Objects on Harvester Ant Mounds

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.


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.


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.


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?


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?


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.

Escargot Anyone?

You can tell a lot about a society by examining what its members throw away.

For example, here’s the trash heap or midden of a Pogonomyrmex colony.



Shell collection?

Water condensation devices?

Taste for escargot?

Garlic and butter anyone?

Where do ants go in the winter?

Once again, our post has been inspired by a children’s book. This time it is Bugs and Bugsicles:  Insects in the Winter by Amy S. Hansen and Robert C. Kray (illustrator). The authors follow different insects, including pavement ants, as they prepare for winter. I posted a review of this book at Wrapped In Foil and insects in winter activities at Growing With Science.bugs-and-bugsicles

When you see ants and other insects coming out in the spring, you may wonder “Where do ants spend the winter?” The answer is, it all depends on where in the world the ants live, and which of the over 12,000 species you are studying.

Where below-freezing temperatures are common, ants exhibit a number of strategies to get by.

Under the ground:

Some ants, like wood ants (Formica), can adjust the structure of their nests to help regulate the internal temperature. The huge mounds act as solar-collectors, increasing the temperature inside. When it becomes too cold, wood ants retreat to deep underground, below the frost line.


In wood:
Camponotus carpenter ants, live in nests in wood. Although wood is a good insulator, it still freezes inside during the winter. Carpenter ant species that live in temperate climates must have a provision for overwintering. The ants enter a state of slowed metabolism called “diapause.” Generally, the queen stops laying eggs. The workers develop large fat bodies, which can be seen as their gasters swell in size. The workers begin to aggregate more than before. In the two species I studied in upstate New York, Camponotus pennsylvanicus and Camponotus novaeboracensis, the larval stage also overwintered in the nest, but pupae and eggs did not.

Although I kept my laboratory colonies at constant temperatures and light conditions, they still periodically went into diapause. It appeared that colonies required exposure to temperatures below 15° C for about 60 days to exit diapause. Without cold temperatures, the colonies would remain in a suspended state for extended periods.


In acorns:
Acorn ants spend the winter inside acorns on the ground. These tiny ants form small colonies. When Joan Herbers and Christine Johnson took a look at how the colonies did over winter, they found low survivorship overall. Why do the ants stick it out in acorns instead of heading underground? Some evidence suggests that by spring acorns are relatively rare, and by staying inside their prize home over winter, the acorn ants are assured of a summer home. (for acorn activities for kids, see Growing With Science).

Winter ants

One species of ant, Prenolepis imparis, has earned itself the name of winter ant because it is often out foraging in temperatures near freezing. Walter Tschinkel showed that in northern Florida these ants actually are active from November to March and then workers seal up their underground nests  and don’t come out until the following fall. Leave it to a species of ant to do things completely the opposite to most other insects.


Photograph of Prenolepis imparis from antweb.org (No photographer named)

No winter?

Ants that live in the tropics or hot climates don’t tend to react much to winter, although they may shut down temporarily during a dry or wet season instead. Some ants have extensive nests with elaborate ventilation systems, where the environment inside has uniform temperature and humidity year around. Now that’s the way to live.

Are ants active where you live yet?


Joan M. Herbers and Christine A. Johnson. 2007. Social structure and winter survival in acorn ants. Oikos. 116(5): 829-835.

Tauber, MJ, CA Tauber and  S. Masaki. 1986. Seasonal adaptations in insects. Oxford University Press, New York.

Walter R. Tschinkel. 1987. Seasonal life history and nest architecture of a winter-active ant, Prenolepis imparis. Insectes Sociaux. 34(3): 143-164.

Animal Architects: Don’t Forget Ants

When we are not doing experiments, my son and I have been reading Animal Architects: Building and the Evolution of Intelligence by James R. Gould and Carol Grant Gould. animal-architects

We have enjoyed the book so far. It is not an easy read, but the stories of the different animals are fascinating. The book is much more than a review of animal construction techniques, it is how different behaviors reflect an animal’s cognitive abilities. It also reveals how researchers interested in cognition measure an animal’s potential.

The only point that has disappointed us has been the relatively thin coverage of ant architecture. Aside from a brief overview of the nests of army and weaver ants, the Goulds pretty much skip the ants, giving the excuse that what ants do is mostly underground and hard to study.

If you are interested in animal architecture, there are ant nests that do deserve attention. Take a look, for example, at this leafcutter ant nest. It definitely rivals that of the fungus-growing termites in its complexity and size.

Another excellent example of elaborate engineering by ants is found in Holldobler and Wilson’s Superorganism book on pages 338-339. Harpegnathus saltator ants build a nest that comes complete with “wallpaper,” and is thought to withstand flooding that occurs during the monsoon season.

Ants are also capable of making decisions about potential new nests sites, a similar issue that faces honey bees when swarming. The nest emigrations of tiny acorn ants of the genus Temnothorax have been studied extensively. Evidence suggests that scout ants investigating potential new nest sites actually have a way to “measure” the interior of a cavity to determine if it is suitable.

Here’s an example experimental set up:


And finally, check out Alex Wild’s wonderful photographs of examples of ant architecture.

What do you think? Are ants capable of intricate architecture?