Hair fibers were examined by atomic force microscopy, nanoindentation. By indenting along (longitudinal) and across (transversal) the fiber, we evaluated the Young's modulus and its dependence on the moisture content (relative humidity) of the environment. The ratio of the two values collected for Young's modulus, at a given relative humidity, is defined as the anisotropy index (IA) of the fiber and the acquired results give the evolution of the index of anisotropy with the relative humidity. The use of the model of composite materials allowed us to relate the anisotropy index to the fiber internal architecture. The evaluation of the results acquired on the components of the fiber, within the frame of this model, ultimately points to a possible micro-structure of exocuticle, hindered under usual circumstances by its heavy cross-linking and only noticeable when the absorbed moisture swells the surrounding network and annuls, in this way, its effect.