Complex sentence: Because of the size and weight of their complex digestive systems, many bovids have a solid, stocky build--the more gracile species tend to have more selective diets, and be browsers rather than grazers.
Split into simpler sentences: Because of the size and weight of their complex digestive systems, many bovids have a solid, stocky build. However, the more gracile species tend to have more selective diets, and be browsers rather than grazers.