To Bob or Not to Bob: Context Dependence of an Antipredator Response in Harvestmen (Arachnida: Opiliones)

Publication Date


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Biological Sciences


Ben Sadd

Mentor Department

Biological Sciences


Many potential prey species exhibit antipredator responses to avoid predator detection or consumption. These responses include behavioral mechanisms that can be employed to confuse predators or deter attack. Yet, the utility of such mechanisms may be dependent on the context in which the behavior is performed, and natural selection will act on potential prey to only perform such behaviors under the conditions in which they are beneficial. Gregarious behavior itself may be an antipredator behavior, but the effectiveness of other defensive behaviors may be group-size dependent. Harvestmen are nocturnal arachnids that form aggregates as an antipredator response during the daytime, and have been reported to perform a bobbing behavior as a defense mechanism in combination with aggregation. While the existence of the bobbing behavior has been documented and proposed as being more effective when performed in groups, empirical evidence on the influence of aggregation on the propensity to perform the behavior is lacking. We addressed the hypothesis that individual harvestmen in aggregations will be more likely to engage in antipredator behaviors that are postulated to be more effective when employed by groups, rather than individuals. Using experimental chambers, groups and individual harvestmen were exposed to a predator stimulant, and subsequently monitored for the antipredator bobbing response. Harvestmen within groups were significantly more likely to bob than individuals. We further demonstrated that the propensity to bob is not influenced by sex ratio or harvestman density, leaving group size as the most parsimonious explanation for the observed differences. We propose that in groups the bobbing of individuals en masse serves to deter or confuse predators, but this antipredator behavior will have reduced effectiveness when performed by isolated individuals, and may even visually attract predators. Our results support the hypothesis that organisms will be selected to perform antipredator behaviors with context dependent benefits only under restricted environmental conditions.



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