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PEEC Amateur Naturalist: Observing Harvester Ants—a Story

on July 31, 2013 - 11:02am
PEEC Amateur Naturalist: Observing Harvester Ants—a Story
By Robert Dryja

“Ah, Watson, there you are! The game is afoot. The Pogonomyrmex are now at their peak of summer activity.” 

Holmes always became restless in the summer when so much ecological activity was occurring on the Pajarito Plateau. The mesas were of particular interest to him. “But what are Pogonomyrmex? Some type of bird?” I asked. 

Holmes looked surprised. “Really Watson, they are a genus of ant, commonly called harvester ants.”

“But ants are so small and inconsequential to my mind,” I said.

“Ah, remember when we discussed the difference between seeing and observing for cactus plants?” Holmes replied. “Clearly you need to do some serious observing of harvester ants. They are not at all inconsequential.” 

“Very well. Where shall we go?” I replied.

“We shall go to Kwage Mesa. We will follow the dirt utility road behind the horse stables.” 

And so we were off, in pursuit of harvester ants. I was about to learn that these ants have the most curious kinds of behavior. We started walking along the side of the utility road, but I initially saw nothing of interest, much less an ant.

“Have you seen any harvester ants yet?” Holmes asked. “None,” I replied. 

“Then look down in front of you. What are you about to step on?” Holmes asked. I looked closely and realized that I was about to step onto a pile of small gravel. “I just see some sand and stone,” I replied. “Look closer,” Holmes said. “Perhaps getting down on your hand and knees will help improve your observation skills.” I proceeded to do so.

“Ah, this mound is actually composed of bare sand with ants wandering around on it. Now I understand why these ants are called harvester ants. They must clear the mound over their nest of all plant life. I also see a horned toad eating any ant that walks too close to it,” I said. 

“Very good,” Holmes said. “You are beginning to observe. By the way, a horned toad is a type of lizard, not a toad. Certain species of horned lizards only eat ants.”

“You may want to stand up and away from the mound now. These ants have a nasty sting.” Holmes then said, “Can you tell me if you see anything else that is curious about this mound?”  

I stood up and realized something else. There was not a bit of plant life for almost three feet in all directions from the peak of the mound. I was looking at a nearly perfect circle. Plant life began abruptly at the edge of the circle. “Holmes!” I exclaimed. “How on earth do these ants know how to measure so accurately? I would need a ruler and compass to draw such a circle. Ants clearly do not use such tools.”

“An interesting question,” Holmes said. “Now get back down on your hands and knees and really look at that so-called sand. What do you observe now?”   Once more I looked closely and realized something new. The sand was actually composed of a peculiar kind of small gravel that looked like tiny bits of quartz, all of the same size. I watched as one ant laboriously dragged a piece of this quartz out of the entrance hole of its nest. It dragged the quartz down the slope of the mound, apparently intent on keeping a desired slope.

How do the ants know how to clear all plant life away to form a nearly perfect circle around their mound? Photo by Robert Dryja

“Holmes, this is remarkable. The mound is made of quartz-like gravel but the soil outside of the clear circle looks like typical dirt—no obvious quartz is showing elsewhere. These ants are selecting a particular kind of rock to take out of their nest for some reason.” 

“Yes, and it becomes more curious if you imagine that you are an ant in the nest somewhere underground,” Holmes said. “There is no light under ground.  How does an ant know the color of the rock it is thinking of taking to the surface?” 

Quartz bits from a harvester ant mound. Photo by Robert Dryja

Harvester ants at the entrance to their mound. Photo by Robert Dryja

A horned lizard. Photo by Jennifer Macke

Holmes now asked, “Are you ready to do some orienteering and a little mathematics? Did you observe anything peculiar about the nest entrance? Where is the entrance located for this mound and in what direction is it pointed?”

“Why, the entrance is low down on the side of the mound and it is opening to the east,” I said. We then walked the length of the utility road, keeping track of the entrances of the harvester ant mounds as we came upon them.  

“How many mounds have we encountered?” Holmes asked as we reached the end of the road. “We have found 16 mounds so far,” I said. “Again you amaze me, Holmes. Eleven of the mounds have a single entrance that faces to the east. Another two mounds have two entrances—one entrance again faces to the east and the other faces to the south. Therefore a combined total of 13 of the 16 mounds have an entrance that faces to the east. The last three mounds have a single entrance and it faces to the south. Oh, and the entrances always are low down on the sides of the mounds.”

“Yes, these are curious creatures,” Holmes said. “They somehow can measure distance and select the kind of rock to take from their nest. They also have a preferred direction and location for the entrance of their nest.”

And so I learned that ants appear to be small and inconsequential only if you “see” them. However there are surprises in store if you instead decide to “observe” them carefully.  

Want to learn more about why harvester ants may select quartz for their mounds? Anna Trugman wrote an article on her research of harvester ants in White Rock and concluded that the quartz helped to regulate the temperature and humidity level in a nest. You can read her article at:

http://www.amnh.org/learn-teach/young-naturalist-awards/winning-essays2/2006-winning-essays/environmental-engineering-of-pogonomyrmex-harvester-ant-mounds

Now, does the position of the nest entrance also affect temperature?      

 


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