Evidence of positive fitness for asymmetric armor phenotypes in stickleback

Subtle morphological asymmetries, common in most species, typically originate from developmental noise rather than being adaptive responses. However, diverse taxa exhibit left versus right biases in behavioural traits due to hemispheric specialization, raising the possibility that subtle morphological asymmetries have functionality. Here we provide a rare test of this hypothesis by investigating the extent to which subtle left and right departures from bilateral symmetry in the defensive apparatus of threespine stickleback (Gasterosteus aculeatus, Linnaeus, 1758) influence: (1) susceptibility to predator capture and (2) fitness of asymmetric relative to symmetric fish. Using large samples (N = 10,126), we found that left-biased asymmetrics had lower frequencies of predator-induced injuries than did symmetrics, while the latter had lower frequencies than did right-biased asymmetrics. This pattern occurred in four of five years and in each sex. Relative fitness based on ontogenetic shifts in frequencies within six cohorts showed both left and right-biased asymmetrics often had a fitness advantage over symmetrics, but this varied among years in conjunction with shifts in bird versus trout predation. The results suggest that subtle departures from symmetry can comprise an axis of functional phenotypic space maintained by fluctuating ecological and selective processes.