Parent Record: Social monogamy in the fiddler crab Uca capricornis

Fiddler crabs are not generally considered subjects for the study of monogamy as they tend to live in dense mixed sex colonies with numerous neighbours and individually defended territories. Here, a series of experiments have been conducted demonstrating the unique relationship between neighbouring males and females in the species Uca capricornis. These experiments illustrate that this species of fiddler crab is socially monogamous. Six experiments were conducted in relation to this investigation in the East Point Reserve, Darwin from November to January in 2002-2006:

1) Natural male-female interactions and territoriality - Twenty-one pairs were filmed from overhead for a period of 30 minutes. Any activity was noted, as was any interactions with intruders. Territoriality was also examined, providing the overall area covered by the crabs within the half hour. By aligning the male and female territories with their respective burrows it was possible to determine the area overlap between them.

2) Sex ratio and distribution - The purpose of this experiment was to determine whether females may be a limited resource within a population of Uca capricornis. The sex ratio in relation to size was determined. The location of burrows was also recorded to determine whether the males and females were distributed independently of each other. The distance to, and sex of, the nearest resident for each individual was also measured to compare mean distances between male-female neighbours and between male-male neighbours.

3) Burrow tenure -To determine whether male-female pairs were longer lasting than male-male pairs, we documented the number of crabs that were still present in their same burrow after 24 hours.

4) 'Neighbour' and 'stranger' relations - This experiment was designed to demonstrate how a male fiddler crab responds to different individuals tethered near to his burrow. The response of a male crab to the neighbour and a stranger (with similar physical characteristics) was observed.

5) Response of paired crabs when faced with intruders of differing sex. 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.

6) Size-assortative mating -This experiment was designed to examine whether the nearest female or male neighbour was correlated with the size of a focal male. This investigation was based upon data collected during the other experiments.