Neck muscle motoneuron anatomy in woodpeckers: An exploration of morphological specializations linked to extreme beak behavior.

The avian head and neck often work together as a singular unit, where they act as a "surrogate limb" to accomplish a variety of ecologically important tasks. Functional diversity of this system can emerge through various anatomical specializations, though details of these specializations remain underexplored. Woodpeckers offer a prime example of unique head and neck behavior, as they drill and drum on hard woody surfaces to forage, excavate nests, and communicate. Here, we investigate neuroanatomical features of the neck that may relate to woodpeckers' highly specialized behavior. Assuming that specialized behavior can be associated with neuroanatomical modifications, we use retrograde tract-tracing to map spinal motoneuron characteristics in downy woodpeckers. We focus on two neck muscles that drive head protraction (longus colli ventralis, LCv) and retraction (longus colli dorsalis, LCd), and we compare motoneuron features for these muscles to those of the scapulohumeralis, a shoulder muscle that is not implicated in drumming. We also perform cross species comparisons with motoneurons in house sparrows, a similarly sized non-drumming bird. Our results show that motor pool size, density, and location for all three muscles tested are consistent between woodpeckers and sparrows. Yet, we find that motoneuron soma size varies by muscle, such that cells innervating neck muscles are smaller on average than those innervating the shoulder, regardless of species. We conclude that specializations for woodpecker drumming and drilling are not explained by simple neuroanatomical differences. Rather, adaptations for motor control of the woodpecker head and neck likely emerge at the physiological level.