This hypothetical marsupial species raises young in a manner loosely similar to seahorses. Only the males have pouches: after gestation the female deposits the fetus into his pouch to compete development. Is this adaptation competitive?
EDIT: Here are some revisions to address problems brought up by answers:
- Nutrition: The male's pouch has nipples and lactates.
Evolution: I would assume that they developed along a loosely similar path to seahorses when developing into mammals. As reptiles that fertilized internally and then laid eggs, they developed a strategy of attaching the eggs to the male, then as time went on the male developed a brood pouch and nipples like a marsupial while the eggs degenerated and the burden of gestation was foisted on the male.
Advantage: The intended advantage is that the female foists the burden of pregnancy on to the male, allowing her to devote energy to producing more offspring at lower cost. Polyandry would probably be common.
Avoiding parasites: If necessary, perhaps the female lays a soft shelled egg in the male's pouch to prevent parasites from attaching to the fetus. The shell later dissolves to allow the fetus to attach to a nipple.