Parasites-The real winners of Halloween in the Animal Kingdom
It’s the spooooookiest month of the year. (Well, not for me personally, we don’t celebrate Halloween in India). And I think this is the perfect time to take a deep dive into the macabre, the parasites of the animal kingdom. From chest burster wasps straight out of Alien, to mind controlling protozoans, they have it all. Mind control and zombies are often the trope of science fiction and fantasy, but, many a time, nature is even stranger than the wildest science fiction tale.
Parasitism is defined as a relationship in which one organism (the parasite), causes a direct negative effect on the other organism (the host), by utilizing the host as a habitat and for resource acquisition. Parasitism, you’d be surprised to know is the norm, rather than the exception in the animal kingdom. Of the nearly 7 million known animal species, almost half are parasitic. (And this is in all likelihood, an underestimate.) It is such a successful lifestyle mode that it has evolved independently at least 223 times.
Let’s start with this little guy, the tongue eating louse. Yes, Cymothoa exigua does exactly what its name says. The louse swims in through the fish’s gills and settles at the base of tongue. It then sucks blood out, eventually depleting all the blood in the tongue, causing the organ to atrophy, wither away and die. And the end of this rather gruesome process is a large louse securely attached to whatever remains of the poor animal’s tongue.
Next up is Ophiocordyceps unilateralis, a fungus that infects carpenter ants in the Amazon rain forest. When a spore infects an ant, it grows through its body, draining it of nutrients and taking over its body. It eventually forces the ant to leave the safety of its colony, and ascend a plant stem to a height with the optimum temperature and humidity (typically a height of 25 cms). It then forces the ant to lock its mandibles around a leaf (called a death grip), and the fungus eventually grows out as a spore filled bulbous stalk from the ant’s head. Interestingly, researchers have discovered that the fungus controls the ant without ever infecting the brain. Instead, the fungal cells invade the ant muscle cells and encircle it, allowing it direct control over the limbs, while also secreting certain compounds that interfere with the ants nervous system. The fungus essentially controls the muscle cells like a puppeteer controlling a puppet, leaving the ant a prisoner in its own body.
The parasitoid wasps are an entire family of truly nasty insects, that were allegedly the inspiration behind the chest busters in the Alien franchise. The female wasp essentially lays its eggs on an unsuspecting insect, the eggs hatch, devour the insect alive from the inside out, and eventually burst out of the hapless insect’s body.
An interesting member of this group is the gorgeous looking emerald cockroach wasp. The female wasp stings the cockroach twice, delivering venom. The first sting is merely to reversibly paralyze the cockroach, and they then administer a second sting right to the brain, in the region responsible for the escape reflex. The venom blocks the neurotransmitter octopamine, and essentially disables the escape reflex of the cockroach. The cockroach can still walk, or even fly, if it chooses to do so, but with the escape reflex disabled, it can no longer initiate these actions on its own. The wasp grabs the disabled cockroach by its antennae and leads it back to its burrow, almost like a dog on a leash. It then lays an egg on the roach, which hatches, eats the roach alive from the inside out, and eventually a fully grown wasp emerges from the cockroach’s body to begin this cycle anew.
Glyptapanteles glyptapanteles is another parasitoid wasp that parasitizes caterpillars, with the whole ‘eat you alive and burst out of you’ sequence. There is an interesting twist however. When the larvae emerge from the caterpillar, they form cocoons around the caterpillar. The larvae are able to induce changes in the caterpillar’s behavior wherein it remains close to the cocoons and defends them by lieu of violent head swings against anything that come too close. The wasp larvae effectively have their own bodyguard; a silent guardian, a watchful protector. A zombie caterpillar.
Finally, let’s take a look at something that infects organisms a little closer to home, mammals. Toxoplasma gondii is a protozoan, that spends part of its lifecycle in rats and part of it in cat’s gut. It grows in the rat’s brain, and reproduces in the cat gut. T. gondii settles in the rats amygdala, the part of the brain responsible for fear, and causes rats to lose their natural fear of the smell of cat urine. This odor also activates sexual arousal pathways in the brains of infected male rats. The dual messages of “there’s no need to fear that cat” and “that cat might just be the lady of my dreams” cause the rats to run headfirst into a cat’s territory, and gets eaten which is exactly what the parasite needs to reproduce. The mechanism of how the parasite alters brain activity is unknown, but there is an apparent link to the production of dopamine, a neurotransmitter responsible for reward and motivation.
Toxoplasma gondii also latently infects about a third of people worldwide. Studies have shown correlation between infected individuals and risky behavior, with latently infected individuals being at a higher risk of car accidents. Infected individuals also appear to have decreased reaction time. There is also a correlation between schizophrenia and being infected with Toxoplasma. However, I’d like to add the caveat that correlation is not causation, and till date, there is no firm evidence as to whether Toxoplasma can definitively change human behavior.
Well, that brings me to the end of my little discourse on the parasites of the animal kingdom. There are plenty more creepy ones out there, from barnacles that castrate crabs and turn them into barnacle making factories, to worms that force crickets to drown themselves. Happy Halloween y’all!
