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Although it seems as though Joe cares for the mule because he pays five dollars to protect it, it becomes clear that he is only exploiting the mule for further self-aggrandizement. He literally uses its carcass as a platform for the "great eulogy" that he performs. Joe prevents Janie from attending the funeral, so no one is there to speak out against the mule's desecration.
Nature, in the form of buzzards, is able to articulate Janie's rage, and speak for the mule. The chief buzzard is seems like a religious figure; Hurston refers to him as the Parson. When the Parson asks what killed the mule, the other buzzard's answer "fat." The reader could interpret this reply as meaning that Joe killed the mule by freeing it because it was fed too much too suddenly.