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.

15 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.

The 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.

角招潮蟹（Uca capricornis）的相邻雌雄个体之间存在独特的互动关系。本实验旨在探究雌雄个体是否因伴侣会主动驱逐同性入侵者，而无法与多个配偶建立关联。实验于2002至2006年的11月至次年1月在达尔文市东点保护区（East Point Reserve）开展。
研究人员共定位到15对雌雄个体，并以随机顺序分别封堵雄性个体或雌性个体的洞穴（burrow）。在两个洞穴的中点位置，用细绳拴住一只体型匹配的同性或异性入侵者（intruder），投放顺序同样随机。待目标个体（focal individual）出洞后，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。每轮试验（trail）结束后，解除洞穴的封堵，让配对个体恢复正常互动10分钟，再开展下一轮试验。通过该流程，我们可观测到配对双方对同性及异性入侵者的反应。因此每对受试个体需完成4组试验：原配雌性vs入侵雌性、原配雌性vs入侵雄性、原配雄性vs入侵雄性、原配雄性vs入侵雌性。
试验结果表明，雌雄个体对同性入侵者发起攻击性反应的概率无显著差异；二者对异性入侵者发起搏斗行为的概率均显著更低。