Neural correlates of seed-eating in songbirds.

Some granivorous songbirds remove the hard husk of seeds before swallowing. This dehusking behaviour is a complex sensorimotor task that requires precise coordination of the beak and tongue, and extensive tactile feedback. Here, we tested whether the evolution of dehusking behaviour is linked to changes in brainstem nuclei controlling beak-tongue movements and intra-oral somatosensory inputs. We measured the volumes of the principal sensory nucleus of the trigeminal nerve (PrV), as well as the trigeminal and facial motor nucleus, across 92 passerine species. We found that species that dehusk seeds have enlarged sensory, but not motor nuclei. This selective enlargement is best predicted by the presence of seed dehusking behaviour rather than by the percentage of seeds in the diet or beak shape. Moreover, seed dehusking species display a disproportionate expansion of the lingual representation within PrV, indicating enhanced somatosensory processing of tactile inputs from the tongue. These results suggest that the evolution of dehusking in songbirds is associated with increased sensory, rather than motor, specialization in the trigeminal system. Our findings highlight convergent principles linking dexterous food manipulation and the expansion of neural substrates for haptic feedback across vertebrates.