Tit for Tat: sticklebacks (Gasterosteus aculeatus) "trusting" a cooperating partner

Individual three-spined sticklebacks (Gasterosteus aculeatus) moved closer to a predatory trout when a "cooperator" stickleback, which the test fish could see through a one-way mirror, swam up to the predator than when a "defector" stickleback appeared to swim only half as close to the predator. After four training runs with both types of partners, the former cooperator. also defected. The test fish continued to move closer to the predator in the presence of the former cooperator even though both the former cooperator and the defector now appeared to stop in their approach to the predator at the same distance. This shows that probable partners build up trust. [Behav Ecol 1990;1:7-11]