Harvester Ants - July

 Harvester Ants - July

Several months ago we decided to build a section of the backyard into a raised bed for native flowers. In the process we redirected a small footpath and moved a few wheelbarrows of soil. Unfortunately, a young mound of harvester ants (Pogonomyrmex occidentalis) happened to be in the way. I tried placing stones around the mound so that it wouldn’t be buried, but in the end we had to sacrifice the ants. When we were finished with the project, the ants were buried under four to five inches of sandy soil. 

Then several days later, to my surprise, I discovered that the ants were still alive and had a new entrance connecting to their underground chambers. It wasn’t on top of the new flower bed but at the base of the new rock retaining wall. Now, these several months later, I think of their remarkable persistence every time I walk by their new mound. 

And then this last week, I learned a little bit more about the ability of harvester ants to move small stones and piles of dirt. I was studying insects on the east slope of the Pine Valley Mountains northwest of Leeds (Utah) when I discovered what first looked like an abandoned mound. You can tell that a mound is a harvester ant mound quite easily by the many small pebbles that form an asymmetrical cone and by the disk surrounding the cone that is bare of vegetation. Sometimes the ants will add bits of dried plants or dead insects on the mound as well. I have a hunch that they are drying them out to preserve underground for a future time of need. 

In this case, the small pebbles and thin blades of grass were present but were no longer covering a mound. They were lying flat and there were divots where there should have been an even convexity covered with red ants. This is not an unusual finding in the deserts of the Southwest where ant mounds are born and die as if they were individuals themselves. In fact ants (and sometimes their mounds) are often called superorganisms because of this. Workers are genetically the same and do not reproduce, and yet they are essential for the survival of the queen and the production of new ants. The individual ants really do act only for the benefit of the colony. A worker that will never have offspring of its own will recklessly defend the colony against any and all threats without giving a second thought about sacrificing her own life. The survival of the colony is all that counts. 

And yet the mound “dies” when the queen expires and no new ants are born. Eventually there is a gradual attrition of the existing workers. So when I saw the flattened mound, I didn’t think much of it. But, as it turned out, the mound wasn’t really dead. Out of the corner of my eye, I noticed a bit of movement. There were a few of my little red friends still occupying the nest. How sad, I thought, their efforts are doomed to failure. It seemed that the queen had died and these were the last survivors.

Then I noticed something else. A small black ant - maybe a third the size of the harvester ants - jumped on the back of a larger ant and began biting its neck. The harvester ant did not tolerate this at all, and in less than a second managed to fling it to the ground. I was quite surprised by the affrontary of such an undersized ant. What chance did it have against the much larger harvester ants? But I soon learned that I had misjudged the situation. The little dark ants (Conomyrma insana) were launching a full attack. 

They were harder to see than the harvester ants. But once I adjusted my vision to the smaller insects I could see that there were over a thousand milling around the erstwhile mound. The few harvester ants that remained were all concentrated around the two openings defending themselves, and what remained of their home, from the attackers. 

I watched this battle play out for several minutes, adjusting my position every few seconds so the black ants wouldn’t start climbing up my pants. When the opportunity presented itself I managed to get close enough to the colony entrance to take a few pictures. That is when I noticed the seriousness of the situation. 

There were harvester ant heads circling one of the openings. The bodies were in the shadows within and were harder to see. But the heads, with their imposing mandibles front and center, were packed as close together as possible - kind of like musk oxen of the far north protecting their young in a ring. Any black Conomyrma would have had quite a challenge getting inside that entrance. 

Then as the sun dipped lower in the west, the ring of heads began to break. The warriors became porters. Pebbles the size of the ants themselves were picked up in their heavily sclerotized mandibles and placed in front of the opening. The ants were closing up the mound for the night and I was left wondering how this would all play out. How, in fact, had this habit even evolved. 

The immediate scene suggested that the closure was an act of defense. But the scientific literature includes several examples of harvester ants closing up for the night when there is no apparent threat nearby. I know this is the case with the ants in my own yard. Mounds can be closed up in the middle of the day if it is too hot or too cold. And sometimes they are active in the middle of the night, while at other times they are closed. I decided to go looking for other nocturnal insects and leave the ants to their nests.

The sun was low in the horizon and because there had been a short rainstorm earlier in the day, the desert air was pungent with the aromatic scents of manzanitas and live oaks. The western sky was orange tinged with scarlet, reminding me of how lucky I was to be looking for small creatures in such a beautiful place. 

A young mantis - one of our native brown species (in the genus Litaneutria) - was mincing its way between bunchgrasses looking for whatever might be suddenly moving at this cooler time of the day. So much comes alive in the desert at night. Insects, scorpions, and a multitude of other creatures bury themselves in the loose terra cotta sands during the day when the ground surface temperature is often in excess of 120 degrees fahrenheit. Then, when the sun sets and stars begin to shine through the dimming cyan sky, they crawl out of their hiding places and become active. Predatory insects like mantises need to get a head-start and are likely to emerge first. It could be a good night for this one, with many species ready to emerge from a much longer sleep because of the rain. 

A large longhorn beetle (Prionus californicus) showed up on the sheet I placed near the truck below an ultraviolet light. It was two inches long with a thick dark brown body and strong antennae. I can only wonder at the scents it might be picking up. After admiring it, I encouraged it to fly back into the deepening shadows of the night.

Twilight was over and the milky way was fully visible as a band of luminescence across the sky. I ambled over to the harvester ant mound and found that it was now deserted. Both the black ants and the red harvester ants had gone to bed. Moreover, the tunnel openings had been completely sealed. My thoughts turn to the mound I buried all those months ago and how I had under-estimated the earth-moving abilities of this insect. I learned that a developed mound has enough tunnel space to sequester very large amounts of soil if needed. If the entrance tunnels get covered, the ants inside the mound can move the soil into one (or several) of their excavated chambers until they have made another opening. Then, if needed, they can move the soil out of the mound once the entrance is cleared.

This is an impressive ability all by itself. But it is also impressive to me how these ants get by with the change in atmosphere inside the closed system of tunnels. Scientists have shown that oxygen levels go down and carbon dioxide goes up. In the case of the ants in my yard (with several inches of sandy soil dumped over their mound) they were completely covered for at least two days before I saw them emerge from a new entrance. How did they manage to survive in this harsh environment for so long?

This is a question that has not been adequately worked out for harvester ants (or for many other ants for that matter). An extreme example of a low oxygen environment occurs in the mangrove swamps of Australia where a large ant (Camponotus anderseni) makes its tunnels. Twice each day, the tunnels get submerged with the high tides and the larger ants plug up the entrances to the tunnels with their heads (Nielsen et al., 2006). This happens day-in and day-out all year long. 

Many other kinds of insects also have to deal with low oxygen (hypoxia) or no oxygen (anoxia) conditions. Some tiger beetle larvae living in riparian areas get inundated periodically during flood season. Insects living in dung heaps, in carrion, in decaying wood, or even as parasites within a digestive tract of other animals all have to deal with extensive periods of oxygen deprivation (Hoback and Stanley, 2001). These adaptations seem to have evolved several different times among insects. And it probably occurs in a lot of situations that we are not aware of. Harvester ants are very likely one of these groups. 

Such an explanation would be extremely improbable for a mammal or other warm-blooded animal. Our energy demands are just too high. But insects are not warm-blooded and perhaps they can use a process similar to fermentation to get them through longer periods when they are sealed off from the outside world. One interesting by-product of the fermentation process is alcohol - which would be somewhat toxic to certain dangerous organisms that might flourish in a damp and oxygen-deprived cave. Do harvester ants occasionally seal themselves off in order to fumigate their nests from molds? Who knows? There is an interesting story here that needs to be worked out. 

We have been studying harvester ants for a few generations now. We have learned quite a bit about its troglodytic ways. It is clear the ants do very well in desert soils. They specialize in gathering seeds and a few small arthropod corpses now and then. The main harvester ant in our garden is the Western harvester ant. We have six or seven harvester ant species in the state of Utah. Most of them are only known from a couple of counties in the southern part of the state. But the Western harvester ant occurs in every county and from elevations between 3,500 and 9,000 feet (Allred, 1982). 

Earlier in the week I decided to go out and look at the ants in the yard. It was early afternoon and hot. My thermometer read 96 degrees in the shade. I shouldn’t have been worried about the ants in this heat. But their pebbly mounds are taking the full brunt of the sun and I know that some of the ants will be out foraging or guarding the nest in uncomfortable conditions. 

I sit on the dirt path next to one of the mounds. Normally this is not a good idea because it is covered by ants - ants that soon begin climbing onto my legs. In this heat, however, I don’t need to worry. Almost all the ants are underground. The few that are out are behaving frenetically. They seem to be running in random directions and incredibly fast, as if they were frantically delusional. 

A few of their sister ants are running in and out of the mound entrance, seemingly uncertain as to what they should do. One of the outside ants hurries inside. As soon as she is in the shade of the mound one of the other ants runs outside as if to take her place. Scientists have not been able to confirm that ants are intentionally coordinated this way. Their actions generally seem completely random. And yet out of this apparent confusion there is an order that emerges as if by magic. I would not be surprised if there is some sort of tag-team communication that takes place between these few hardy ants that are out on patrol on such a hot day. Why would any of them be outside under such conditions if these insects weren’t adapted to defend their mound? There seems to be some way of taking turns.

Western harvester ants make their mounds in such a way that captures the morning sun (Romney, 2002). The southeastern side is longer and slopes more slowly than the other sides - at least in the majority of mounds. In the morning it is noticeably warmer than the other sides even though, by midday, all sides are about the same temperature. Inside the mounds, adult ants stay busy moving the grub-shaped larvae into different chambers depending on temperature. By evening, almost all of the immature ants have been moved to the brood chambers on the southeast side - anticipating the first warming rays of the morning sun. 

What occupies the ants inside the mound on such a hot day? Were they moving the brood to lower chambers where it was cooler? When I buried the colony several months ago, did I limit their underground options? How many of the young ants died because there was no place to move them?

I began thinking of a motivational aphorism I learned in Sunday School many years ago. It was written by George A. Smith about a conversation he had with his cousin Joseph Smith, Jr. (the 19th Century founder of the Church of Jesus Christ of Latter-day Saints). It was about not giving up. Joseph knew quite a bit of discouragement in his short life and he told George to, “never get discouraged, whatever difficulties might surround [you]. If I were sunk into the lowest pit of Nova Scotia [Joseph said] and all the Rocky Mountains piled on top of me, I ought not to be discouraged, but hang on, exercise faith, and keep up good courage, and I should come out on the top of the heap.”

It’s funny how memories from long ago can present themselves at unexpected times. But having a giant wheelbarrow worth of dirt dumped over a human-occupied system of tunnels would seem fatal for sure. There is little chance that humans could dig themselves out of such a predicament. But then again, maybe some of us could. Perhaps the truly remarkable nature of the ant-hill exhumation is that it could be repeated any number of times by harvester ants all over the American Southwest.

Now, in July, with temperatures in the upper 90’s every afternoon, I am watching a different kind of perseverance. The ants know how to hunker down in the heat. They bury themselves on purpose. Perhaps their metabolism has to adjust. Maybe they have to sequester themselves from predators and other invading ants for a while. But they are survivors. Sometimes, it seems, it is wise to just close the doors and wait for better times.  

References

Allred, Dorald M. (1982). Ants of Utah. The Great Basin Naturalist 42(4), 415-511. 

Hoback, W. W., & Stanley, D. W. (2001). Insects in hypoxia. Journal of insect physiology, 47(6), 533-542.

MacMahon, J. A., Mull, J. F., & Crist, T. O. (2000). Harvester ants (Pogonomyrmex spp.): their community and ecosystem influences. Annual Review of Ecology and Systematics, 31(1), 265-291.

Nielsen, M. G., Christian, K., Henriksen, P. G., & Birkmose, D. (2006). Respiration by mangrove ants Camponotus anderseni during nest submersion associated with tidal inundation in Northern Australia. Physiological Entomology, 31(2), 120-126.

Romey, W. L. (2002). Does the harvester ant, Pogonomyrmex occidentalis, shape its mound to catch the morning sun?. The Southwestern Naturalist, 47(2), 175-181.

Smith, George A. (1997) The Teachings of Joseph Smith, ed. Larry E. Dahl and Donald Q. Cannon (Salt Lake City: Bookcraft), 195.