Combat in the Fiddler Crabs Uca Pugilator and U. Pugnax: a Quantitative Analysis

Abstract

The aggressive behavior of two temperate species of fiddler crabs (Uca pugilator and U. pugnax) was studied in the field. Most of the fights wer between Residents (crabs owning burrows) and Wanderers seeking to displace them, though less commonly, fighting occurred between Residents occupying adjacent burrows. Over 400 fights were observed in each species. The two species showed differences in the sequences of acts performed during fights. Uca pugnax also performed two acts never observed in U. pugilator. Wanderers were rarely successful in displacing Residents of the same size or larger, but they won a small percentage of fights when they were larger than the Resident. Uca pugilator Wanderers select Residents that are slightly smaller, but we found no evidence for size selection in U. pugnax. Strongly motivated Wanderers repeatedly performed a key act (Downpush), which was also strongly correlated with successfully displacing Residents. Combat duration and number of acts were not related to temperature, the time of day, or the time in relation to low tide. However, the incidence of fighting increases in the early afternoon. Most of the fights occur within 1-3 hours after low tide. Differences between the species in the tempo and length of fights were observed. An hypothesis, based upon habitat differences and the value of burrows as a resource, is presented to account for these differences. We compared our data to CRANE'S (1967) for U. rapax, a tropical species. There are some important differences between her findings and ours, even though U. pugnax is closely related to U. rapax. We suggest that at least one of her opinions on the significance of aggression in tropical fiddlers is in conflict with evolutionary theory. Some of her other ideas do not apply to temperate species, where climatic factors may be responsible for behavioral convergence. Our data also indicate that the differences and similarities between species are sufficiently provocative to warrant further comparative studies.