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Why do some marsupial species lack a fully developed pouch?

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Red kangaroo (Macropus rufus) — marsupial Red kangaroo (*Macropus rufus*). The home range of this species spans much of Australia's interior, and it is the largest member of the family Macropodidae.

While the marsupium—or pouch—is a defining characteristic for which marsupials are named, it is not a universal feature across all species.

The structure of the marsupium varies significantly among the order. In many species, such as kangaroos, it is a well-developed pocket, while in others, it may exist only as a simple fold of skin or mere remnants surrounding the nipples. A few species, such as the South American caenolestids (rat opossums), lack a marsupium entirely.123

In species that lack a fully developed pouch, the newborn young simply cling directly to the mother's teats for protection and nourishment. Regardless of whether a pouch is present, all marsupials share the reproductive strategy of giving birth to young in a vulnerable, embryonic state, which then must remain attached to the mother's nipples for a considerable period to complete their development.21