Zombies. You’ve seen them in the movies– creatures devoid of consciousness and self-awareness, but able to move and respond to to the environment around them. No longer in control of theirown bodies or minds, the undead are merely a vehicle for a vicious brain infection that wants one thing, and one thing alone—-BRAIIIIIIINS! Or in the case of zombie ants, a death grip on leaf veins in the rainforest understory.
Zombie ants may look like ants, but what they really represent is a fungal genome expressing fungal behavior through the body of an ant. This is pretty much the case with any manipulating parasites- they change or manipulate the behavior of their hosts, eliciting behaviors that ensure the life cycle of the parasite continues. For instance, salt marsh killifish serve as a second intermediate host to Euhaplorchis californiensis, which ultimately want to find their way into their final host, egrets. E. californiensis encysts on the brain of the killifish, and cause it to exhibit conspicuous swimming behaviors, making the fish surface, jerk, shimmer and contort to maximize the chance that the egret notices and eats the infected killifish. Voila! Parasite is now where it is supposed to be, inside the bird. The same sort of thing happens with Toxoplasma gondii. Ultimately the parasitic protozoan wants to be inside a cat, so what does it do? It takes over the brain of rats and mice, and makes them drawn to, rather than fearful of the scent of cats. That’s right…the little buggers run straight towards the cats, ensuring that they become a tasty snack for kitty. I digress…back to the zombie ants!!!
The ants in question are Camponotus leonardi, a tropical canopy-dwelling carpenter ant from Thailand. They live pretty routine lives, rarely descending to the canopy floor, and only traveling along well-defined paths on those rare occasions. When infected by the parasitic fungus Ophiocordyceps unilateralis, all of that changes– random wanderings like a drunk unable to find their way home, and repeated convulsions that make the ants fall to the canopy floor, ensuring that they remain in the understory and attach themselves to leaves low to the ground.
At solar noon, the fungus synchronizes ant behavior, forcing the zombie ants to bite the main vein on the underside of the leaf. At that final moment, when the ant is under fungal control and biting into the leaf vein, its head is filled with fungal cells, and its mandibular muscles are atrophied. The multiplying fungal cells in the ant’s head cause fibers within the muscles that open and close the ant’s mandibles to become detached, resulting in lock-jaw, which ensures that the ant will remain attached to the leaf after death.
The death grip of the zombie ants seem to have no purpose other than to facilitate fungal reproduction, which is only possible after the growth of a large stalk from the back of the ant’s head, followed by the release of spores which can then be picked up by another worker ant. The location of death in the humid understory is adaptive for the fungus, because it provides optimal conditions reproduction to occur. Inevitably the ant dies from the fungus, and it is necessary that this happens outside of the colony because ants quickly remove dead nest-mates, and that would not allow enough time for stalk growth and the release of spores.
The synchronization of the death grip suggests either a direct solar cue or an indirect via correlated temperature or humidity, and warrants further study to determine the neurobiological and molecular mechanisms behind such adaptations. Fungi are well known to have clock genes useful in synchronizing activity.
Perhaps these poor ants didn’t remember the rules. They didn’t do their cardio. They didn’t limber up. They forgot the double tap. But most of all, they forgot to get in the good graces of Ed Yong, who is someone you want to have on your side in the event of a zombie invasion.
David P Hughes, Sandra Andersen, Nigel L Hywel-Jones, Winanda Himaman, Johan Billen, & Jacobus J Boomsma (2011). Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection BMC Ecology, 11 (13) : doi:10.1186/1472-6785-11-13
Images are from Wikimedia Commons.