Why would a male marsiupial agree to accept responsibility for carrying half of his mate's joeys?

I'm looking to create a species of marsupial that is heavily K select, which is to say they put significant effort into raising their young after birth (both of them :P ) in which both the male and female carry joeys.

My general idea is that the female would get pregnant with more children then she could raise personally. Then after they develop to joey phase half the joey would move into the female pouch, as with most marsupials, but half would instead be transferred to the male's pouch. The male would also produce milk and both sexes would carry their joeys to term.

Notice I'm suggesting the male carry only half the joeys, the female would still carry half. This way the two sexes are equally sharing energy cost of pregnancy (or near equally, the cost of producing eggs is minimal in comparison), rather then trying to move all the cost over to the male. If the male is unavailable or refuses to accept his 'half' of joeys presumably the female will abort/abandon the joey(s) the male would have received rather than attempting to carry all of them; since she knows she is unable to care for that many, and may not even have pouch space or sufficient nipples to do so. thus the male doesn't benefit from trying to force the female to carry his half of the joeys.

The idea would be to allow production of more joeys at one time. The evolutionary advantage for the female seems obvious in this situation. However, I'm struggling with justifying why the male would accept the responsibility of a 'pregnancy'. While his carrying half the joeys would allow production of more joeys from the single mating it seems potentially insufficient advantage compared to the opportunity cost for the male of not focusing on trying to impregnate multiple females.

For the male, there are a number of potential disadvantages to this system.

1. By committing to carrying children he is passing up a chance to mate with other females, and committing resources to the children that could be utilized to grow larger and more competitive towards achieving mates next mating season.

2. He risks cuckoldry, being tricked into carrying another male's young. Presumably, the species is either purely monogamous or engages in mate guarding during mating season to lower this risk.

3. Transferring the Joeys to the male would have increased risk of the joey being harmed, joey migrating to the pouch of the female is already a dangerous period without two different individuals needing to coordinate the transfer.

4. In a heavily K select species the majority of energy is committed to raising the young after birth, so sharing resources by having both sexes carry the child seems only a marginal advantage since pregnancy is only a small part of the overall contribution spent on raising the child(ren).

I'm looking for methods that can be used to make this an evolutionary adventurous option for the male. I'm looking for hard science and detailed analysis of how such a system could evolve if possible.

I'm willing to accept a system where males only agree to accept responsibility for carrying joeys under certain circumstances, such as young males offering to share carrying responsibility in order to earn mating rights while more dominate males focusing on mating multiple females. However, such a possibility would half to address how the female is able to ensure a male actually intends to carry when they mate, to avoid males 'lying' about willingness to help carry to secure more matings.

Inspired by my answer/comments on this question

posted about 5 years ago

dsollen‭

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