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In page Macropodidae:

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Furthermore, the act of hopping in kangaroos and wallabies is associated with their breathing process. The movement of their feet off the ground helps to expel air from their lungs, while bringing their feet forward for landing replenishes their lungs with air, resulting in greater energy efficiency. Studies conducted on these animals have shown that hopping at faster speeds requires only a minimal increase in effort beyond the energy required to hop in general, which is significantly less than what would be required in other animals like horses, dogs, or humans. Additionally, it has been observed that carrying extra weight requires little additional energy, which is particularly important for female kangaroos and wallabies carrying heavy pouch young.[citation needed]