The contributions of thermoelastic coupling and fluid motions to viscoelastic response in bone are calculated, using the theories of Zener and Rusch, respectively. Fluid motion and homogeneous thermoelasticity produce mechanical losses which are size dependent. Inhomogeneous thermoelastic coupling results in thermal currents between regions of different stiffness, such as osteons and lamellae, and around voids. Losses of this type display no size effect. Fluid motion, homogeneous thermoelasticity, and inhomogeneous thermoelasticity resulting from stiffness variations, yield losses which are significant enough to be measurable. An expression is derived for the contribution of piezoelectric-like coupling to the mechanical loss. This loss is calculated from published data, and for dry bone it is negligibly small. The role of inhomogeneous deformation and molecular modes in collagen as viscoelastic mechanisms is considered and experimental evidence is discussed.