Parasite Rex by Carl Zimmer, Free Press, $26, ISBN 0684856387
PARASITES have driven the evolution of complex life, perhaps leading to the
evolution of sex鈥攁nd thus to the all-singing, all-dancing displays of
courtship. We and all Earth鈥檚 ecosystems are probably partly dependent on
parasites for our health.
Contrast this view with those of Victorian biologists. They thought of
evolution as a progressive process, ascending from lower forms to humans.
Parasites seemed to arrest this process, even leading to degeneration. In
Parasite Rex, Carl Zimmer quotes one Henry Drummond, according to whom
parasitism 鈥渋s one of the great crimes in nature. It is a breach of the law of
别惫辞濒耻迟颈辞苍鈥�.
Advertisement
This is the grand theme in Zimmer鈥檚 book, subtitled The Secret and
Bizarre World of Nature鈥檚 Most Dangerous Lifeforms. Over a century, our
understanding of parasites has turned almost full circle. Nowadays we consider
them to be as exquisitely well adapted as any other form of life. We may not
always like having them around, but too few parasites in us may lead to asthma
and autoimmune disease. 鈥淧erhaps some day,鈥� Zimmer suggests, 鈥渁long with polio
vaccines children will get parasite proteins, so that their immune systems will
be trained not to fly out of control.鈥�
And it isn鈥檛 just people that need be concerned. If there are too few
parasites in an ecosystem it may lead to instability and monoculture. Zimmer
illustrates this with superb stories about biological control. He describes, for
instance, how the introduction of a parasitic wasp from Paraguay has saved the
cassava crops of subSaharan Africa from the cassava mealybug. It is biological
control on an almost military-industrial scale. The wasps are factory-bred
at the rate of 150 000 per month, anaesthetised, crammed into cylinders, loaded
in aircraft, and then dropped like bombs on cassava plantations. It seems to
work.
Parasite Rex鈥檚 main appeal lies, though, not in its argument with
Victorian biology, but with stories about the parasites themselves. I鈥檇 suggest
that Victorian ideas about progress are one of the least influential ideologies
of our time. These stories may be lacking narrative unity but they are a
non-stop delight.
Zimmer is a colourful writer, and takes full advantage of the macabre natural
history of parasites. Some parasites 鈥渂urst out of caterpillars as if their
hosts were great birthday cakes.鈥� Caterpillars, however, have developed a
cunning trick to defend themselves. Parasitic wasps are thought to track down
caterpillars by their droppings. The trick then, is to keep a safe distance
between yourself and your excrement. Some caterpillars have evolved what Zimmer
calls 鈥渁n anal cannon鈥�, the mechanism of which鈥攊nvolving a hinged plate
and a build-up of anal blood pressure鈥攈as recently been puzzled out. The
result is that the caterpillar blasts out its droppings at high velocity 鈥渋n a
soaring arc that carries them up to two feet away鈥�.
Zimmer has visited many of the scientists whose work he writes about. Many
experts insist that parasites are beautiful.
One visit he describes was to the National Parasite Collection at the
Agricultural Research Service in Beltsville, Maryland. He was shown round by
parasitologist Eric Hoberg. 鈥淎t one point,鈥� Zimmer says, 鈥渨hen Hoberg was
showing me a nematode鈥攖hick as a finger, long as a riding crop, the colour
of blood鈥攚hich was curled up inside a fox鈥檚 kidney, I couldn鈥檛 help
myself. I said, `Gross.鈥� . . . Hoberg was disappointed: `I get irritated by the
yuck factor,鈥� he said. `What鈥檚 being missed is how incredibly interesting these
补谤别鈥�.鈥�
But later Zimmer enjoys a small revenge. They come to a jar containing
horse鈥檚 stomach, studded with botfly larvae. This time, 鈥淚 was fascinated, but
Hoberg flinched. `That鈥檚 one thing I can do without.鈥� I was glad to see that
even a parasitologist has his limits,鈥� says Zimmer.
Some of the best bits are the examples of parasites manipulating their hosts.
For example, a worm that lives in mayflies needs to reach water for the next
stage of its life. These worms are well set up if they inhabit a female mayfly,
because female mayflies end their lives by laying their eggs in water. But males
drop dead in the grass after mating, and the worms in them have a bit of a
problem. The solution is to feminise the male mayfly. This makes the male 鈥渄rop
down to the stream, even going so far as to try to lay imaginary eggs as the
parasite bursts out of its body鈥�.
Stories of this kind are not just hypotheses (or just-so stories, in the
language of the scoffers). Zimmer stresses this by describing the carefully
controlled experiments of Janice Moore at Colorado State University, on worms
that parasitise woodlice. Yet another kind of parasitic worm, or fluke, lives in
killifish. As the flukes develop, they need to get inside the waterbirds that
feed on the fish. The flukes modify the fish鈥檚 behaviour, making them dance
about at the water surface. The consequence is that it is now thirty times
easier for a bird to catch its dinner.
This raises a new thought. Zimmer quotes the ecologist Armand Kuris: 鈥淲hat
blew me away was the conservative estimate that they increased the
susceptibility to predation by thirty times. Thirty times. So now I
step back, and I look at the birds flitting around out there and think, could we
have those birds out there if it were thirty times harder for them to get their
food?鈥� Anyone who admires waterbirds on salt marshes鈥攁nd perhaps anyone
who admires birds at all鈥攕hould be grateful to parasitic worms.
Zimmer鈥檚 section on host manipulation includes a great mix of anecdotes about
individual parasites, rigorous science, amazing large-scale ecological
conjectures and light relief at the movies. For some reason, Hollywood has
concentrated on alien parasites that are transmitted sexually,and turn their
hosts into sex maniacs.
Zimmer represents a healthy trend in science writing. In contrast to many
writers, he popularises science on its own terms rather than twisting it into
one of the clich茅s of the marketplace. There are no apocalyptic,
sensational or quasi-religious themes in Zimmer鈥檚 writing; the science can be
enjoyed鈥攁s Zimmer clearly does鈥攆or what it is.