the nest

One of the major workers had the severed head of a Solenopsis aurea, a Golden Fire Ant, clamped to her antenna.

Pheidole xerophila major worker emerges from a different nest entrance. Note the severed Solenopsis aurea head attached to her antenna. You can see a P. xerophila minor worker entering the nest in the background.

Eventually, the Pheidole minors started to mob the Pogonomyrmex. Again, not with any real persistence…more in a sort of “Go away now, please!” way.

Pheidole xerophila minor workers bite Pogonomyrmex desortum worker as the Pogonmyrmex takes seeds from the area around the Pheidole nest
Pheidole xerophila minor workers and Pogonomyrmex desortum. The Pogonomyrmex is holding one of the pilfered seeds in her mandibles. You can see other seeds scattered in the dirt.

The Pogonomyrmex did walk away from the nest entrance, and most of the Pheidole workers clambered off of her. One Pheidole, however, stayed on the Pogonomyrmex. This worker was underneath the Pogonomyrmex’s thorax. While hanging upside down, she excreted a droplet of liquid.

Pogonomyrmex desertorum worker with Pheidole xerophila minor on her underside. Note the fluid droplet at the tip of the Pheidole’s gaster – I’m assuming this is a defensive compound of some sort.
Close-up of the Pheidole xerophila minor worker discharging a fluid droplet (see red arrow) while clambering on the underside of the Pogonomyrmex desortum

I’m assuming that the droplet was some sort of defensive compound. The Pogonomyrmex walked around a little bit more and gathered a number of seeds. She held them using both her mandibles and the scoop formed by her psammaphore – the “beard” of long hairs on the underside of her head.

Pogonomyrmex desertortum carrying seeds away from a Pheidole xerophila nest.

Then the Pogonomyrmex walked away from the Pheidole nest, presumably back to her own.

I’m fascinated when I see encounters such as these  between species. It seems that the participants all know about each other, and what to expect from each other.

Sources:
Cole, Arthur (1968). Pogonomyrmex harvester ants; A study of the genus in North America. University of Tennessee Press.

Johnson, Robert A. (2000). Seed-harvester ants (Hymenoptera: Formicidae) of North America: An overview of ecology and biogeography. Sociobiology 36(1):89-122.

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