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.