Reuben Kaufman and graduate student Brian Weiss have found that for females, the secret to staying slim is staying virginal. Fortunately for males, human and otherwise, this works only if you're a special type of tick. Kaufman and Weiss have isolated the engorgement factor protein (EF) in the semen of the tick family ixodidae; this protein inspires gluttony in inseminated females. They have dubbed it "voraxin," from the Latin vorare, to devour.

Tick blood lust is ghastly: Some can consume up to 4,000 ml of blood. Nearly 120,000 ticks were once found on one unfortunate Canadian moose. (That species, the winter tick, hates human blood.)

EF was first postulated in 1972 when researchers irradiated male ticks and proved that even in the nonpotent ones, something was causing the females to engorge. It took modern molecular biology techniques to find out what that was. "The only thing they could have...

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