Some nonhuman animals form adaptive long-term cooperative relationships with nonkin that seem analogous in form and function to human friendship [1, 2, 3, 4]. However, it remains unclear how these bonds initially form, especially when they entail investments of time and energy. Theory suggests individuals can reduce the risk of exploitation by initially spreading out smaller cooperative investments across time [e.g., 5] or partners [6], then gradually escalating investments in more cooperative partnerships [7]. Despite its intuitive appeal, this raising-the-stakes model [7] has gained surprisingly scarce empirical support. Although human strangers do “raise the stakes” when making bids in cooperation games [8], there has been no clear evidence for raising the stakes during formation of social bonds in nature. Existing studies are limited to cooperative interactions with severe power asymmetries (e.g., the cleaner-client fish mutualism [9]) or snapshots of a single behavior within established relationships (grooming in primates [10, 11, 12, 13]). Raising the stakes during relationship formation might involve escalating to more costly behaviors. For example, individuals could “test the waters” by first clustering for warmth (no cost), then conditionally grooming (low cost), and eventually providing coalitionary support (high cost). Detecting such a pattern requires introducing random strangers and measuring the emergence of natural helping behaviors that vary in costs. We performed this test by tracking the emergence of social grooming and regurgitated food donations among previously unfamiliar captive vampire bats (Desmodus rotundus) over 15 months. We found compelling evidence that vampire bats selectively escalate low-cost grooming before developing higher-cost food-sharing relationships.

一些非人类动物与非亲属形成适应性长期合作关系，其形式和功能似乎与人类友谊相似[1,2,3,4]。但是，目前尚不清楚这些债券最初是如何形成的，特别是当它们需要时间和精力的投资时。理论表明，个人可以通过首先在时间上分散较小的合作投资[例如5]或合伙人[6]，然后逐步增加对更多合作伙伴的投资[7]来降低剥削的风险。尽管它具有直观的吸引力，但这种筹码模型[7]却获得了令人惊讶的稀缺的经验支持。尽管人类陌生人在合作游戏中竞标时确实“提高了赌注” [8]，但没有明显的证据表明在自然界中形成社会纽带的过程中提高赌注。现有研究仅限于具有严重权力不对称的合作互动（例如，清洁者与客户之间的鱼的共生[9]）或已建立关系中单个行为的快照（灵长类动物的修饰[10、11、12、13]）。在关系建立过程中增加风险可能涉及升级为更昂贵的行为。例如，个人可以通过以下方式“测试水域”：首先进行聚类（无成本），然后有条件地进行梳理（低成本），最后提供联合支持（高成本）。要发现这种模式，就需要引入随机的陌生人，并测量成本不同的自然帮助行为的出现。我们通过跟踪以前不熟悉的圈养吸血蝙蝠中社交修饰和反哺食物捐赠的出现来执行此测试（Desmodus rotundus）超过15个月。我们发现有力的证据表明，吸血蝙蝠在发展高成本的食物共享关系之前会选择性地升级低成本的美容。