biostudies-literatureSurapaneni VAUniversity Grants CommitteeHuman Frontier Science ProgramCity University of Hong Kong, Strategic Interdisciplinary Research78-91https://www.ebi.ac.uk/biostudies/studies/S-EPMC11829324biostudies-literatureUnknown1544(1)Among hornbill birds, the critically endangered helmeted hornbill (Rhinoplax vigil) is notable for its casque (a bulbous beak protrusion) being filled with trabeculae and fronted by a very thick keratin layer. Casque function is debated but appears central to aerial jousting, where birds (typically males) collide casques at high speeds in a mid-flight display that is audible for more than 100 m. We characterized the structural relationship between the skull and casque anatomy using X-ray microtomography and quantitative trabecular network analysis to examine how the casque sustains extreme impact. The casque comprises a keratin veneer (rhamphotheca, ∼8× thicker than beak keratin), which slots over the internal bony casque like a tight-fitting sheath. The bony casque's central cavity contaiAnnals of the New York Academy of SciencesThe helmeted hornbill casque is reinforced by a bundle of exceptionally thick, rod-like trabeculae.PMC11829324F‐CityU103/21‐Joint Research Scheme with the Consulate General of France in Hong Kong7020042CityU11102022 ‐ General Research Fund GRFG-CityU103/22-Germany/Hong Kong Joint Research Scheme with the German Academic Exchange ServiceF-CityU103/21-Joint Research Scheme with the Consulate General of France in Hong KongCityU11102022 - General Research Fund GRFG‐CityU103/22‐Germany/Hong Kong Joint Research Scheme with the German Academic Exchange ServiceRGP0010‐2020RGP0010-2020Hayat KBaum DSchindler MFlaum BSurapaneni VADean MNDoube MWolfer JHu RKong TFfalseThe helmeted hornbill casque is reinforced by a bundle of exceptionally thick, rod-like trabeculae.Among hornbill birds, the critically endangered helmeted hornbill (Rhinoplax vigil) is notable for its casque (a bulbous beak protrusion) being filled with trabeculae and fronted by a very thick keratin layer. Casque function is debated but appears central to aerial jousting, where birds (typically males) collide casques at high speeds in a mid-flight display that is audible for more than 100 m. We characterized the structural relationship between the skull and casque anatomy using X-ray microtomography and quantitative trabecular network analysis to examine how the casque sustains extreme impact. The casque comprises a keratin veneer (rhamphotheca, ∼8× thicker than beak keratin), which slots over the internal bony casque like a tight-fitting sheath. The bony casque's central cavity contai2025-01-01T00:00:00Z2025 Feb2026-05-29T15:06:32.268Z2025-04-04T02:48:05.554ZS-EPMC118293243976137310.1111/nyas.15254