Background: Paranasal sinuses and mastoid air cells have been attributed to multiple functions—such as voice resonance, cranial lightening, and pressure regulation—yet their potential role in local thermal homeostasis remains underappreciated. The thermoregulatory hypothesis, first proposed in the mid-twentieth century, was largely abandoned after the mid-century, when anthropological findings of climate-correlated variation seemed contradictory. Hypothesis: We propose that pneumatized skull regions form a three-component craniofacial biothermal system that maintains thermal stability in the ocular vitreous and vestibular endolymph, two avascular, temperature-sensitive structures that lack intrinsic thermoregulatory capacity. This represents a novel integration that explicitly links paranasal and mastoid pneumatization into a coordinated system that protects sensory organs, distinct from previous brain-cooling hypotheses. Mechanism: The system comprises: (1) passive thermal insulation via air spaces, providing ~15-fold greater thermal resistance than bone; (2) active cold protection via mucosal heat delivery (estimated 2–5 W capacity); and (3) active heat dissipation via evaporative cooling (estimated 0.3–0.5 W capacity). This architecture provides asymmetric protection, with cold buffering exceeding heat dissipation by approximately 5- to 15-fold, consistent with thermodynamic constraints and putative evolutionary priorities. Evidence: Preliminary observations consistent with this hypothesis include the anatomical proximity of pneumatized regions to the vitreous and labyrinth, intranasal selective brain cooling studies, and clinical observations after mastoidectomy showing preserved pressure buffering but reduced vestibular thermal insulation under extreme stimulation. Climate-correlated pneumatization patterns are consistent with bidirectional thermal adaptation. Implications: We present five falsifiable predictions that can be tested with thermographic imaging, pharmacological manipulation, and computational modeling. Validation could inform surgical planning, explain postoperative thermal-sensitivity symptoms, and provide evolutionary insights into craniofacial adaptation.
Avraham et al. (Sun,) studied this question.