Bearded Harvester Ant
Hello, ladies! These fine girls are Pogonomyrmex barbatus, this time in a group photo and showing off their beards (look at the lower left ant in the larger image). Don’t let their beards fool you: all ants are female except for the males. I realize that this is a bit of an obvious statement, but what I mean is, males have wings and are only reared when it is time for the queen to mate. I should also point out that all female ants are sterile workers except for the queen.
The photo was taken through the side of my AntWorks™ ant habitat, which is full of a clear blue gel that the ants can tunnel in and remain visible. (It also contains moisture and is edible, so it’s sort of an all-in-one substrate for ants.)
So, now for the bit of amazing trivia I promised you on Sunday. According to the UF Book of Insect Records, genus Pogonomyrmex posesses the most toxic venom, with Pogonomyrmex maricopa (a black harvester ant commonly found in Arizona) topping the list. P. maricopa venom has an LD50 in mice of 0.12mg/kg. The lovely ladies up there in the photograph, P. barbatus, clock in around 0.5mg/kg, or one-fourth the potency of their maricopa cousins.
LD50 is a measure of toxicity. It means “Lethal Dosage for 50% of the population” and it is usually expressed in terms of a mass ratio: the amount of toxin compared to the mass of the victim. So, for example, 0.12mg of P. maricopa venom is about 50% likely to kill a 1kg animal (such as a rat). To kill a 10kg dog, you’d need 1.2mg, and to kill a 100kg person, you’d only need 12mg of the stuff.
Allow me to put those numbers in perspective for you: Honeybee venom has an LD50 of 2.8mg/kg–five times less toxic than our ants here (and 20 times less toxic than P. maricopa).
Why are these ants so deadly, and how come you haven’t heard about this on the news with them rampaging across the countryside stinging people to death?
Ant nests are full of nutrition if you’re a digging rodent. Harvester ant nests, in addition to being full of yummy larvae and pupae, tend to be full of seeds. It’s like getting a side salad with your grubs. Rodents, therefore, have evolved to dig up ant nests. Harvester ants, having naturally selected in favor of being tasty and nutritious, would not survive long as a species without some form of defense. They have taken this to an extreme: a single sting from P. maricopa can kill a small mouse. More importantly, the sting hurts like crazy, so the mouse is immediately going to run away; for the purpose of defending the nest the ant has done its job and it is of no matter whether or not the mouse dies (except, perhaps for the possibility of slowly breeding out a tendency to dig up ant nests¹).
Now why aren’t they killing people left and right? The answer is in the dosage: a single sting delivers about 60 micrograms of venom. That’s enough to put you into dangerous LD50 territory if you weigh half a kilogram. If, like me, you clock in somewhere closer² to 100kg, you’d need to get stung two hundred times to get all the nutrition in one bowl of Total be at risk of death by ant stings. More importantly, the ant sting has evolved to be very effective against vertebrates, and as mentioned, they really hurt. My father reads water meters for a living and frequently finds himself working on the ground near ant nests. In his professional opinion, harvester ants “really know how to put the hurt on ya.”
I have a friend down in Moab named Howard who is a retired chemist. When I first got started talking to him about these ants, I had found the UF records page about ants but could not find any data on P. barbatus specifically. Howard, knowing how the academic world works on account of having a Ph.D. himself, contacted Dr. Schmidt directly and just asked him, who recalled the 0.5mg/kg figure from memory. The world is an awesome place when you have awesome friends. Thank you, Dr. Schmidt, and thank you, Howard.
ADDENDUM: I should point out that the UF Record for venom toxicity is openly believed to be incorrect by many biologists. There is a caterpillar in Brazil that can actually kill you if you lean up against a group of them (apparently this happens 5 or 6 times a year down there) and get stung all along your arm. The toxicity has never been measured, but we’re talking about hair coating here, not injections from a gland, so the dosage is believed to be on the order of a few micrograms per kg. Furthermore, in Australia (why is it always Australia?) human researchers became sick just from being in the room where a new species of ant was being handled. None of them were stung and some of them never even touched the ants. The study was aborted by a wise administrator who realized that, without understanding the toxic agent involved or how it was communicated, there was no way to safely study those insects. Their venom has also not been studied. And so, biologists concede that Pogonomyrmex spp. will enjoy this title for just a while longer, but quietly tell themselves that it’s called a “world record” not just for being the most toxic, but for being the most toxic on record. Someday those caterpillars or those ants will get studied, and our little ant friends above will be relegated to a distant second place.
¹ The careful reader will note, however, that some of these mice will survive, which also means the ants slowly breeding into mice a tendency to survive ant stings. Evolution is dangerous stuff, and when you pick up one end of the stick for half a million years, you also pick up the other.
² On the low side, thank you very much. But not before I dieted and lost about 20kg. I gained 6kg back, but I’m still in the mid-90’s.