Testing the influence of crushing-surface variation on seed cracking performance between beak morphs of the African seedcracker Pyrenestes ostrinus [RESEARCH ARTICLE]

Nicola S. Heckeberg, Philip S. L. Anderson, and Emily J. Rayfield Extreme phenotypic polymorphism is an oft-cited example of evolutionary theory in practise. Although these morphological variations are assumed to be adaptive, few studies have biomechanically tested such hypotheses. Pyrenestes ostrinus (the African seedcracker finch) shows an intraspecific polymorphism in beak size and shape that is entirely diet driven and allelically determined. Three distinct morphs feed upon soft sedge seeds during times of abundance, but switch to specializing on three different species of sedge seeds that differ significantly in hardness during lean times. Here we test the hypothesis that beak morphology is directly related to consuming seeds of different hardness. We used a novel experimental analysis to test how beak morphology affects the efficiency of cracking sedge seeds of variable hardness. We found that neither mandibular ramus width nor crushing surface morphology had significant effects on the ability to crack different seed types. It is likely that feeding performance is correlated with other aspects of beak size and shape such as beak depth and strength, muscle force, or gape. Our results highlight how even seemingly straightforward examples of adaptive selection in nature can be complex in practice.
Source: Journal of Experimental Biology - Category: Biology Authors: Tags: RESEARCH ARTICLE Source Type: research