15.13: Parasitism is a form of predation.

For most people, the word “predation” conjures images of a large animal such as a cheetah, chasing and killing another large animal such as a gazelle. But there is another world of predation, largely unseen but equally deadly: parasitism, defined as a symbiotic relationship—that is, a close and commonly long-term relationship between individuals of different species—in which one organism (the parasite) benefits while the other (the host) is harmed. In fact, this hidden world of parasites is thriving with activity: parasites are probably the most numerous species on earth, with three or four times as many species as non-parasites!

Two general types of parasites make life difficult for most organisms. Ectoparasites (ecto = outside) include organisms such as lice, leeches, and ticks. One species of Mexican parrot is all too aware of ectoparasites: it has 30 different species of mites living on its feathers alone. And many of the parasites even have parasites themselves! Endoparasites are parasites that live inside their hosts (endo = inside). They are equally pervasive. Endoparasites infecting vertebrates include many different phyla of both animals and protists, the single-celled eukaryotes (FIGURE 15-27). In all of these parasite-host interactions, as with all predator-prey interactions, the predator or parasite benefits and the prey or host is harmed.

Even though they are considered predators, parasites have some unique features and face some unusual challenges in comparison to other predators. The most obvious of these features is that the parasite generally is much smaller than its host and stays in contact with it for extended periods of time, normally not killing the host but weakening it as the parasite uses some of the host’s resources. Being located right on your food source all the time can be advantageous. But this also leads to what is perhaps the greatest challenge that parasites face: how to get from one individual host to another—after all, a parasite can’t survive long once its host dies. The methods by which parasites accomplish such dispersal are surprising. Many of their complicated life cycles involve passing through two (or more) different host species (and could have come about only through coevolution with each of the host species). These life cycles are likely to give us a new appreciation for the ingenuity of these microorganisms—or rather, for the evolutionary process that produced them. Let’s look at a few representative examples.

Case 1: Parasites can induce foolish, fearless behavior in their hosts. During their evolution, rats have developed a protective wariness of cats, as well as areas in which cats have been roaming. Toxoplasma is an organism that changes this. This single-celled parasite of rats must also spend part of its life cycle in cats. It does this by altering the brain of its rat host so that the rat no longer fears cats. In fact, Toxoplasma-infected rats not only lose their fear of cats, they become attracted to them. Is this an accident? No. This behavioral change, while quite dangerous for the rat, is exactly the change that increases the likelihood that the rat will be attacked by a cat, spreading the parasite in the process.

Case 2: Parasites can induce inappropriate aggression in their hosts. Rabid animals don’t behave normally. They froth at the mouth and become unusually aggressive (FIGURE 15-28). Is this an accident? No. Rabies is caused by a virus that infects warm-blooded animals, mostly raccoons, skunks, foxes, and coyotes. It is passed from one host to another via saliva. Inducing these “rabid” behaviors, of course, is exactly the change in behavior that will most help the virus to spread.

Case 3: Parasites can induce bizarre and risky behavior in their hosts. The lancet fluke is a parasitic flatworm. It has also been described as a “zombie-maker.” This fluke spends most of its life in sheep and goats, but the fluke’s eggs pass into snails that graze on vegetation contaminated by sheep and goat feces. Once inside the snail, the fluke eggs grow and develop, eventually forming cysts that the snail surrounds with mucus and then excretes. Continuing on their complex life cycle, the fluke cysts find their way into ants that eat the snail mucus. The flukes’ journey back to a sheep or goat is now expedited by the so-called zombie-making. In an infected ant, the flukes grow into the ant’s brain, altering its behavior. Whereas ants normally remain low to the ground, when infected by the lancet fluke, an ant climbs to the top of a grass blade or plant stem and clenches its mandibles on a leaf. This behavioral change puts the ant at greater risk of being eaten by a grazing mammal—a bad outcome for the ant, but just what the parasite needs to complete its life cycle.

These parasite-induced behavior modifications are among the most dramatic, but there are many others that are more subtle yet equally effective at allowing a parasite to thrive attached to or within its host’s body. This subset of predation is an active and exciting area of ecological and physiological investigation.