If gut bacteria can sway their hosts to be selfless, it could answer a riddle that goes back to Darwin. Evolutionary theorists have puzzled over such altruistic behaviors, particularly among unrelated creatures, because selfishness often seems like a better survival strategy in the context of natural selection. A new theory suggests that parasites might tip the odds in favor of host altruism for their own gain.
Parasites are among nature’s most skillful manipulators — and one of their specialties is making hosts perform reckless acts of irrational self-harm. There’s Toxoplasma gondii, which drives mice to seek out cats eager to eat them, and the liver fluke Dicrocoelium dendriticum, which motivates ants to climb blades of grass, exposing them to cows and sheep hungry for a snack. There’s Spinochordodes tellinii, the hairworm that compels crickets to drown themselves so the worm can access the water it needs to breed. The hosts’ self-sacrifice gains them nothing but serves the parasites’ hidden agenda, enabling them to complete their own life cycle.