In an upcoming paper, I show that when a female bat feeds another bat, this allows her to add another possible donor to her own ‘social safety net’. There’s an obvious benefit to her: bats with larger sharing networks are more successful at getting fed.
But there’s potentially a more subtle benefit. If a hungry bat is fed by only 1 donor (say her daughter), then her daughter pays the entire cost of feeding her. But if that hungry bat is fed by, say, 5 donors (her daughter, her sister, and three non-relatives), then the feeding costs are reduced for her daughter and sister.
Sharing investments in non-kin do not necessarily detract (at least directly) from sharing investments in kin, unless both kin partners and non-kin partners are in need on the same day.
Kin selection might therefore favor bats that establish non-kin bonds because it will reduce the burden on their close relatives. If so, that would be an interesting means by which that kin selection could paradoxically favor non-kin helping.
To make an anthropomorphic analogy, this would be like saying that natural selection rewards families where the children quickly develop strong cooperative relationships with others in the tribe outside the family, so that they won’t be socially dependent 100% on their parents (and their parents can therefore invest more in other kids).
Or am I making a logical error? Has anyone made a formal social evolution model of something like that? It seems like someone would have. Let me know in the comments.