POPULATION AND SEX-DIFFERENCES IN ANTIPREDATOR RESPONSES OF BREEDING FATHEAD MINNOWS (PIMEPHALES-PROMELAS) TO CHEMICAL STIMULI FROM GARTER SNAKES (THAMNOPHIS-RADIX AND T-SIRTALIS)

We conducted a predator bite survey on a population of fathead minnows (Pimephales promelas) considered to be under substantial predation pr essure by western plains garter snakes (Thamnophis radix). Scarring, d ue to failed predation attempts by garter snakes and crayfish (Orconec tes virilis), was observed significantly more often in breeding males than in breeding females and nonbreeding minnows. Likely, territorial nest defense under the edges of rocks along the water's edge, a habita t occupied by crayfish and frequented by snakes, caused the breeding m ales to be differentially vulnerable to predation. Under controlled la boratory conditions, breeding males from this population exhibited an antipredator response to chemical stimuli from live snakes (T. sirtali s and T. radix) significantly more often than breeding female minnows from the same population and breeding minnows of both sexes from a pop ulation that was presumed to be under lower predation pressure from sn akes.