Social monogamy in the fiddler crab Uca capricornis: The response of paired crabs when faced with intruders of differing sex

A unique relationship exists between neighbouring males and females in the fiddler crab Uca capricornis. This experiment was designed to determine if males and females are unable to associate with multiple partners because their partners aggressively repel same-sex intruders. Experiments were conducted in the East Point Reserve, Darwin from November-January in 2002-2006. \n\n15 male-female pairs were located and the burrow of the male or the female (in a random order) was blocked. We tethered a size-matched same-sex or opposite sex intruder (in a random order) half way between the burrows. Once the focal individual emerged we documented whether it pushed or grappled (fought) with the intruder within 5 minutes. After each trail, we unblocked the burrow and allowed the pair to interact normally for 10 minutes before running the next trail. In this way we observed the reactions of both pair members to same- and opposite-sex intruders. Therefore for each pair, 4 experiments were undertaken: original female vs. intruder female, original female vs. intruder male, original male vs. intruder male, original male vs. intruder female. \n\nThe results indicated that males and females were equally likely to respond aggressively to intruders of the same sex. Both were less likely to fight intruders of the opposite sex.

邻近的雌雄角招潮蟹（Uca capricornis）之间存在一种独特的关联模式。本实验旨在探究雌雄个体是否因伴侣会主动驱离同类入侵者，而无法与多个配偶建立社交联系。实验于2002至2006年的11月至次年1月，在澳大利亚达尔文市的东点保护区（East Point Reserve）开展。

研究人员共找到15对雌雄配对个体，并以随机顺序封堵雄性或雌性的洞穴。随后在两个洞穴的中点位置，以随机顺序拴住一只体型匹配的同性或异性入侵者。当目标个体出洞后，研究人员会记录其在5分钟内是否会对入侵者发起推挤或扭打（搏斗）行为。每次实验结束后，我们会解除洞穴封堵，让配对个体恢复正常互动10分钟，再开展下一次实验。通过该流程，我们可观察到配对双方对同性及异性入侵者的反应。因此每对个体需完成4组实验：原配对雌性vs入侵者雌性、原配对雌性vs入侵者雄性、原配对雄性vs入侵者雄性、原配对雄性vs入侵者雌性。

实验结果显示，雌雄个体对同性入侵者发起主动攻击的概率并无显著差异；二者对异性入侵者发起搏斗行为的概率均相对更低。