David P. Barash writes: 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. [Continue reading…]