1219 INTRODUCTION Prey animals constantly have to make life-or-death decisions. While ensuring escape from all real predatory threats, they also have to avoid costly false alarms. Because of the limitations of sensory systems, the information available to make these decisions is usually imperfect and incomplete (Hemmi, 2005a; Hemmi, 2005b; Koops, 2004; Koops and Abrahams, 1998; Lima, 1998; Luttbeg, 2002; Martin and Lopez, 1999; Sih, 1992). For instance, many animals cannot reliably determine important risk-related cues such as the distance, approach direction or speed of potential predators (Collett and Harkness, 1982; Hemmi, 2005a; Hemmi, 2005b; Hemmi and Zeil, 2005). Instead, they have to rely on simple rules- of-thumb to estimate predation risk (Bouskila and Blumstein, 1992; Koops and Abrahams, 1998; Welton et al., 2003). These rules-of-thumb are not necessarily based on cues reflecting general properties of dangerous situations. Rather, the cues should be commonly available in the presence of a natural predator, and reliably detectable against unpredictable background noise. Differences in appearance and behaviour between commonly encountered dangerous and harmless animals, for example, can be reflected even in highly degraded signals or those viewed by comparatively simple sensory systems. Although these cues may not directly indicate risk, they can help determine a potential predator’s identity or behaviour, and thus correlate with the relative risk it poses. By combining several such cues – some of which may be readily available but unreliable, while others may take longer to obtain but provide more risk-related information – prey animals can create robust and efficient anti-predator strategies even with poor sensory information. In a previous study (Smolka et al., 2011), we compared the signals that dangerous and harmless flying animals present to fiddler crabs in their natural habitat. The results showed that the spatio-temporal statistics of these signals, despite background noise, differ enough for the crabs to partially distinguish dangerous from harmless events. The cues they use to make this distinction include not only retinal speed and elevation (Hemmi, 2005b), but also flicker, which can be created, for example, by flapping bird wings. The main goal of this study is to understand how flicker is used as part of a multi-cue response criterion to estimate the threat an object poses. We therefore aim to show (1) that flicker is used as a response criterion, (2) how flicker interacts with retinal speed and (3) that flicker does not trigger an immediate ‘panic’ response at the time of detection, but contributes to a more complex risk assessment strategy. Understanding how fiddler crabs weigh their response decisions against different criteria, and how these tie in with different stages of the escape response, will help us understand how prey animals in general might employ multi-cue, multi-stage escape strategies to respond to predators in an efficient and adaptable way. MATERIALS AND METHODS Animals, apparatus and video analysis Experiments were conducted in October 2006 with the fiddler crab Uca vomeris McNeill 1920 (Ocypodidae: Brachyura: Decapoda) on intertidal mudflats near Cungulla (19°24′S, 147°6′E), south of SUMMARY Predator avoidance behaviour costs time, energy and opportunities, and prey animals need to balance these costs with the risk of predation. The decisions necessary to strike this balance are often based on information that is inherently imperfect and incomplete because of the limited sensory capabilities of prey animals. Our knowledge, however, about how prey animals solve the challenging task of restricting their responses to the most dangerous stimuli in their environment is very limited. Using dummy predators, we examined the contribution of visual flicker to the predator avoidance response of the fiddler crab Uca vomeris. The results illustrate that crabs let purely black or purely white dummies approach significantly closer than black-and- white flickering dummies. We show that this effect complements other factors that modulate escape timing such as retinal speed and the crabʼs distance to its burrow, and is therefore not due exclusively to an earlier detection of the flickering signal. By combining and adjusting a range of imperfect response criteria in a way that relates to actual threats in their natural environment, prey animals may be able to measure risk and adjust their responses more efficiently, even under difficult or noisy sensory conditions. Key words: sensory constraint, Uca vomeris, escape behaviour, vision, flicker, multi-cue response strategy. Received 13 June 2012; Accepted 3 December 2012 The Journal of Experimental Biology 216, 1219-1224 © 2013. Published by The Company of Biologists Ltd doi:10.1242/jeb.076133 RESEARCH ARTICLE Flicker is part of a multi-cue response criterion in fiddler crab predator avoidance Jochen Smolka 1,2, *, Chloé A. Raderschall 1 and Jan M. Hemmi 1,3 1 ARC Centre of Excellence in Vision Science, Evolution, Ecology and Genetics, Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia, 2 Department of Biology, Lund University, Sölvegatan 35, 22362 Lund, Sweden and 3 School of Animal Biology and the UWA Oceans Institute, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia *Author for correspondence (jochen.smolka@biol.lu.se) THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY