Houston suffers its second wave of unicolonial ants in 30 years: the crazy rasberry ants have arrived

John Lennon, more than anyone, both soothed and validated the outrage of the 1960s, as we discovered the injustices of the grown-up world. I’m thinking of Americans, because I am one. Vietnam, racial crimes, and women’s inequality were our big issues. We hadn’t yet noticed environmental disasters much past DDT. One song, more than any other, expressed our outrage in the form of longing, John Lennon’s Imagine, released in 1971, shortly after I entered university at Michigan.

Today, that song means something else to me, for there are organisms on the planet living this particular dream, or at least one stanza of it:

Imagine there’s no country
It isn’t hard to do
Nothing to kill or die for
And no religion, too
Imagine all the people
Living life in peace

The unicolonial ants do this. First of all, let me get one important thing straight. This is not a species or a group of related species. It is a way of living that crops up in very different ants. It is more than anything a suite of behaviors, or the loss of standard behaviors.

So, what do unicolonial ants do? What they don’t do is defend their territory, their nest, or their queen. They have no country, nothing to kill or die for, live their lives in peace. Sorry, I know nothing about ant religion, except, for some people, ants are their religion. Here we are talking within the species, just as John Lennon meant it, for they do kill for food, just as we do, plants or animals. We wrote a review of these peaceful ants with Heikki Helanterä in Trends in Ecology and Evolution, in 2009, which you can get from our website, or here.
We can get an idea of the cost of territoriality by comparing a unicolonial ant species to its nearest relative. The answer is astounding, because unicolonial ants can take over a habitat, become the top predator as their nests transform the landscape, while their territorial relatives take a modest place in their ecological community. When the unicolonial habit crops up, it spreads like the proverbial wildfire. Clearly John Lennon was right, for this is a highly successful way to live. Because these ants have no territory, no fidelity to nest or queen, and never know an enemy, they forage and forage, bringing their prey to the nearest brood. All the energy that might have been expended defending a territory, or nest, from others of the same species is translated into more baby ants.
If you call all the cooperators one colony, there is a colony of Argentine ants that stretches from the northwestern coast of Italy, through France, and right around Spain, with only a smaller different colony in Catalonia. How do the researchers know this? Tatiana Giraud, Jes Pedersen, and Laurent Keller, of Lausanne, Switzerland, at the time, know this because they tested distant colonies for aggression. This is one of the best studied species of unicolonial ants, but it is not the one that invaded Houston.
The first unicolonial ant to hit Houston was the imported fire ant Solenopsis invicta. It came from Brazil to Mobile, Alabama in the 1930s and then spread. I believe the early invaders did not have the unicolonial habit. It developed later, in several places. In Houston there still seem to be both unicolonial fire ants and monocolonial fire ants. If you drive past a cattle pasture on one of those lovely Texas farm roads, and see mound after mound of fire ant nests, it will be a unicolonial population. If the nests are not so dense, they may be single queen, with territoriality. The unicolonial nests have many queens, and little dimorphism among workers; they are all small. The monocolonial nests have a single queen and workers that can be large soldiers or small foragers.
A single monocolonial nest in your yard is your best defense against fire ants, for it will spend a lot of energy killing all the other budding colonies. You can even see what kind of fire ant you have by simply sticking a long stick into a nest, getting it covered in ants, then carrying it to another colony. Watch carefully and see if the introduced ants fight with the new ants or not.
Fire ants have caused billions of dollars of damage, killed cattle, made running barefoot through the grass a risky behavior in Texas, and diminished or wiped out native ants, small rodents, and threatened prairie chickens, as I wrote about here.
But now we have something new, the ants Houstonians are calling rasberry ants, after an exterminator, Tom Rasberry. It is a kind of crazy ant, named because of how it runs around so fast. Scientifically, it is Nylanderia species near pubens. They are smaller than fire ants, and do not sting or bite the way fire ants do. But they get in everything, and apparently interfere with electrical junction boxes. I had heard they were in Pearland a few years ago, and meant to get out to see them. Now I don’t need to go to Pearland, for they were everywhere on the Rice campus when I last visited in October. They were most visible in the early morning as they ran back and forth across every wooden expansion joint in Rice’s pebbled sidewalks. These pictures don’t really capture their numbers or their speed. Now the fire ants are way less common on campus, as this new invader takes their place, perhaps out-competing them for food with their sheer numbers.
If this unicolonial behavior crops up so easily, and is so successful, then why is there anything on the planet except unicolonial ants? The answer is clear, and comes straight from Darwin. You see, unicolonial ants are like any other ant in that they have workers and queens. The workers bring in the food and tend the babies, while the queen pumps out the eggs. Evolution through natural selection will favor the queen that produces the most eggs, and this kind of selection will still occur in unicolonial ants. But evolution through natural selection will also favor the colonies with the best workers, those that are most efficient at bringing in the food for those babies. But this kind of evolution can only work at a one-step remove, for the workers themselves do not reproduce. The worker traits are favored when they share more genes with the babies they rear than with babies they do not rear, in other colonies. This is called kin selection, and requires that helpers be related to the brood they rear. If they are not related, then there can be no evolution for better worker traits. The genes for those traits cannot be passed on directly through the sterile workers, but if the workers raise young they are related to, then those traits can be passed on indirectly.
Since workers rear any baby in unicolonial ants, there is no possibility for worker traits to be evolutionarily favored. This means that mutations that reduce worker expertise will not cause their bearers to fail, so ultimately, unicoloniality is a short-lived strategy that crops up, spreads wildly, and then fails as ants with better workers take over. I guess it is just like the world-wide peace and equality we so desperately wanted in the 1960s and now.

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About Joan E. Strassmann

Evolutionary biologist, studies social behavior in insects & microbes, interested in education, travel, birds, tropics, nature, food; biology professor at Washington University in St. Louis
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