Lineages with independent evolutionary histories often differ in both their morphology and diet. Experimental work has improved our understanding of the links between the biomechanics of morphological traits and foraging performance (trait-utility). However, because the expression of foraging-relevant traits and their utility can be highly context-specific, it is often unclear how dietary divergence arises from evolved phenotypic differences. Here, we explore the phenotypic causes of dietary divergence between two genetically and phenotypically divergent lineages of threespine stickleback (Gasterosteus aculeatus) with independent evolutionary histories of freshwater colonization and adaptation. First, using individuals from a line-cross breeding design, we conducted 150 common-garden foraging trials with a community of multiple prey species and performed morphological and behavioural analyses to test for prey-specific trait-utility. Second, we tested if the traits that explain variation in foraging performance among all individuals could also explain the dietary divergence between the lineages. Overall, we found evidence for the utility of several foraging traits, but these traits did not explain the observed dietary divergence between the lineages in common garden. This work suggests that evolved dietary divergence results not only from differences in morphology but also from divergence in behaviours that underlie prey capture success in species-rich prey communities.

We conducted a common-garden feeding experiment, in order to understand the functional basis of dietary divergence between two divergent lineages of European stickleback (Gasterosteus aculeatus). In the experiment, 150 lab-raised individuals, derived from a line-cross breeding design, foraged freely on a diverse prey community. The feeding trials were video recorded and analyzed using the event-logging software BORIS to obtain data on the foraging behavior (number and targets of feeding strikes, non-foraging behaviors). After the conclusion of the feeding trials, fish were sacrificed and dissected for gut content analyses. We counted, identified and weighed the prey items contained in the guts. Finally, we used the software phenopype to measure the functional morphology.