Rattling motions in Type I clathrates (Sr8Ga16Ge 30 and Ba8Ga16Ge30) are studied by heat capacity measurements. Using several models, the lattice contributions to the heat capacity are shown to be different among the alkaline-earth elements. The contributions of the anharmonic motions of the endohedral atoms to the heat capacity are found to be larger in Sr8Ga16Ge30 than those in Ba8Ga16Ge30. The experimental data for Ba8Ga16Ge30 are fitted to standard Debye and Einstein models. Extracted charge carrier mean free paths indicate that conduction electrons are not strongly scattered by endohedral atom vibrations while heat transfer in Sr8Ga16Ge30 is greatly reduced.