The adaptive significance of dorsal spine variation in the fourspine stickleback, Apeltes quadracus. III. correlated traits and experimental evidence on predation

Summary Apeltes quadracus is polymorphic for the number of dorsal spines, the variation is heritable, and it is subject to natural selection. Here we investigate selective predation favouring the higher-spined morphs. We predict that populations with greater mean spine number will have longer spines, and we so find. However, the morphs do not differ for spine length within populations, and we conclude that spine number and spine length are selected independently. Populations with greater mean spine number also have longer and deeper bodies, thus having a greater defensive circumference. Again, the morphs do not differ within populations. Experimental tests demonstrate selective predation on the morphs for four fish species in the laboratory, and for a fish and a bird species in the field. However the experiments also show that: the higher-spined morphs are not always favoured vegetation plays a role in selective predation factors other than spine number are involved these factors operate before Apeltes is captured (behavioural/ecological) the factors are correlated with spine number. We conclude that predators are selective agents on the polymorphism, but that the relationship between fitness of the morphs and predators is complex.