In a flash, schools of male longfin squid can turn from peaceful gatherings to violent mobs. One minute, individuals are swimming together in peace; the next, they’re attacking one another. The males give chase, ramming each other in the sides and grappling with their tentacles.

These sudden bouts of violence are the doing of the female squid. Males are attracted to the sight of eggs, and females lace the eggs with a chemical that transforms the males into aggressive brutes. Yesterday, I wrote about scientists who could instigate aggression in mice with pulses of light. The longfin squid do the same thing with a rage-inducing chemical

Hanlon found that males are attracted to the sight of the mops – they’re a