When observing animal behavior, we tend to assume that the animals are acting of their own accord. But what if there was another creature in charge of their actions? Though this sounds too sinister to be commonplace, it is exactly that.
Parasitic infection is prevalent in most species, even humans. Over 60 million American humans have the CDC’s top 5 parasitic infections alone1 and it is estimated that nearly all animals are host to at least one parasite in their lifetimes.2 In fact, parasites are so abundant that in some environments, they collectively outweigh the total biomass of all that ecosystem’s top predators.3 Some of these parasites may cause few negative effects for their host animals, but there are plenty of parasites that are not such low-key tenants. These organisms use their hosts in ways that are often rather disturbing, including taking control of their minds and directly manipulating their behavior.
An example of such can be found in the food that gives flamingos their signature pink coloring – brine shrimp. Scientists did an experiment4 in which they infected groups of these crustaceans with one cestode and two microsporidian parasites, leaving others uninfected as control groups. They found that the shrimp who were infected with the cestode parasite swarmed far more than the uninfected shrimp. They also appeared to have a more intense red coloration. In the shrimp who were infected with microsporidians, the researchers observed that they not only swarmed more, but also came to the surface more often than the uninfected did.
These changes in both appearance and behavior actually increase the chance that the shrimp will be seen and eaten by the birds – whom the scientists believe to be the parasite’s intended host. If the parasites manage to get the shrimp eaten, they will have achieved their ultimate goal of infecting the flamingos. If the results of this study are any indication, it could be that parasites are actually behind the group-forming behavior of some animals, rather than it being their own natural tendency.
This isn’t the only case of parasites effectively driving animal hosts to their death. It has been found that hairworms can induce “suicidal” tendencies in crickets 5 by releasing proteins into their brains that motivate them to jump into water, the only place where the hairworms can mate. Once the crickets have jumped into the water, they drown. But it matters not for the worm, who wriggles itself out of the cricket’s body to freedom.
If toxo(plasmosis) gets into the body of a rat, it turns it into a cat-seeking missile.
– Ed Yong
Parasites can also induce “suicide” by causing their hosts to have feelings of affection for their predators. Rats that have been hijacked by the Toxoplasma parasite develop an ill-fated sexual attraction to cats.6 If an infected rat picks up the scent of a feline nearby, it will be overwhelmed by feelings of lust, rather than normal feelings of fear. The rat seeks out the cat, who is usually only interested in the rat as a meal. Devoured, the rat’s body releases the parasite into the cat’s intestines, where it can finally reproduce.
Some species of mushroom can be just as deadly. Parasitic fungi like a number of the Cordyceps genus invade the bodies of insects and take over.7 Once the spores have successfully infected the brain of the insects, the fungus directs them to find a suitable place for setting up residence. After the insects have been steered to a spot with the perfect temperature, humidity, and orientation, they attach themselves to a nearby plant with their mandibles (the hook-like appendages on their mouths). Within hours, the insects die, and the fungus begins to sprout from the bodies, feeding off the nutrients of the corpses. Eventually, the fungus will release more spores into the air, which will infect any insects that are unlucky enough to be nearby. And so the repeats the cycle.
Watch one of these parasitic fungi at work in the video below:
By now, you may be thinking that if parasites had personalities, they’d be quite selfish (and a little psychopathic). But not all parasites use their hosts for their sole benefit. Many also do it for their offspring.
A Costa Rican wasp known as Hymenoepimecis argyraphaga places its larvae into the care of a predator most other insects avoid at all costs: a spider.8 After breeding, the female wasp stings a spider in order to paralyze it. She lays her egg on its abdomen, where the hatching larva can derive as much sustenance from the spider’s body as it needs. When the larva is ready to metamorphose, it manipulates the spider (with an injection of chemicals) into weaving the perfect web for holding its cocoon. Then, the spider is killed. The larva will drain the spider’s carcass of all remaining nutrients, deriving all the energy it needs to build its cocoon and undergo its transformation into a wasp. If the wasp is a female, it too will find another unfortunate spider to host its egg when the time comes.
Similar dynamics are at play in the case of another wasp, Dinocampus coccinellae, and its host of choice, ladybugs. The females of this wasp species also seek out a “babysitter” when they are ready to lay their eggs.9 A wasp will lay her egg on the underbelly of the ladybug, and in about a week, the larva will hatch – with quite an appetite. It will gorge itself on the innards and nestles inside the body of the ladybug, but doesn’t kill it. Surprisingly, the ladybug is able to continue life as usual until the larva paralyzes the ladybug, emerges, and builds a cocoon. At this point, the larva spins the threads of its cocoon around the legs of the ladybug, anchoring it into place. The ladybug will serve as a guardian of the cocoon, deterring predators with its twitching and bright coloration.
After having a majority of its body devoured, the ladybug may not seem to be a likely survivor. However, about 1/4 of all ladybugs whom are parasitized by these wasps are able to return to normal life after the larva has metamorphosed and left the cocoon.
Many may wonder why parasites like the two aforementioned wasps require such gruesomeness in order to successfully reproduce. For the wasps, it’s a numbers game. In the case of the one that uses ladybug hosts, far more of their offspring are able to survive if a living ladybug is standing guard over the cocoon. Only about 1/3 are eaten by predators. Keeping the ladybugs alive yet under their control is key. If the ladybugs attached to the cocoons are dead, 85% of the larvae still fall prey to other animals.9
From Lankaster to Lorenz, scientists have gotten it wrong. Parasites are complex, highly adapted creatures that are at the heart of the story of life. If there hadn’t been such high walls dividing scientists who study life – the zoologists, the immunologists, the mathematical biologists, the ecologists – parasites might have been recognized sooner as not disgusting, or at least not merely disgusting.
If parasites were so feeble, so lazy, how was it that they could manage to live inside every free-living species and infect billions of people? How could they change with time so that medicines that could once treat them became useless? How could parasites defy vaccines, which could corral brutal killers like smallpox and polio? 10
― Carl Zimmer
Parasites might seem like evil masterminds, but as far as we know, most aren’t particularly intelligent. Microparasites (protozoans, viruses, and bacteria) lack the complexity to even possess the sort of sentience we attribute to animals with nervous systems. Essentially, parasites evolve to out-maneuver their host’s defenses. In response, the hosts evolve better defenses, and the resulting feedback loop is a veritable arms race known as co-evolution.11 Because parasites can usually reproduce more quickly than their hosts, they are often able to continually outpace their hosts’ adaptions. This allows parasites to maintain the upper hand, keeping a firm role as the masters of their “zombie slaves”.
It’s impossible to cover all examples of parasites controlling the minds and bodies of other animals. Indeed, there may be many cases that have yet to be discovered. Only with more research can we begin to get a picture of the full extent of parasitic influence on the cognition and behavior of animals.
For an informative and entertaining introduction to the subject, watch science writer Ed Yong’s TED Talk below:
This article was written by Amanda Pachniewska, founder & editor of AnimalCognition.org
Photo credit: Chrysidid (Emerald) Wasp by USGS Bee Inventory and Monitoring Lab
No changes made. License https://creativecommons.org/licenses/by/2.0/
1 – Centers for Disease Control and Prevention
Neglected Parasitic Infections in the United States
2 – Peter W. Price
Evolutionary Biology of Parasites
Princeton University Press
3 – Armand M. Kuris et al.
“Ecosystem energetic implications of parasite and free-living biomass in three estuaries”
Nature, July 2008
4 – Nicolas O. Rode et al.
Why join groups? Lessons from parasite-infected Artemia
5 – David Biron et al.
‘Suicide’ of crickets harbouring hairworms: A proteomics investigation
Insect Molecular Biology, December 2006
6 – Patrick K. House, Ajai Vyas, Robert Sapolsky
Predator Cat Odors Activate Sexual Arousal Pathways in Brains of Toxoplasma gondii Infected Rats
7 – Sandra B. Andersen et al.
The Life of a Dead Ant: The Expression of an Adaptive Extended Phenotype
The American Naturalist
8 – William G. Eberhard
Spider Manipulation by a wasp larva
9 – Fanny Maure et al.
The cost of a bodyguard
10 – Carl Zimmer
Parasite Rex : Inside the Bizarre World of Nature’s Most Dangerous Creatures
11 – R. Dawkins, J. R. Krebs
Arms Races between and within Species
The Royal Society