The evolution of the form and function of the head of snakes

The vertebrate head serves a diversity of functions, from energy intake to interactions with the biotic and abiotic environment. The loss of limbs in snakes placed additional selective pressures on the head as it must compensate for the functions that were fulfilled by the limbs such as manipulation, locomotion, or defense against other organisms. Rather than limiting snake diversification, selective pressures inherent to specific functions of the head allowed them to innovate, permitting the exploration of new anatomical and behavioral strategies. This chapter aims to highlight the richness in form, function, and adaptations of the head of snakes from an integrative and comparative perspective, in the light of their evolution and ecology. First, we discuss the defining features of the snake skull, followed by a review of what fossils can tell us about how snakes acquired their unique skull anatomy and the ecological origins of snakes. Next, we explore the macroevolution of skull diversity in extant snake species in the light of their ecology. From a functional perspective, we review data on the morphology and biomechanics of the head. We will then explore the relationships between external, osteological, and endocranial morphology and the sensory ecology of snakes. Finally, we will examine novel or unexplained shapes and behaviors involving the head and propose questions for future investigations.