A keeper jokingly told me the other day why, early in his career, he chose to work with large mammals: “I like to actually be able to find and count all the animals in my care.” It made me laugh—who wouldn’t? But at that time I didn’t know that I would soon be counting—at long last—one of the most notorious and elusive animals in the San Diego Zoo’s insect collection and that it would inspire more anxiety than relief!
Tracking animals in the Zoo’s entomology collection affords little of the numerical certainty that helps to manage other animal groups. The reality is that in some of our invertebrate cultures, there are just going to be too many individuals to count, and certainly too many to count each day. This is not to say that we don’t keep track of them—in fact, we adhere to very strict permit conditions issued by the U.S. Department of Agriculture (USDA) in order to display and rear the regulated exotic invertebrates in our collection. When they recommend, as a containment measure, the presence of queen–excluding mesh on our leafcutter ant queen’s accommodations, that’s what we provide. Huh? Allow me to explain…
Paige prepares to moves the young colony and queen into the queen chamber in 2007.
All ant species have a reproductive division of labor, an overlap of at least two generations, and cooperative care of young. For our leafcutter ants (one of the most highly specialized ant species on Earth), it translates to one enormous queen ant, which accomplishes all the egg-laying in the colony, and hundreds of thousands of varied-age sterile worker ants that do everything else. (This is an immense oversimplification, but a treatment of leafcutter ant biology and behavior is another blog entirely!)
Leafcutters are quite famous for the long, green rivers of cut plant material that they produce while traveling back to their nests. They use the cuttings as a substrate to maintain and grow a fungus within their nests that they use for food. In the areas where they occur—particularly Central and South America—they are widely regarded as agricultural pests. So it is no surprise that our regulatory agencies would like to feel secure that though we may not always be able to see the queen ant, we always know where she is. Enter the queen excluder!
The queen ant in the tropical species that we hold, Atta cephalotes, is a big girl. In ant terms, she towers over the other colony members at a size of about one inch in length. But it is the widest, most rigid part of the queen’s body that the excluder must contain—the thorax. Our queen-excluding mesh is ¼-inch x ½-inch, big enough for the largest ants to access her chamber, but sufficient to keep Her Majesty in one spot.
And in one chamber she has remained for the five years she has been with us. We always feel confident that she is doing well based on the colony behavior and health of the fungus gardens (particularly the royal one), and that has to be enough, as we have not actually SEEN her for four years. Buried within the royal chamber, she is busy laying eggs (close to 30,000 per day), and participates in no other daily aspects of colony life. As long as she is producing continual reinforcements for the short-lived adult ants, the work of the colony carries on.
Here’s the containment area in the Insect House, showing the current housing of the two leafcutter ant colonies at the San Diego Zoo. Individual fungus gardens for each colony are maintained within the larger terraria.
Early in November, we noticed that the ants were reducing the amount of fungus in the queen chamber, little by little. Knowing that as the queen goes, so goes the colony (for leafcutter ants are not capable of producing a “backup” queen if their foundress dies), this was an unsettling turn of events. Each day we would check the garden and try to triangulate the height and shape of the fungal peaks (“Do you think it looks smaller today? I think it looks smaller.”), until we could no longer deny that something was changing. And that change seemed likely to include the relocation of the queen, since the ants had already started the demolition of her quarters.
The ants, of course, are quite oblivious to our needs, wants, and regulations. If they encounter an obstacle in nature, they do what it takes to surmount it. So here we were, faced with the unconquerable result of a decision rule (the obvious desire to move her), a very large queen, and a port of exit too small to safely accomplish the task (the excluder mesh). This made me very nervous, because I knew they would try to get the queen where they wanted her to be, but the outcome of that effort might be MOST unsatisfactory to ALL parties involved.
When I came in on Sunday, senior keeper Barb told me that the ants had been trying to move the queen out through the mesh, royal head first. Panic! There are anecdotal reports of other captive colonies, faced with this very circumstance, giving it a try and ending up with the head of the queen on the outside of the chamber and the rest of her body inside—effectively excluded, but with a huge price. Barb covered the entrance with a piece of plastic, and I headed for the toolbox in a hurry.
The young colony's fungus garden was only the size of a softball when it arrived at the San Diego Zoo.
As long as the excluder keeps the queen in a controlled area (one that could be examined if need be), we are well within the limits of our permit. Since the back area of our display colony houses several fungus chambers, including the queen’s, we decided to give her access to all of the other chambers in the off-exhibit area and move the excluder to the port that leads into the exhibit. In this way, the ants would have some freedom to choose where to house the queen, and all we would lose is the ability to know exactly in which of the 11 chambers she resides. Though that specific knowledge is a huge benefit for long-term management of the colony, I decided that we could live without it—after all; there is no long-term management for a colony with a headless queen.
In fits and starts, scraping angry, biting ants off of each hand as I went, I snipped away the steel excluder mesh at the chamber entrance, placed a fitting with a new excluder on the exit tubing, and removed the plastic barrier that Barb had placed earlier that morning. Within an hour or so, I got a call from lead keeper Kelli: “They are taking her out!” I ran back to the containment room, and we all watched a few supermajors (also known as soldiers) emerge with an entourage of medium-size workers carrying the queen! They made a direct line for a chamber on the bottom row, and after a few gut-wrenching (for us) tries to get her in there sideways, they figured it out, and she was in. And then she was GONE! During the transport, she remained still, but once she hit the ground in the chamber, she made her way quickly to the interior.
Seeing the queen of a leafcutter ant colony is a really unique, fascinating, and rare experience. It is a glimpse of a hidden world, and for insect lovers, akin to seeing a celebrity—only way better! But in managed care, it can be nerve-racking, because it could mean that the colony is rejecting something about their current living arrangement. So it appeared here, and we are happy to have been able to read the signs and facilitate the transition.
A few days later, there is harmony again, with all chambers receiving new leaf material and business resuming as usual. Leafcutter queens can live at least a decade, laying more than ten million eggs per year. Assuming there was no disease or failing in fertility that precipitated this event (or any that follow it), I hope she stays with us for the rest of her long life. And please understand that I say this in the most respectful way: I would gladly count lions, eagles, or stick insects all day long, but I hope I NEVER see Her Royal Majesty again!
Paige Howorth is an animal care manager at the San Diego Zoo. Read her previous post, Off with his Leeeeg!