The Aresan Clan is published four times a week (Tue, Wed, Fri, Sun). You can see what's been written so far collected here. All posts will be posted under the Aresan Clan label. For summaries of the events so far, visit here. See my previous serial Vampire Wares collected here.

Sunday, July 31, 2011


I just finished reading Parasite Rex by Carl Zimmer, a fascinating book about parasites. It's only been fairly recently that there's been real serious study of parasites in biology. Add that to the inherent difficulty of studying parasites (since they live inside other living things) and we discover that we really don't know a lot about parasites overall.

What we do know, though, is very surprising. For one, there are tons of species of parasites. An estimated 75% of all species are parasites. All free living animals have to deal with parasites, and many animals have to deal with multiple parasites, if not whole arrays of parasites. We should mention that there's no reason to distinguish an infectious disease from a parasite: cold viruses and malaria protozoa are just as much parasites living at your expense as a tape worm or hook worm. Secondly, many parasites have very sophisticated life cycles, not uncommonly passing through two or three hosts over the course of a life and perhaps equally as many life stages (such as the way a butterfly goes through the stages of caterpillar, chrysalis, butterfly). There's a parasite, for example, the lancet liver fluke (Dicrocoelium dendriticum) that lives in sheep, but is passed to snails (through the cow dung) to ants (through a slime the snail secretes) back to cows (when the cows accidentally eat the ants while eating grass). Thirdly, many parasites are very good at manipulating their host for personal gain. This may be as simple as the cold virus causing us to sneeze (in the hope that we'll sneeze out the cold virus and it'll land on a new host) to parasites that change animal behavior or color to make them more vulnerable to prey. It'd be like if you were infested with a parasite that's life-cycle depended on being passed on to bears, and to further this purpose it gave infected persons a sudden urge to start a fistfight with a grizzly bear. Or (perhaps more realistically) it'd be as if a venereal disease, in order to spread itself more rapidly, made you much more horny and uninhibited in propositioning people for sex (some think Herpes might even do something like that, see Survival of the Sickest, pp 113-14). In the case of the lancet liver fluke that goes from sheep, to snails to ants, it causes infected ants to climb up to the top of a blade of grass and wait there all night (in the hope that the ant will be eaten by a cow that's feeding on grass).

There's an interesting theory out there that parasites are the reason for the development of sexual reproduction. As nice as sex is, it's not entirely obvious, from an evolutionary perspective, why it's advantageous. Without sexual reproduction, living things can reproduce more quickly and abundantly and without all the time and resources devoted to mating. Sexual reproduction has its advantages, but the sheer abundance of asexual species, such as bacteria, suggest that asexual reproduction might be better. On theory about how sexual reproduction evolved came about when a researcher, Curtis Lively, found a New Zealand snail, Potamopyrgus antipodarum, that was able to switch between sexual and asexual reproduction. He found that the snails that were more afflicted with parasites were using sexual reproduction, whereas the ones that were mostly parasite free, used asexual reproduction. Sexual reproduction allows the snails to change more rapidly, producing children that are more dissimilar to their parents and thus less likely to be afflicted by parasites. It may not necessarily be the case parasites are what pushed species in the past to adopt sexual reproduction; but it certainly is the case with these snails and may be the case with many other species.

Much of this shows that, our traditional hierarchies of food chains aren't entirely accurate. Preying on the predators at the top of the food chain are any number of parasites, which might be as ecologically important for culling the herd (in this case, reducing the number of predators) as the predators are for reducing overgrazing by herbivores they prey on. It's always interesting how even us humans, masters of the planet, are brought low by the ravages of disease and parasites.

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