Sticklebacks receive some protection from predators fish through their spines. In this research, pike and perch initially snapped up, but then rejected sticklebacks. After a few experiences, the pike and perch learned to avoid the sticklebacks altogether. The scientists found that when spines were removed from sticklebacks meant their protection disappeared.
"Sticklebacks are rejected when, after being snapped up, their spines hurt the predator’s mouth"
"After very few experiences both Perch and Pike become negatively conditioned to the sight of sticklebacks and avoid them before they have made contact"
"On a number of occasions a completely swallowed stickleback was regurgitated alive after some minutes."
"Usually it was rejected immediately after it had been snapped up, and the Pike would make violent coiling movements, and would "cough" intensely several times."
"After eating, the Perch frequently gulps, gapes, or belches, and we have even seen the whole body vibrate with rapid jerks after a stickleback had been swallowed." "Sometimes a half-swallowed stickleback that has become lodged in the throat of the Perch may be dislodged by a combination of gaping movements and vigorous sideways shaking of the head."