3294 INTRODUCTION Dominance hierarchies for many species are determined through a combination of both signalling and fighting. When the costs of fighting are high, it is mutually advantageous for opponents to passively establish dominance using signals of potential fighting ability (Maynard Smith, 1974; Sneddon et al., 1997; Martín et al., 2007). Agonistic interactions over resources should then be decided through an asymmetry of the signal, which represents an individual’s resource holding potential (RHP) (Parker, 1974; Maynard Smith and Parker, 1976). As a receiver’s response determines the evolution of a signal, it is expected that most agonistic signals should be honest indicators of an individual’s quality (Dawkins, 1976; Dawkins and Guilford, 1991; Maynard Smith and Harper, 2003). Signal honesty (or reliability) should then be maintained by two non- mutually exclusive mechanisms (Searcy and Nowicki, 2005): (1) performance-based signals, and/or (2) signal costs. Performance- based signals are those that cannot be maintained by low- condition individuals as they are mechanistically linked to the inherent quality being advertised [e.g. stotting in gazelles (FitzGibbon and Fanshawe, 1988)] (see also Rohwer and Ewald, 1981; Enquist, 1985; Lappin et al., 2006). Signal costs – the costs of producing or maintaining signals – can ensure only high-quality individuals can afford to pay the extra investment, and thus they reliably convey accurate information about an individual’s quality (Maynard Smith and Harper, 2003). Dishonest (or unreliable) signalling occurs when the signals used to establish dominance become disassociated from an individual’s actual quality or fighting ability. Although unreliable signalling is theoretically expected, it should only occur at low frequencies in natural populations (Gardner and Morris, 1989; Johnston and Grafen, 1991; Számadó, 2000; Searcy and Nowicki, 2005). Despite such theoretical expectations, there are many reported cases of unreliable signals of both fighting ability and mate choice across a range of species, including fiddler crabs (Backwell et al., 2000; Lailvaux et al., 2009), hermit crabs (Elwood et al., 2006), snapping shrimp (Hughes, 2000), stomatopods (Steger and Caldwell, 1983; Adams and Caldwell, 1990) and freshwater crayfish (Seebacher and Wilson, 2006; Wilson et al., 2007). Moreover, recent studies of male slender crayfish, Cherax dispar, revealed dishonest signals are routinely used during agonistic interactions and are far more common than previously predicted (Wilson et al., 2007; Bywater et al., 2008). In this system, male slender crayfish use relative chela size to determine dominance (Seebacher and Wilson, 2007), but chela size is a poor predictor of chela strength (a measure of fighting ability), and strength is unrelated to social dominance (Wilson et al., 2007). It is important to note that when fights do occur between males of the slender crayfish it is the individuals with the stronger The Journal of Experimental Biology 214, 3294-3299 © 2011. Published by The Company of Biologists Ltd doi:10.1242/jeb.056754 RESEARCH ARTICLE Social control of unreliable signals of strength in male but not female crayfish, Cherax destructor Gregory M. Walter, Vincent O. van Uitregt and Robbie S. Wilson* School of Biological Sciences, The University of Queensland, St Lucia, QLD 4072, Australia *Author for correspondence (r.wilson@uq.edu.au) Accepted 16 July 2011 SUMMARY The maintenance of unreliable signals within animal populations remains a highly controversial subject in studies of animal communication. Crustaceans are an ideal group for studying unreliable signals of strength because their chela muscles are cryptically concealed beneath an exoskeleton, making it difficult for competitors to visually assess an opponent’s strength. In this study, we examined the importance of social avenues for mediating the possible advantages gained by unreliable signals of strength in crustaceans. To do this, we investigated the factors that determine social dominance and the relative importance of signalling and fighting during aggressive encounters in male and female freshwater crayfish, Cherax destructor. Like other species of crayfish, we expected substantial variation in weapon force for a given weapon size, making the assessment of actual fighting ability of an opponent difficult from signalling alone. In addition, we expected fighting would be used to ensure that individuals that are weak for their signal (i.e. chela) size would not achieve higher than expected dominance. For both male and female C. destructor, we found large variation in the actual force of their chela for any given weapon size, indicating that it is difficult for competitors to accurately assess an opponent’s force on signal size alone. For males, these unreliable signals of strength were controlled socially through increased levels of fighting and a decreased reliance on signalling, thus directly limiting the benefits accrued to individuals employing high-quality signals (large chelae) with only low resource holding potential. However, in contrast to our predictions, we found that females primarily relied on signalling to settle disputes, resulting in unreliable signals of strength being routinely used to establish dominance. The reliance by females on unreliable signals to determine dominance highlights our poor current understanding of the prevalence and distribution of dishonesty in animal communication. Key words: biting, performance, sexual selection. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY