— Inside the Bizarre World of Nature’s Most Dangerous Creatures
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parasites make up the majority of species on Earth. According to one estimate, parasites may outnumber free-living species four to one. In other words, the study of life is, for the most part, parasitology.
And only now are they realizing that parasites have been a dominant force, perhaps the dominant force, in the evolution of life. Or perhaps I should say in the minority of life that is not parasitic.
Sukhdeo has earned the respect of other parasitologists for having shown that there is a behavior to parasites, that they make their way through the unique inner ecology of their hosts’ bodies, and that you can figure out the rules they obey.
Parasites have colonized the most hostile habitats nature has to offer, evolving beautifully intricate adaptations in the process. In this respect, they’re no different from their free-living counterparts,
3 The Thirty Years’ Warhow Parasites Provoke, Manipulate, And Get Intimate With Our Immune System
Toxoplasma wants to keep its intermediate host alive, so it uses its host’s immune system to hold itself in check.
A few go even further: their hosts are born with the virus’s DNA already embedded in their own genes and transmit it to their children.
4 A Precise Horrorhow Parasites Turn Their Hosts Into Castrated Slaves, Drink Blood, And Manage To C
The fly lowers its front legs, tilting its abdomen away from the surface. It flaps its wings for a few minutes before locking them upright. The fungus has meanwhile pushed its tendrils out of the fly’s legs and belly.
But when the snails are infected by the fluke, they change their behavior. They grow restless; they wander onshore or onto sandbars during low tides and linger there while healthy snails keep to the water. They shed their flukes on the sand, putting the parasites so close to the fiddler crabs that they can easily burrow into them.
The ant scurries back down to the ground and spends the day acting like a regular insect again. If the host were to bake in the heat of the direct sun, the parasite would die with it. When evening comes again, it sends the ant back up a blade of grass for another try.
They draw a quick reaction that brings immune cells rushing to them, making the skin around them swell and blister. The easiest way for a victim to get some relief from the hot pain of the wound is to pour cool water on it or just stick the leg in a pond. The juveniles that have already escaped their mother inside the blister respond to the splash by swimming free.
A mosquito with ookinetes in it will give up trying to take a blood meal more easily than a parasite-free one. Once the parasite has reached the mosquito’s mouth, though, it wants the mosquito to start biting as much as possible.
In some cases, they even took a special interest in the spot and came back to it over and over again. By turning rats into rodent kamikazes, Toxoplasma probably increases its chances of getting into cats.
Psychologists have found that Toxoplasma changes the personality of its human hosts, bringing different shifts to men and women.
Parasites have mastered the vocabulary of their hosts’ neurotransmitters and hormones.
There are boxcars of parasite biomass here.” He points to the snowy constellation of bird droppings along the bank. “Those are just packages of fluke eggs.”
If the bird scrupulously avoided infected killifish, it might stay healthy, but it would also go hungry. The parasites make so much food available to it that their benefits far outweigh their costs.
An infected polyp can’t feed or reproduce, so allowing a fluke to fester inside it is a drain on the colony, slowing its growth. If a butterfly fish prunes the coral, it can perform as well as a healthy coral. It’s to the coral’s advantage to get rid of its sick polyps, which may mean that the coral is actually contributing to the color or spikes in order to make it easier for the butterfly fish to spot.
The thinning of the herd is an illusion, not the service of the predator but the side effect of a tapeworm traveling through its life.
5 The Great Step Inwardfour Billion Years In The Reign Of Parasite Rex
If you think of parasitism in terms of Dawkins’s definition of genetic interests, then a fetus is a sort of half-parasite. It shares half its genes with its mother, and the rest belong to its father. Both mother and father have an interest, evolutionarily speaking, in seeing the fetus get born and live a healthy life. But some biologists have argued that parents also have strong conflicts on how the fetus grows.
The maternal genes try to slow down the growth of the fetus, to control this parasite within her. Meanwhile, the paternal genes clamp down on these maternal genes and silence them, letting the fetus grow faster and draw more energy from its host.
There is more than one way to look back at the dawn of humanity. You can go to Ethiopia and sift the dust for stone tools and scoured bones, but you can also go to the National Parasite Collection, find the right jar, and stare at a fellow traveler.
6 Evolution From Withinthe Peacock’S Tail, The Origin Of Species, And Other Battles Against The Rule
“We do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey.”
Hosts and parasites may evolve together in a continual escalation (what biologists call an arms race), but in many cases their evolution can look more like a merry-go-round.
Parasites force their host to go through a huge amount of change without going anywhere in particular.
“Your immune system is a sort of parasite of the parasite,” says Lively. Like parasites, T cells and B cells multiply into many different lineages, and the most successful killers get to reproduce themselves the most. Like their hosts, parasites can defend themselves by having sex and diversifying their genes.
The snail lineages that carried the heaviest burdens of parasites in a given year had been the most common snails a few years before, and now they were declining.
They’ve stuck with visible cues like rooster combs and lizard pouches because they’re easy to measure. But among the channels of communication between the sexes, vision may not be all that important.
“The scent of a male mouse,” writes one biologist, “is the chemical equivalent of a peacock’s plumage.”
It appears that parasites have essentially driven flies to evolve a habit of mating more when death—and no more matings—seems imminent.
Study arms races long enough, and you start to imagine that hosts and parasites could carry each other into the clouds, each driving the evolution of its counterpart so hard that they become all-powerful demigods hurling lightning bolts at each other.
Kraaijveld set his flies selected for wasp-fighting in competition against regular flies for food and found that they fared badly. They grew more slowly than the flies that were still vulnerable to the wasps, they died young more often, and when they grew into adults they were smaller. Evolution doesn’t have an infinite arsenal to offer hosts, and at some point they have to relent, to accept that parasites are a fact of life.
Specializing on particular host species makes parasites even more diverse. A coyote will eat just about anything on four legs, and partly as a result, there is only one species of coyote in all of North America.
7 The Two-Legged Hosthow Homo Sapiens Grew Up With Creatures Inside
Since allergies serve no good purpose, immunologists could only look on IgE as one of the rare shortcomings of the immune system. But then they discovered that IgE can be good for something: fighting parasitic animals.
But even though vaccines work well against some viruses and bacteria, there’s no commercially available vaccine against a eukaryote. None.
For some parasites, it may actually make more sense to find a better coexistence than to try for eradication.
Eradicating parasites may even create new diseases.
Parasitologists have found that intestinal worms can nudge the immune system from a poison-spouting, cell-engulfing frenzy to a gentler sort of attack. In this mellower mood, the immune system can still keep bacteria and viruses in check, but the parasitic worms can live unmolested.
Without parasitic worms exerting their influence, our immune systems may be prone to overreacting to harmless bits of cat dander and mold.
To fight these diseases, we may need to acknowledge our long marriage to parasites.
Perhaps some day, along with polio vaccines children will get parasite proteins, so that their immune systems will be trained not to fly out of control. It would be a supreme final twist to the story of parasites in humans. They may not always be the disease. In some cases they may be the cure.
8 How To Live In A Parasitic Worlda Sick Planet, And How The Most Newly Arrived Parasite Can Be Part
There’s something that the plant releases that attracts the wasps, a cry for help.”
“They’re absolutely beautiful parasites,” Lafferty said. He had me picture a big, opaque pouch with a mouth at one end, carrying a collection of golden eggs inside. “It’s hard to describe them. They look like—God, they don’t look like anything you could ever imagine.”
The other scientists who worked at Guanacaste looked on Brooks as something of a vampire, a scientist interested in the beautiful animals of the forest only if he could slit them open. But I had never seen someone mourn a dead animal so deeply.
Cordyceps, a fungus that invades insects and sprouts flowerlike stalks out of its body, is the source of cyclosporin, an important antibiotic. Hookworms produce molecules that clasp perfectly with clotting factors in human blood, and biotechnology companies are putting them through trials as blood thinners for surgery. Ticks can also tamper with our blood to make their drinking easier, using chemicals that not only dissolve clots but reduce inflammation and kill bacteria that try to enter a wound.
Parasites are actually a sign of an intact, unstressed ecosystem, and the opposite, as strange as it may sound, is true: if the parasites disappear from a habitat, it’s probably in trouble.
In other words, overgrazing automatically triggers an outbreak and scales back the herd, allowing the grass to recover. Soon the sheep population bounces back as well, but thanks to the management of the parasites, it never gets large enough to turn the grassland into desert.