Can friendships reduce the burden on family?

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.

6 thoughts on “Can friendships reduce the burden on family?

  1. I already know there is great work on the inclusive fitness benefits of kin dispersal to avoid kin competition, but I’m not sure about non-kin social network centrality leading to indirect fitness benefits.


  2. Gerry, that’s a crazy idea. (Also brilliant. 🙂 I doubt anyone has done that but I have no actual information!
    I see a possible cost to developing non-kin cooperative bonds — by relying on non-kin you could weaken your bonds with kin, which might in general be more reliable. I suspect that all social bonds depend somewhat on the frequency with which the interaction happens. If there is lots of food sharing between two individuals they might be more likely to share again in the future.
    The benefit to kin of outsourcing their food sharing would have to be pretty high to offset the selfish cost of security from kin. Otherwise kids wouldn’t be so demanding on their parents. 🙂


  3. Someone needs to make a model! Yes– that’s an interesting possible cost, which sorta combines two of Trivers’ big ideas: reciprocity and parent-offspring conflict (for case of mother-daughter pairs). I suspect you might not weaken kin bonds by investing elsewhere, because 1) direct competition would require both bats missing a meal on the same night and 2) kin sharing might be more altruistic and unconditional. For example, in cooperative breeding fish, dominant breeders tolerate unhelpful kin on their territory. But in contrast– unhelpful non-kin are ousted. In other words, kin tolerance is unconditional. But non-kin must “pay-to-stay”.

    I shouldn’t say that though, because I’m staying at your house for free right now. 🙂


    1. Humans live in strange new worlds where we are surrounded by strangers and “social networks” can refer to fake constructs on the internet that provide very little of the social benefits of real face-to-face social interactions. Because of the internet, the social networks we form are more independent of location in time and space than they have ever been. This is good for culture. But I think we are missing a lot in terms of personal and emotional benefits. “Facebook friends” will never be like the housemates or co-workers you see everyday.


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