Feeding experiments with Pike (Esox lucius L.) and Perch (Perca fluviatilis L.) as predators, and Three-spined Sticklebacks (Gasterosteus aculeatus L.), Ten-spined Sticklebacks (Pygosteus pungitius L.), Minnows (Phoxinus phoxinus (L.)), Roach (Rutilus rutilus (L.)), Rudd (Scardinius erythrophthalmus (L.)) and Crucian Carp (Carassius carassius (L.)) as prey species show that both species of stickleback enjoy a demonstrable degree of protection from these predators. This protection is much better for Gasterosteus than for Pygosteus; tests with de-spined sticklebacks show that it is mainly due to the spines. A description of the predatory behaviour of Perch and Pike is given; analysis of the experiments shows that (1) sticklebacks are rejected when, after being snapped up, their spines hurt the predator's mouth; (2) after very few experiences both Perch and Pike become negatively conditioned to the sight of sticklebacks and avoid them before they have made contact. As a result of this conditioning, the last links of the predators' feeding chain drop out; in Pike, mere fixation with two eyes was often sufficient to recognise the prey as unwanted. The possession of few and large spines in Gasterosteus must be regarded as more specialised than that of many small spines as found in various other species of stickleback. There is a correlation between the extreme development of this anti-predator device in Gasterosteus and (1) its boldness, (2) its tendency to select a more open nesting habitat, (3) the schooling and wandering tendencies of the females in the spawning season, and (4) its relatively conspicuous nuptial colours; these correlations suggest an interrelationship between anti-predator devices and reproductive behaviour and structures; the evolution of these two systems must have occurred in conjunction with each other.
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