Subjective Fear Dilutes as Group Size Increases

According to Hamilton’s Selfish Herd Theory, a crucial survival benefit of group living is that it provides a ‘risk dilution’ function against predation. Despite a large literature on group living benefits in animals, few studies have been conducted on how group size alters subjective fear or threat perception in humans, and on what factors drive preferences for being in groups when facing threats. We conducted seven experiments (N=3,838) to test (A) if the presence of others decreases perception of threat under a variety of conditions. In studies 1 to 3, we experimentally manipulated group size in hypothetical and real-world situations, to show that fear responses decreased as group size increased. In studies 4 to 7 we again used a combination of hypothetical, virtual and real-world decisions to test (B) how internal states (e.g. anxiety) and external factors (e.g. threat level, availability of help) affected participants’ preference for groups. Participants consistently chose larger groups when threat and anxiety were high. Overall, our findings show that group size provides a salient signal of protection and safety.

The effect of aggregation on visibility in open water

Aggregation is a common life-history trait in open-water taxa. Qualitative understanding of how aggregation by prey influences their encounter rates with predators is critical for understanding pelagic predator–prey interactions and trophic webs. We extend a recently developed theory on underwater visibility to predict the consequences of grouping in open-water species in terms of increased visual detection of groups by predators. Our model suggests that enhanced visibility will be relatively modest, with maximum detection distance typically only doubling for a 100-fold increase in the number of prey in a group. This result suggests that although larger groups are more easily detected, this cost to aggregation will in many cases be dominated by benefits, especially through risk dilution in situations where predators cannot consume all members of a discovered group. This, in turn, helps to explain the ubiquity of grouping across a great variety of open-water taxa.

Cooperate or compete? Influence of sex and body size on sheltering behaviour in the wall lizard, Podarcis muralis

Reptile sheltering behaviour, despite profound life history ramifications, remains poorly investigated. Whether or not individuals share a suitable shelter or, conversely, exclude conspecifics may depend on associated costs (resource partitioning, sexual harassment, disease or parasite contamination) and benefits (predation risk dilution, thermal resilience, information sharing). We performed two experiments on field caught wall lizards (Podarcis muralis), a highly territorial species, to investigate the relative roles of sex and body size in night sheltering. In the first experiment, random pairs of lizards were offered two identical shelters. Lizards either shared a shelter, or sheltered separately. In the second experiment, different random pairs of lizards were offered only one shelter so as to elicit a share or compete response. Body size and sex both appeared as significant drivers for sheltering patterns. Unexpectedly, wall lizards often chose to share shelters. When only one shelter (too small to accommodate two adult lizards) was available, many lizards rejected the sheltering option in preference for aggregation. Such aggregative behaviour was not sex dependant, and may reflect thermoregulatory or anti-predatory benefits. Our results nevertheless suggest that cooperative behaviour may exist in wall lizards.

Neighbour density, body size and anti-predator hiding time in the New Zealand mud-crab Austrohelice crassa

Many preys retreat into a refuge as a response to the presence of a predator, a behavioural strategy which guarantees safety but is also costly due to a trade-off between hiding time and time spent in other essential activities. The balance between costs and benefits of hiding, which are influenced by different factors, determine the hiding duration. In a field experiment, the anti-predator behaviour of the mud crab Austrohelice crassa was studied to assess the effects of two factors, body size and neighbour density, on the time spent hiding in burrows following a predator threat. Hiding times, body size (estimated from burrow diameter) and neighbour density (number of other burrows within a 30 cm radius) were measured for 158 individual crabs during a snorkelling survey. Regression analyses showed that hiding time of individual crabs significantly increased with increasing body size, and decreased with increasing neighbour density. These trends are the result of three main selective pressures: size-biased predation risk, dilution of predation in dense clusters of burrows, and more intense competition in dense clusters. Variation among individual crabs probably reflects differences in the balance between costs and benefits for crabs differing in body size and neighbour density.

Perceived Risk Dilution with Multiple Warnings

The application of multiple warning labels on consumer products and household appliances is now commonplace with increased public attention on safety issues. In the present study, the number of on-product warning messages was manipulated to examine perceived risk. Risk assessment for individual warning messages associated with a product and the overall perceived risk level for the product were examined and compared. We hypothesized that the number and contents of warnings may influence the perceived risk level for both the individual warning message and the overall perception of danger. Warning messages were presented in the format of either discrete messages or as messages as part of a group, then compared with regard to their perceived risk. The results showed that the perceived risk declined in a stepwise manner as a function of the number of low-criticality warning messages, thereby producing a dilution effect. This dilution effect appeared to take place as the number of low-risk warnings increased beyond five messages. The implications for safety and product design are discussed.