CONTEST AND POST CONTEST BEHAVIOUR OF THE SOUTHEAST ASIAN MANGROVE CRAB PERISESARMA EUMOLPE (CRUSTACEA: SESARMIDAE), AND THE VIBROACOUSTIC COMMUNICATION OF PARASESARMA AND PERISESARMA SPECIES

Abstract

Fighting, though an effective means to gain access to limited resources, often incurs high cost on individuals. Thus, fights in animals often involve strategies, at different stages of the contest, to avoid any unnecessary cost accrual. In Perisesarma eumolpe?a species of mangrove crab?contests are sometimes followed by stridulatory events. Stridulation is achieved by the rubbing of specialised structures found on the chelae. These structures, characteristic to genera of Parasesarma and Perisesarma, are known to be species-specific. This thesis attempts to answer the following questions: Is stridulation also present among other crabs of the genera, Parasesarma and Perisesarma? Is it a cost-mitigating strategy in Pe. eumolpe contest and, if so, how does it reduce the cost involved in conflict resolution? Which assessment strategy does P. eumolpe employ to avoid protracted contest? To begin, the prevalence of stridulatory behaviour in two other related species, P. fasciatum and Parasesarma ungulatum were investigated. Observation of intra-specific interactions showed that these species also use the structures on their chelae to stridulate. Although the stridulatory frequency spectra were similar, the signals obtained from all three species were markedly different in their temporal features. Stridulation in P. eumolpe was dubbed by Michael Tweedie (1954) as ?a gesture of defiance and triumph?. In P. eumolpe, this is hypothesized as a post-contest cost-mitigating strategy?more specially a victory display. Evaluation of P. eumolpe stridulation against the three established criteria of victory display supports this hypothesis. In P. eumolpe, stridulation was never observed in other contexts except to assert victory. While it is uncertain how victory displays actually mitigate the cost of fighting, two possible functions have been suggested: to advertise recent victory to nearby eavesdroppers or deter recent losers from future contest. Empirical support for these purported functions is generally lacking. While some evidence has been provided for the advertising function of victory display, no study has yet supported the browbeating function of victory display. To investigate this function, the influence of stridulation on losers was examined. The results suggest that stridulation deterred losers from starting a new fight. While mainly observational, this is probably the first empirical work that supports the browbeating function of victory display in animals. Future fights could be averted by a stridulatory victory display, but how does P. eumolpe avoid drawn out of a current contest? The two main strategies animals have evolved to prevent protracted fighting are self and mutual assessments. However, distinguishing between these assessments has not been easy. Studies examining contest assessment of various animals have yielded findings that are not consistent with the predictions of either assessment type. The mismeasure of resource holding potential (RHP) in relation to contest duration is a potential cause of the inconsistency. In this study, contest behaviours were the only trait that predicted contest outcome and contest duration. Thus, they were used to evaluate the contest assessment strategy of P. eumolpe. The fights of P. eumolpe were consistent with the predictions of the cumulative assessment model (CAM), a self assessment contest model.