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Over at my Growing with Science blog I have been doing a long-running series about insects called Bug of the Week, as well as a series about identifying seeds, called Seed of the Week.  "Ant of the Week" seems to be inevitable.  So, without further ado, our first Ant of the Week is Messor pergandei.

These sleek, black beauties are a type of harvester ant. The photographs were taken at South Mountain Park in Phoenix, Arizona.

As harvesters, Messor pergandei workers gather seeds from local plants. In their book, The Ants, Hölldobler and Wilson list it as relying primarily or exclusively on a diet of seeds. It is thought their seed storing behavior might be why they can withstand living in areas that are very dry or experience prolonged droughts.

Looking at the trash heap, or midden you can see the semi-circular rim of spines from a bur clover, Medicago sp, probably Medicago poymorpha. (See, Seed of the Week does come in handy :-)). Dale Ward reported Messor ants gathering creosote (Larrea) and gold poppy in similar habitats. Rissing found combseed, Pectocarya platycarpa, and 35 other species of plants in harvester ant middens.

Hölldobler and Wilson also suggest that the ant can survive in harsh desert conditions because of their flexible foraging strategies. Single workers search for seeds when food is in short supply and when a patch of suitable seeds is encountered, a large number of workers are recruited.

Foraging workers are known stridulate to recruit to seed sources. Ants of all castes in the genus Messor have the ability to stridulate, even the males.

Harvester ants like Messor do more than simply eat seeds, they also may help disperse them. Rissing (1986) found six species of plants were more likely to occur around harvester ant mounds, and two of those plants showed a 6 to 15 fold increase in fruits or seeds when they were growing near a nest versus away from a nest.

Messor pergandei has been receiving a lot of attention lately because scientists have discovered that in certain regions colonies have only one queen as a result of a single queen founding a colony, but in other areas multiple queens start a nest together and then fight one another until only one queen is left. In still other areas, colonies have multiple queens that cooperate (called primary polygyny). (Cahan, et. al., 2005). Sounds like some exciting avenues for further research.

This is just a brief summary of this fascinating species. For more information, try:

Dale Ward on Messor pergandei, including videos

Alex Wild has a fabulous photograph of a Euryopsis spider catching a Messor pergandei worker.

Cahan, S. Helms and Rissing, S. W. (2005). Variation in queen size across a behavioral transition zone in the ant Messor pergandei. Insectes Sociaux. 52(1): 84-88.

Donato A. Grasso,  Marco Priano,  Gianni Pavan,  Alessandra Mori,  Francesco Le Moli. 2000. Stridulation in four species of Messor ants (Hymenoptera, Formicidae). Italian Journal of Zoology, Volume 67, Issue 3: 281 – 283.

Steven W. Rissing. (1986). Indirect effects of granivory by harvester ants:  plant species composition and reproductive increase near ant nests. Oecologia. 68:231-234. (free .pdf)

Note:  In the older literature the genus name of this ant was Veromessor.

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Remember the post a few weeks back showing the snail shells in the harvester ant midden? Let's take a little closer look at what kinds of objects harvester ants collect and put on their mounds.

At first glance a harvester ant mound looks a bit like a pile of rubble.

If you start to study the mound, however, you begin to notice that the pebbles are roughly the same size. Harvester ants (Genus Pogonomyrmex) are known to gather various objects and deposit them around their nest entrances. Harvester ants in the western United States often gather pebbles, among other things.

Fossils

if you are interested in harvester ants, you should pick up the fascinating article by Daniel Adams in Smithsonian Magazine from 1984.  Adams described how paleontologist John Hatcher discovered a terrific place to find tiny fossils, such as the teeth of mouse-sized mammals. In fact he went from finding an average of 2 per day to over 87 per day.  What was his secret? Hatcher discovered that harvester ants pick up ant-sized fossils and drop them on their mounds. Hatcher simply had the locate ant mounds and sort through the piles. Much of what is known about the mammals that lived during the time of dinosaurs is due to the diligent collecting behavior of harvester ants.

Paleontologists and archaeologists both still use mounds as a source of tiny fossils today. The University of Colorado Museum of Natural History has an exhibit honoring the contribution of the western harvester ant (Pogonomyrmex occidentalis), called Tiny Collectors: Harvester Ants. This page has a photograph of a mound and more information about how the fossils are collected.

Most of the finds are in Montana, the Dakotas, Wyoming, and Colorado. I'm not sure, but there might even be something in the mound from Arizona.

Can you spot it in the upper right hand corner? To me it looks a bit like a tooth.

Beads

In 2009, Schoville et al. distributed beads of various sizes and colors around harvester ant mounds at measured distances to investigate how far harvester ants move artificial material. They found ants would bring back beads from as far as 48 m away, but most were collected within 20 m of the main entrance. They were interested in how harvester ants moving artifacts potentially effects archaeological findings.

Charcoal

Deborah Gordon first studied deposits of charcoal in Pogonomyrmex badius middens in 1984. She established that the pieces of charcoal were not incidental, because if she removed the charcoal bits, the ants quickly began replacing them. She concluded that the charcoal probably marked the ants' territories and deterred other ants.

Smith and Tschinkel re-visited what they called "non-food collection" by harvester ants. They evaluated mounds in Florida and found that pieces of charcoal were the most common objects, as Gordon had. They suggested that the objects reflect a significant amount of material, and that once again, the collecting behavior probably isn't incidental. They found a slight increase in temperature in mounds with charcoal versus those without, but found no evidence other ant species avoided the pieces.

Snail Shells

It turns out that other people have also noticed snail shells on harvester ant mounds.

PÁLL-GERGELY and SÓLYMOS are malacologists working in Turkey who found that harvester ant mounds can be a significant source of taxonomic material, especially for more cryptic species of snails.

They noted that the ants tended to collect the smaller species of snails, and the juveniles of larger species. They did not observe the ants feeding on snails, but noticed some shells were cleaned out and some were not. They emphasized that harvest ants are known to collect and feed on seeds.

Some ants do eat snails. Mark Moffett has a photo of a Basiceros singularis worker feeding a tiny snail to its larvae. (You might have to scroll through a few photos to find it.) Do harvester ants do the same?

Conclusions:

The bottom line is that harvester ants have some interesting and unusual things on their mounds, and as of yet, we don't have a very clear picture exactly why or how. In any case, we humans have found ways to use their activities to our benefit.

What unusual items have you found on a harvester ant mound?

References:

Adams, D.B. (1984). Fossil hunters best friend is an ant called pogo: paleontologists use insects to find
small bones. Smithsonian, 15: 99-104.

Gordon, D. M. (1984), The harvester ant (Pogonomyrmex badius) midden: refuse or boundary? Ecological Entomology, 9: 403–412.

PÁLL-GERGELY, B. and P. SÓLYMOS. (2009). Ants as shell collectors: notes on land snail shells found around ant nests. Malacologica Bohemoslovaca, 8:  14–18.

Schoville, B. J. Burris, L. E. and L. C. Todd. (2009).  Experimental Artifact Transport by Harvester Ants (Pogonomyrmex sp.): Implications for Patterns in the Archaeological Record Journal of Taphonomy, 7 (4):  285-303.

Smith C. R. and W. R. Tschinkel. (2005). Object Depots in the Genus Pogonomyrmex:  Exploring the “Who,” What, When, and Where. Journal of Insect Behavior, 18 (6):  859-879.

Smith C. R. and W. R. Tschinkel. (2007). The adaptive nature of non-food collection for the
Florida harvester ant, Pogonomyrmex badius. Ecological Entomology, 32:  105–112.

While investigating the Dorymyrmex bicolor ants I posted about on Monday, I spotted a few other ants that looked similar, mostly because of their red-brown and black coloration.

harvester-ant

harvester-ant4

On closer observation, however,  these ants were obviously another species. First of all, they were almost double the size.

harvster-ant2

The long hairs on the underside of their heads gave it away that these were harvester ants in the genus Pogonomyrmex. The 'beard" of hairs, called a psammophore, is characteristic of the genus. In fact, the name Pogonomymrex means “bearded one.”  Psammo comes from the word for sand in Greek, so the psammophore acts like a basket or an extra pair of hands to help the ants move sand, dirt, and probably some types of food as well.

pogo-californicus

These turned out to be a color variant of Pogonomyrmex californicus. Cole (1968) describes the species as concolorous light ferrugineous red in coastal California, moving to concolorous black or brown to the eastern part of its range in southern Texas.

Ants of the Southwest has a good page about these harvester ants that shows some of the the color variations.

It seems that Dorymyrmex bicolor and Pogonomyrmex californicus are often found in the same open, arid environments. It's interesting that at this Maricopa, AZ site their coloration is so similar.