Dept. of Biological Sciences, National University of Singapore
PhD Defense Seminar cum Oral Examination
“Contest and post-contest behaviour of mangrove crab Perisesarma eumolpe (Crustacea: Sesarmidae), and the vibro-acoustic communication of Parasesarma and Perisesarma species.”
By Chen Zijian Paul
Supervisor: Professor Ng Kee Lin, Peter
Mon 07 Sep 2015: 10.00am
@ DBS Conference Room (S3, Level 5,#05-02)
All are welcome
Abstract – Fighting in animals, though an effective means to gain access to limited resources, often incurs high cost on individuals. Thus, fights 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 with the rubbing of specialised structures found on the chelae. These structures, characteristic to the genera Parasesarma and Perisesarma, are known to be species-specific. This thesis attempts to answer the following questions:
- Is stridulation also present among other species of crabs of the genera, Parasesarma and Perisesarma?
- Is stridulation a cost-mitigating strategy in Pe. eumolpe contests?
- If so, how does stridulation 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 M.W.F. Tweedie 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.
While future fights could be averted by a stridulatory victory display, how does P. eumolpe avoid drawing-out 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.