Do you think juvenile vampire bats would regurgitate food to their hungry parents?

The benefits of helping: direct and indirect

Biologists divide the evolutionary benefits of helping others into two categories. Direct fitness benefits means helpers tend to have more offspring, all else being equal. Indirect fitness benefits (or kin selection) mean that helpers pass on more of their own genes because their relatives have more offspring. Kin selection is the only way to get true reproductive altruism. Due to this semantic division, biologists tend to label cooperative behaviors into “mutualism” or “kin selection”. But the real world is a bit more messy.

There can be complex interactions between direct and indirect fitness benefits. When they  exist together, they might mask each other. For example, members of cooperative animal groups are often related, which makes researchers focus on kinship and kin selection. But then they might miss some of the direct benefits of helping others. For example, cooperative male alliances are often formed between brothers (kin selection!), but by helping the dominant brother, a subordinate male might inherit dominance later (a less obvious direct benefit). Similarly, when biologists think “cooperation in wasps”, kin selection is the first thing that comes to mind, because kin altruism is the main driver of eusociality. But Elli Leadbeater showed that even unrelated subordinate wasps will help the dominant Queen in a primitively eusocial wasp, because being a helper increases the chance that they will acquire Queen status later (direct benefit). So obvious benefits to relatives can mask the importance of subtle benefits to one’s self.

The opposite can also happen: A focus on direct benefits can lead observers to underestimate kin selection. In a recent paper entitled, Kin selection, not group augmentation, predicts helping in an obligate cooperatively breeding bird the authors show that… well the title actually says it all. “Group augmentation” is when helpers get a direct benefit because helping other group members survive, helps their own survival and reproduction. For example, if a marmot feeds other babies in her burrow and help them survive the winter, those warm bodies might help her keep her own baby warm. So more cooperative marmots might do better on average because of group augmentation. Well, people thought group augmentation might be able to explain helping behavior in the Australian chestnut-crowned babbler and other cooperatively breeding birds. Maybe we don’t need kin selection at all? Not so. The authors show quite convincingly that the babblers care more about kinship than group size. This leads us to…

Would juvenile vampire bats feed their mothers?

Direct and indirect fitness benefits can also interact in more complex ways. For example, the potential interaction between reciprocity and kin discrimination in food sharing vampire bats might go in two different directions, depending on the relative costs and benefits to donors and receivers of food.

Option 1: The interaction might be negative if food sharing between relatives tends to be altruistic and asymmetrical, especially if the benefits of help are age-dependent. So maybe bats will reciprocate help from non-kin, but then not reciprocate help from relatives. Why bother if it’s altruistic? For example, older relatives might help the next generation, but not vice versa. Is that what you expect?

Option 2: On the other hand, kinship is inherently symmetrical (reciprocal), so the interaction might be positive if donors reciprocate help more often with relatives, for example, if relatives help each other equally in both directions, even more so than when two unrelated bats help each other. This predicts that offspring might even feed their parents. Do you think that would happen?

Our recent results validate one of these scenarios. Which do you think it is? Let me know in the comments. We are submitting the first results for publication in the next few days hopefully.