Visual signalling and sexual selection in male fiddler crabs Uca tangeri

Similar to many other species of fiddler crabs, the interactions of Uca tan geri are influenced by 2 characteristic visual signals: the waving display performed by males with their enlarged claw, and the building of structures (mudballs) around the burrow entrance. This study focused on male signalli ng, male-male competition and female mate choice. Female choice and male ma ting success were investigated by looking at male quality, male visual sign als (waving activity and mudballs) and male interactions and their outcome. Fieldwork was carried out in June and July 1997 at the Ria Formosa Natural Park, Algarve, Portugal, on a sandy beach with an average population densi ty of 2.06 burrows m(-2). Behavioural observations of focal males were made at low tide, 1 h before and after the peak of low tide, that is during the mudballing phase or interaction phase. This was also when measurements of male characteristics, mudballs and burrow characteristics were taken. The r esults show that the quality of a male's burrow is related to its depth, an d depth is correlated with 4 features: claw size, waving rate, mean distanc e to mudballs and number of mudballs. Therefore, females could use these fe atures as indicators of gallery depth, eliminating the need to enter the bu rrow for sampling. Males compete for the possession of burrows that are mor e visited by females, thus taking over burrows is an alternative strategy t o burrow digging. Male waving activity, mudball distance and claw size can be considered as multiple visual signals available to females for mate choi ce. Females' first choice criterion would seem to be burrow quality.