Data for experiment 2 from Rapid juvenile hormone downregulation in subordinate wasp queens facilitates stable cooperation

In many cooperatively breeding animals, subordinate group members have lower reproductive capacity than dominant group members. Theory suggests subordinates may downregulate their reproductive capacity because dominants punish subordinates who maintain high fertility. However, there is little direct experimental evidence that dominants cause physiological suppression in subordinates. Here, we experimentally test how social interactions influence subordinate reproductive hormones in Polistes dominula paper wasps. P. dominula queens commonly found nests in cooperative groups where the dominant queen is more fertile than the subordinate queen. In this study, we randomly assigned wasps to cooperative groups, assessed dominance behaviour during group formation, then measured levels of juvenile hormone (JH), a hormone that mediates Polistes fertility. Within three hours, lowest ranking subordinates had less JH than dominants or solitary controls, indicating that group formation caused rapid JH reduction in low-ranking subordinates. In a second experiment, we measured the behavioural consequences of experimentally increasing subordinate JH. Subordinates with high JH-titres received significantly more aggression than control subordinates or subordinates from groups where the dominant's JH was increased. These results suggest that dominants aggressively punished subordinates who attempted to maintain high fertility. Low-ranked subordinates may rapidly downregulate reproductive capacity to reduce costly social interactions with dominants. Rapid modulation of subordinate reproductive physiology may be an important adaptation to facilitate the formation of stable, cooperative groups.

在诸多合作繁殖的动物类群中，群体内的从属成员的繁殖能力普遍低于优势成员。相关理论认为，从属成员之所以会下调自身繁殖能力，是因为优势成员会对维持高生育力的从属个体实施惩罚。然而，目前鲜有直接实验证据表明优势成员会对从属个体造成生理抑制。本研究以占氏纸胡蜂（Polistes dominula）为实验对象，通过实验探究社会互动如何影响从属个体的生殖激素水平。占氏纸胡蜂的蜂后通常以合作群体的形式筑巢，其中优势蜂后的生育力高于从属蜂后。本研究中，我们将胡蜂随机分配至合作群体中，在群体形成阶段评估其优势行为，随后检测保幼激素（juvenile hormone, JH）的水平——该激素可介导占氏纸胡蜂的生育能力。实验结果显示，仅在群体形成三小时内，等级最低的从属个体的JH水平便低于优势个体与独居对照组个体，表明群体形成会快速降低低等级从属个体的JH水平。在第二项实验中，我们通过实验提升从属个体的JH水平，并检测由此产生的行为后果。与对照组从属个体、以及优势个体JH水平被提升的群体中的从属个体相比，高JH滴度的从属个体遭受的攻击行为显著更多。上述结果表明，优势个体会对试图维持高生育力的从属个体实施攻击性惩罚。低等级从属个体可能通过快速下调自身繁殖能力，以减少与优势个体之间的高成本社会互动。对从属个体生殖生理的快速调控，或许是促进稳定合作群体形成的一项重要适应性策略。