Abstract: In June 1994, we observed a herd of killer whales (Orcinus orca) attack a school of pantropical spotted dolphins (Stenella attenuata) in the Gulf of Mexico. The killer whales cut out up to three dolphins from the school, then proceeded to take turns chasing a single dolphin and keeping it within a confined area for 1.5 h. They could have killed the dolphin at any time, but apparently chose to prolong the encounter instead. An adult female killer whale appeared to use the opportunity as a training session for her calf. The single adult male present did not participate until the end of the encounter. He made several loud percussions by slapping his flippers, dorsal fin, and flukes against the water surface, then he swam to the dolphin and quickly killed it. Neither the role of the adult male killer whale in cooperative feeding situations nor the significance of the marked sexual dimorphism in this species has ever been adequately explained. We suggest that size differences between the sexes may have evolved as an 'ecological sex trait', allowing groups of related individuals to take a wider diversity of prey sizes.
Key Words: KILLER WHALE; ORCINUS ORCA; PANTROPICAL; SPOTTED DOLPHIN; STENELLA ATTENUATA; PREDATION; SEXUAL DIMORPHISM
Document Type: Research article