Abstract
Many animals benefit from aggregating due to the anti-predator effects associated with living in groups. Hermit crabs are known to form groups, or ‘clusters’, which may occur at sites of high shell availability. Clustering may also have anti-predator benefits, if individuals in larger clusters able to spend less time engaging in defensive behaviours such as hiding in their shells. Here, we test the hypothesis that crabs in larger clusters will emerge faster from their shells after an elicited startle response in the European hermit crab (Pagurus bernhardus). We found that individuals were generally consistent in their emergence times across group sizes (displaying ‘personality’ in relation to emergence time), but that group size influenced emergence time in P. bernhardus. In contrast to the hypothesis, crabs in larger clusters had longer emergence times relative to their own emergence times in smaller clusters. Suggested explanations for this effect include intra-specific competition for the gastropod shells that hermit crabs inhabit, as well as the possible release of chemical cues by crabs in larger clusters.
Highlights
Group-living has been observed across a broad range of animal taxa (Krause and Ruxton, 2002), and group size in particular has a major influence on the outcome of predator–prey interactions, allowing group-living animals to manage their vulnerability to predation risk (Cresswell and Quinn, 2011)
By analyzing the variation in startle responses exhibited by individual hermit crabs across several classes of group size, this study explores whether emergence time is influenced by clustering in P. bernhardus
The results suggest that both individual consistency and group size affect emergence time in hermit crabs
Summary
Group-living has been observed across a broad range of animal taxa (Krause and Ruxton, 2002), and group size in particular has a major influence on the outcome of predator–prey interactions, allowing group-living animals to manage their vulnerability to predation risk (Cresswell and Quinn, 2011). The major costs associated with group-living, such as higher rate of attack from predators due to increased conspicuousness, may be offset by anti-predator mechanisms (Uetz et al, 2002). These mechanisms include the dilution of individual risk (Foster and Treherne, 1981), the confusion of predators, reducing attack success (Miller, 1922; Krakauer, 1995), encounter-dilution (Turner and Pitcher, 1986) and selfish herd effects (Hamilton, 1971). Grouping individuals benefit from collective vigilance, with those in larger groups able to reduce time spent scanning and increase time engaging in other activities (Pulliam, 1973; Cresswell and Quinn, 2011), which can allow for cooperative warning, escape and defence behaviour (Krause and Ruxton, 2002). In P. bernhardus, the structure of these vacancy chains differs in the presence and absence of
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