HA Peru Results for external and internal morphological variation among the four samples are summarized in Figure 2. ANOVA and Tukey post-hoc tests on size-standardized variables were conducted in SPSS 8 to identify significant differences. Results indicate that external craniofacial morphology and nasal cavity size are overall similar among groups; however, Tibetans display significantly larger maxillary sinuses than all groups (Fig 3; Table 3). Reduced major axis (RMA) regressions on distances were conducted in PAST 9 to investigate size relationships. Results indicate regional and/or altitudinal differences between nasal and maxillary sinus morphology (Table 4). Specifically, Tibetans have relatively narrower nasal breadths for nasal height and relatively larger maxillary sinuses for a given craniofacial size (Fig 4). Weak correlation coefficients suggest craniofacial size only partially explains these differences. Tibetans are characterized by enlarged maxillary sinuses and relatively narrow nasal cavities, which likely reflect nasal adaptations to their cold-dry environments. Previous studies show Tibetans also exhibit high levels of nitric oxide 11 . Produced in the sinuses and inhaled into the lungs, nitric oxide acts as an aerocrine messenger to facilitate oxygen delivery 12 . We suggest that relatively larger sinuses of Tibetans may relate to increased nitric oxide storage and/or production as an adaptation to hypoxic environments. Studies increasing current sample sizes and geographic regions are needed to support this hypothesis. High-altitude (HA) populations (>2500m) face increased respiratory pressures due to exposure to colder temperatures, lower humidity, and hypoxia. Theoretically, adaptations in upper respiratory structures could help alleviate some pressures: tall/wide nasal cavities would augment oxygen uptake 1 ; tall/narrow nasal cavities would help in air conditioning processes 2 ; and/or larger sinuses could increase nitric oxide production or storage to facilitate oxygen delivery 3 . However, previous research on high-altitude craniofacial morphology is limited. Using linear dimensions, this initial study explores whether two high-altitude samples exhibit different nasal and/or sinus morphologies compared to their lowland counterparts. Internal craniofacial morphology of high-altitude Tibetans may reflect unique adaptations to hypoxic environments. BACKGROUND Lauren N. Butaric & Ross Klocke, College of Osteopathic Medicine, Des Moines University STATISICAL ANALYSES & RESULTS CONCLUSION FIGURE 2. Summary of results with cut-away 3D models for sampled crania (to scale). See specific results below. HA Asia LA Peru • Tall nasal cavities • Relatively narrow nasal breadth • Large sinuses (height & breadth) • Short nasal cavities • Relatively wide nasal breadth • Smaller sinuses (height & breadth) • Short nasal cavities • Relatively wide nasal breadth • Smaller sinuses (height & breadth) • Short nasal cavities • Relatively wide nasal breadth • ~Intermediate sinuses LA Asia NHM Natural History Museum, London; PBD Peabody Museum, Cambridge, MA; SI Smithsonian Institution, Washington DC. *Crania without signs of artificial cranial modification were carefully selected. FIGURE 4. Bivariate plots with RMA slopes. Note HA-Asians primarily fall above regression lines. Also see Table 4. TABLE 2. Measurements calculated in current study. External Midface Internal Nose Maxillary Sinuses Bimaxillary breadth Upper facial height (na-pr) Facial length (pr-sta) Nasal breadth Nasal height Choanal breadth Choanal height Ethmoidal breadth Craniofacial size (Geo. Mean) Internal nasal breadth Internal nasal height Internal nasal length Nasal floor length Nasal cavity volume Taken on right & left sinuses, then averaged Medial-lateral breadth Inferior-superior height Anterior-posterior length Maxillary sinus volume Distance measures (Table 2) were calculated from selected XYZ coordinates of landmarks shown in Figure 1. All linear distances were size-standardized by the geometric mean of craniofacial size. HA-Tibetans LA-Chinese HA-Peru LA-Peru RMA slope: 7.477 R = 0.520; P < 0.0001 Log (raw) Maxillary Sinus Volume Log (raw) Craniofacial Size (GM) STATS HA Asia HA Asia HA Peru HA Peru LA Asia LA Asia LA Peru LA Peru HA Asia HA Peru LA Asia LA Peru 50.00 55.00 60.00 65.00 70.00 75.00 60.00 80.00 100.00 120.00 140.00 160.00 5.00 15.00 25.00 35.00 45.00 55.00 FIGURE 3. Boxplots illustrating size variables for craniofacial size (left), nasal cavity (middle), and maxillary sinus (right) volumes (ml). P = 0.064 (Not Sig) P = 0.12 (Not Sig) P = 0.006 (Sig) TABLE 3. ANOVA & Tukey post-hoc results for significant variables. Variable F Ratio P-value Tukey Post-hoc Results External Midface Dimensions Bimaxillary breadth 3.711 0.015 LA Peru > LA Asia Facial length 11.054 0.000 LA Peru > LA Asia, HA Tibet, HA Peru Internal Nasal Dimensions Internal nasal height 3.321 0.024 HA Asia > LA Asia Internal nasal length 10.099 0.000 LA Peru > LA Asia, HA Asia, HA Peru Nasal floor length 8.363 0.000 LA Peru > LA Asia, HA Peru Maxillary Sinus Dimensions Maxillary sinus breadth 3.027 0.034 Ha Asia > LA Peru Maxillary sinus height 4.303 0.007 HA Asia > LA Asia, LA Peru, HA Peru Maxillary sinus volume 4.458 0.006 HA Asia > LA Asia, LA Peru TABLE 1. Samples used in current study. Total n = 86, male crania. High Altitude Low Altitude HA-Asia: (n=13) Tibetans NHM LA-Asia: (n=24) Southern Chinese SI HA-Peru: (n=25) Yauyos PBD *LA-Peru: (n=24) primarily Pachacamac SI RMA slope: 0.655 R = 0.305; P = 0.004 Log Internal Nasal Height Log Internal Nasal Breadth TABLE 4. RMA results for internal structures regressed against craniofacial size. Variables Regressed Slope R P-value 95% Slope CI Scaling* Internal nasal breadth 1.845 0.140 0.197 (1.325 - 5.827) - Internal nasal height 1.441 0.489 <0.0001 (1.123 - 1.693) + Allometry Internal nasal length 1.002 0.386 0.000 (0.789 - 1.194) Isometry Nasal cavity volume 2.885 0.468 <0.0001 (2.284 - 3.351) Isometry Maxillary sinus breadth 3.651 0.349 0.001 (2.849 - 5.312) + Allometry Maxillary sinus height 3.204 0.568 <0.0001 (2.598 - 4.14) + Allometry Maxillary sinus length 2.131 0.391 0.000 (1.699 - 2.788) + Allometry Maxillary sinus volume 7.477 0.520 <0.0001 (5.695 - 8.972) + Allometry *Scaling patterns based on expected slopes & 95% confidence intervals 10 ; when regressed against the geometric mean for craniofacial size, expected slopes for linear dimensions = 1 .0, for volumes = 3.0. We thank the museum curators & CT technicians who assisted in obtaining these scans: R Kruszynski at NHM, B Frohlich & D Hunt at SI, & Olivia Herschensohn at Peabody Museum, Harvard University. Funding was provided by Texas A&M’s Vision 2020 Dissertation Improvement Grant; Texas Academy of Science Graduate Student Research Award; & DMU’s IOER grant. We also thank S Athreya, L Wright, DL Carlson, DS Carlson, S Maddux, T Rae, K Kyukendall, A Evteev, C Nicholas, T Yokley, N Holton, & M Bastir who have all contributed interesting discussions on this and related projects. Landmark placement primarily follows Butaric and Maddux 4,5 . 1. Holton N, Yokley T, Froehle A, Southard T. 2014. Amer J Phys Anthropol 153:52-60. 2. Holton N, Yokley T, Butaric L. 2013. Anat Rec 296:414-426. 3. Andersson JA, Cervin A, Lindber S et al. 2002. Acta Oto-Laryng 122:861-865. 4. Butaric L, Maddux S. 2016. Amer J Phys Anthropol 160:483-497. 5. Maddux S, Butaric L. 2017. Anat Rec 300:209-225. 6. Stalling D, et al.. 2005. The Visualization Handbook p 749-767. 7. Sfedorov A, Beichel R, Kalpathy-Cramer et al., 2012. PMID: 22770690 8. IBM Corp. Released 2013. Armonk, NY: IBM Corp. 9. Hammer O, Harper DAT, Ryan PD. 2001. Palaeontol Elect 4(1):9pp. 10. Butaric LN. 2015. Anat Rec 298:1710-1721. 11. Beall CM, Laskowski D, Strohl KP et al. 2001. Nature 414: 411-412. 12. Lundberg JO. 2008. Anat Rec 291: 1479-1484. FIGURE 1. Screen shots from 3D Slicer 4.4 6 for external (left) and internal (right) landmark placement; crania were digitally rendered from CT scans in Amira 7 . HA-Tibetans LA-Chinese HA-Peru LA-Peru