Abstract
Social relationships that involve costly helping occur most often among kin, but in many complex and individualized animal societies, nonkin also demonstrate stable cooperative relationships that share similarities with human friendship. How do such cooperative bonds form between complete strangers? One theory suggests that strangers should ‘test the waters’ of a new relationship by initially making low-cost cooperative investments and gradually escalating them with good partners. This ‘raising-the-stakes’ strategy is evident in humans playing short-term economic games, but it remains unclear whether it applies to the natural helping behaviors that underlie a long-term cooperative relationship. Here, we show evidence that unfamiliar nonkin vampire bats (Desmodus rotundus) selectively escalate low-cost investments in allogrooming before developing higher-cost food-sharing relationships. By introducing females from geographically distant sites in pairs or groups and fasting them repeatedly over 15 months, we observed that bats first established new reciprocal grooming relationships, and that increasing grooming rates predicted the occurrence of first food donations, at which point grooming rates no longer increased. New food-sharing relationships emerged reciprocally in 14.5% of 608 female pairs and formed faster when strangers lacked alternative familiar partners. Our results suggest that ‘raising-the-stakes’ might be easier to detect when tracking multiple types of behavior during relationship formation, rather than measuring a single behavior within an established relationship. The general principle of ‘testing the waters’ might play an underappreciated role across many other social decisions with long-term consequences, such as joining a new social group or choosing a long-term mate.
Significance statement Vampire bats form long-term cooperative social bonds that involve reciprocal regurgitated blood sharing. But how do two individuals go from complete strangers to reciprocal food donors? By introducing unfamiliar bats, we found evidence that low-cost grooming paves the way for higher-cost food donations. Food sharing emerged in a reciprocal fashion and it emerged faster when two strangers could not access their original groupmates. Bats that did form new food-sharing relationships had a history of escalating reciprocal grooming up until the food sharing began. The finding that unfamiliar nonkin vampire bats appeared to gradually and selectively transition from low-cost to high-cost cooperative behaviors is the first evidence that nonhuman animals ‘raise the stakes’ when forming new cooperative relationships.