The heart of the hair fiber: How the inner structure of the coat reflects adaptive variation across color, sex, age, and body region in dromedary camels (Camelus dromedarius)

Understanding how hair fiber structure mediates thermal adaptation in desert animals still remains an unresolved question. In dromedary camels (<i>Camelus dromedarius</i>), the coexistence of dark and white coat colors in hot deserts presents an adaptive paradox.. This study examined how the internal architecture of camel hair fibers-specifically the degree and type of medullation-varies with coat color, sex, age, and body region to assess their potential thermoregulatory significance. Hair samples from 138 Canarian dromedaries were analyzed using an automated medullation detection system and evaluated through non-parametric tests and generalized linear mixed models (GLMMs). Dark and white camels exhibited higher proportions of medullated fibers, suggesting structural adaptations that facilitate both insulation and heat dissipation. Males and dorsal regions (hump and tail) showed greater medullation than females and ventral regions, reflecting combined physiological and mechanical demands. Although age effects were weaker, medullation tended to increase with maturity. These results demonstrate that the dromedary coat operates as a multifactorial thermoregulatory system in which pigmentation, biological traits, and fiber architecture interact to maintain homeostasis under extreme heat. The findings provide new insight into the structural mechanisms resolving the adaptive paradox of dark and white camels in hot deserts.