By David P. Barash, The New York Times, October 6, 2012
That’s right: zombie bees. First reported in California in 2008, these
stranger-than-fiction creatures have spread to North Dakota and, just recently,
to my home in Washington State.
Of course, they’re not really zombies, although they act disquietingly
like them, showing abnormal behavior like flying at night (almost unheard-of in
healthy bees), moving erratically and then dying. These “zombees” are victims
of a parasitic fly, Apocephalus borealis. The fly lays eggs within honeybees,
inducing their hosts to make a nocturnal “flight of the living dead,” after
which the larval flies emerge, having consumed the bee from the inside out.
These events, although bizarre, aren’t all that unusual in the animal
world. Many fly and wasp species lay their eggs inside hosts. What is
especially interesting, and a bit more unusual, is the way an internal parasite
not only feeds on its host, but also frequently alters its behavior, in a way
that favors the continued survival and reproduction of the parasite.
Not all internal parasites kill their hosts, of course: pretty much
every multicellular animal is home to numerous fellow travelers, each of which
has its own agenda, which in some cases involves influencing, or taking control
of, part or all of the body in which they temporarily reside.
And this, in turn, leads to the question: who’s in charge of your own
mind? Think of the morgue scene in the movie “Men in Black,” when a human
corpse is revealed to be a robot, its skull inhabited by a little green man
from outer space. Science fiction, but less bizarre than you might expect, or
want to believe.
Providing room and board to other life-forms doesn’t only compromise
one’s nutritional status (not to mention peace of mind), it often reduces
freedom of action, too. The technical phrase is “host manipulation.”
Take the tapeworm Echinococcus multilocularis, which causes its mouse
host to become obese and sluggish, making it easy pickings for predators,
notably foxes, which — not coincidentally — provide an optimal environment for
the tapeworm to move into the next phase in its life cycle.
Sometimes the process is truly strange. For example, a kind of fluke
known as Dicrocoelium dentriticum does time inside a snail, then an ant,
followed by a sheep. Ensconced within an ant, some of the resourceful worms
migrate to their host’s brain, where they manage to rewire its neurons,
essentially hijacking its body.
The manipulated ant, in response to Dicrocoelium’s demands, then
climbs to the top of a blade of grass and waits patiently and conspicuously
until it is consumed by a grazing sheep. Once in its desired happy breeding
ground, the worm releases its eggs, which depart with a healthy helping of
sheep poop, only to be consumed once more by snails, which eventually excrete
the immature worms for another generation of unlucky ants to consume.
It may be distressing to those committed to “autonomy,” but such
manipulators have inherited the earth. Including us.
Take coughing, or sneezing. It may be beneficial for an infected
person to cough up or sneeze out some of her tiny organismic invaders, although
it isn’t so healthful for others nearby. But what if coughing and sneezing
aren’t merely symptoms but also, even primarily, a manipulation of us, the
“host,” by influenza viruses? Shades of zombie bees, fattened mice and
grass-blade-besotted ants.
Just as Lenin urged us to ask “who, whom?” with regard to social
interactions — who benefits at the expense of whom? — the new science of
evolutionary medicine urges a similar question: who benefits when people show
symptoms of a disease? Often, it’s the critters that are causing the disease in
the first place.
But what about the daily, undiseased lives most of us experience?
Voluntary actions are, we like to insist, ours and ours alone, not for the
benefit of some parasitic or pathogenic occupying army. When we fall in love,
we do so for ourselves, not at the behest of a romance-addled tapeworm. When we
help a friend, we aren’t being manipulated by an altruistic bacterium. If we
eat when hungry, sleep when tired, scratch an itch or write a poem, we aren’t
knuckling under to the vices of our viruses.
But it isn’t that simple.
Think about having a child, and ask who — or rather, what — benefits
from reproduction? It’s the genes. As modern biologists recognize, babies are
our genes’ way of projecting themselves into the future.
Unlike the cases of parasites or pathogens, when genes manipulate
“their” bodies, the situation seems less dire, if only because instead of
foreign occupation it’s our genes, our selves. But those presumably personal
genes aren’t any more hesitant about manipulating our bodies, and by extension
our actions, than is a parasitic fly hijacking a honeybee.
Here, then, is heresy: maybe there is no one in charge — no
independent, self-serving, order-issuing homunculus. Buddhists note that our
skin doesn’t separate us from the environment, but joins us, just as biologists
know that “we” are manipulated by, no less than manipulators of, the rest of
life. Who is left after “you” are separated from your genes? Where does the
rest of the world end, and each of us begin?
Let’s leave the last words to a modern icon of organic, oceanic
wisdom: SpongeBob SquarePants. Mr. SquarePants, a cheerful, talkative —
although admittedly, somewhat cartoonish — fellow of the phylum Porifera,
“lives in a pineapple under the sea... Absorbent and yellow and porous is he.”
I don’t know about the pineapple or the yellow, but absorbent and porous are
we, too.
David P. Barash is an
evolutionary biologist and professor of psychology at
the University of Washington and the author, most recently, of “Homo
Mysterious: Evolutionary Puzzles of Human Nature.”
2 comments:
A member of a society where conscious self-interest and manipulation is the norm projects those traits onto nature. A member of a more communal society might, with the same very facts, decide simply that after all, everything is related to everything. The bacteria in our gut, even those just visiting, are as much part of the "I am" as are our neurons, or other big and minute parts and secretions of our body.
A propos, http://davidhaskell.wordpress.com/2012/10/21/ants/
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