Many of you have probably heard of one of the most bizarre and sinister adaptations in nature: the manipulation of host behavior by parasites in a way that not only kills the hosts, but makes them behave in ways that help spread the fungus’s genes.
This can occur through either fungi making ants climb up high on trees or grass before expiring, facilitating the spread of spores that grow from the ant’s corpse, or, in the case of worms, making an ant resemble a berry before it dies. This attracts birds, which, eating the berrylike ant, become the next host in the parasite’s complex life cycle.
The ability of fungi and worms to control the behavior of insects to the parasites’ benefit is, to me, one of the most remarkable results of natural selection—which acts, as always, to promote the passing on of genes. So, though a process of blind, mindless, and unguided selection (for what kind of a loving god would promote such a horrible scenario?), genes in the fungus that make the ant more liable to spread spores, and genes in the worm that make the ant more susceptible to the parasite’s next host, become the genes that replicate more often.
Rather than just rewrite someone else’s story, here’s the tale, from Wikipedia, of the fungus that is probably the one in the photo below, Ophiocordyceps unilateralis. Read carefully, for there are many marvels of evolution here:
The fungus’s spores enter the body of the insect likely through the cuticle by enzymatic activity, where they begin to consume the non-vital soft tissues. Yeast stages of the fungus spread in the ant’s body and presumably produce compounds that affect the ant’s brain and change its behaviour by unknown mechanisms, causing the insect to climb up the stem of a plant and use its mandibles to secure itself to the plant. Infected ants bite the leaf veins with abnormal force, leaving telltale dumbbell-shaped marks. A search through plant fossil databases revealed similar marks on a fossil leaf from the Messel pit which is 48 million years old.
The fungus then kills the ant, and continues to grow as its mycelia invade more soft tissues and structurally fortify the ant’s exoskeleton. More mycelia then sprout out of the ant, and securely anchor it to the plant substrate while secreting antimicrobials to ward off competition. When the fungus is ready to reproduce, its fruiting bodies grow from the ant’s head and rupture, releasing the spores. This process takes 4 to 10 days.
The changes in the behavior of the infected ants are very specific, giving rise to the term zombie ants, and tuned for the benefit of the fungus. The ants generally clamp to a leaf’s vein about 25 cm above the ground, on the northern side of the plant, in an environment with 94-95% humidity and temperatures between 20 and 30 °C. According to David Hughes, “You can find whole graveyards with 20 or 30 ants in a square metre. Each time, they are on leaves that are a particular height off the ground and they have bitten into the main vein [of a leaf] before dying”. When the dead ants are repositioned in various other situations, further vegetative growth and sporulation either fails to occur or results in undersized and abnormal reproductive structures.
Ophiocordyceps fungus growing from the carcass of a carpenter ant. Note how the ant’s mandibles have gripped the leaf edge, anchoring it in place. This behavior is induced when the fungus takes partial control over the ant’s brain. Jatun Sacha reserve, Napo, Ecuador
This is an ex-ant. Bereft of life, it rests in peace
Now ponder what this fungus does: it produces some chemical that affects the ant’s brain so that it
- Climbs up to a specific height and location on a tree, a location that has a specific direction (north) and humidity
- Makes the ant clamp its mandibles onto a leaf before dying
I find that astounding. Truly, natural selection is cleverer than we are. And here’s some project for an enterprising myrmecologist: find out exactly what chemical the fungus secretes that can control an ant’s brain in such a specific way. The results will surely be surprising.
If anything makes me “spiritual,” it is scenarios like this, gruesome as they are.
Here’s another zombie ant, this one controlled by a nematode worm that not only affects the ant’s behavior, but also its morphology, making it resemble a tasty berry and weakening the junction between the nematode-containing abdomen and the thorax, so that the abdomen is easily plucked off and ingested by the bird predator, its next host. The worm also affects the ant’s foraging behavior, making it search for food outside the nest more often, and thus be more likely to be found and eaten. Text and photo from Mental Floss:
A parasitic nematode named Myrmeconema neotropicum targets the gliding ant Cephalotes atratus in the Central and South America rain forests on its way to infecting birds. The nematodes travel to the ant’s abdomen, and as they mature, cause the abdomen to grow round and bright red! The ants will hold their abdomens, or gasters, high as if to draw attention—which they do. The red gasters look like berries and are eaten by birds who normally don’t eat insects. The nematodes then live in the bird’s digestive tract.