The vestibular system of the inner ear detects themotions of the head and is involved in maintaining balance.For this reason, this organ has been deeply studied and severalscientists have tried to link its morphology with the locomotorbehavior of an animal. Via high-resolution computedmicrotomography and geometric morphometric methods, weanalyzed the intraspecific variation of the 3D morphology ofthe bony labyrinth (inner ear) in four species of primates differingin their locomotor adaptations: two being slow-movingtaxa (Nycticebus and Perodicticus), and two being fastmovingtaxa (Callithrix and Microcebus). Basically, thereare very few analyses of the inter-individual variation of thisorgan in mammals in general, and this approach has neverbeen attempted in primates thus far. Our results show thatvariation of the bony labyrinth morphology is expressed bythe same ways in the different species (e.g., differences in thesize, shape, and orientation of the semicircular canals, and inthe width and height of the cochlea), but that slow-movingtaxa exhibit a higher amount of intraspecific variation thando fast-moving taxa. Our results strengthen support for a previouslypublished hypothesis, according to which a relaxationof the selective pressure applied to the morphology of thebony labyrinth is the likely reason for this higher amount ofintraspecific variation in slow-moving taxa, and that it may berelated to a reduced functional demand for rapid posturaladjustments.