Abstract
BackgroundFor diurnal animals that heavily rely on vision, a nocturnal resting strategy that offers protection when vision is compromised, is crucial. We found a population of a common European jumping spider (Evarcha arcuata) that rests at night by suspending themselves from a single silk thread attached overhead to the vegetation, a strategy categorically unlike typical retreat-based resting in this group.ResultsIn a comprehensive study, we collected the first quantitative field and qualitative observation data of this surprising behaviour and provide a detailed description. We tested aspects of site fidelity and disturbance response in the field to assess potential functions of suspended resting. Spiders of both sexes and all developmental stages engage in this nocturnal resting strategy. Interestingly, individual spiders are equally able to build typical silk retreats and thus actively choose between different strategies inviting questions about what factors underlie this behavioural choice.ConclusionsOur preliminary data hint at a potential sensory switch from visual sensing during the day to silk-borne vibration sensing at night when vision is compromised. The described behaviour potentially is an effective anti-predator strategy either by acting as an early alarm system via vibration sensing or by bringing the animal out of reach for nocturnal predators. We propose tractable hypotheses to test an adaptive function of suspended resting. Further studies will shed light on the sensory challenges that animals face during resting phases and should target the mechanisms and strategies by which such challenges are overcome.
Highlights
For diurnal animals that heavily rely on vision, a nocturnal resting strategy that offers protection when vision is compromised, is crucial
From the 152 nocturnal sightings, 146 are documented cases of resting by hanging by a silk thread, the remaining six spiders were encountered sitting on vegetation
Hanging suspended on a silk thread seems to be a common strategy for both sexes and all developmental stages in Evarcha arcuata, yet these spiders are capable of building typical salticid silk retreats, which we observed both in the field and in the laboratory (Fig. 1a, supplementary methods S1: figure S3)
Summary
For diurnal animals that heavily rely on vision, a nocturnal resting strategy that offers protection when vision is compromised, is crucial. Most animals seek out safe sites during resting and sleeping phases, allowing them to lower their metabolic rates and be minimally vigilant [1]. Resting strategies and sites can reduce exposure to adverse abiotic (e.g., temperature) and biotic (e.g., predators) interactions [2], directly influencing fitness [3]. For both diurnal and nocturnal animals, choosing a resting site with minimal exposure to both predators and environmental conditions is crucial [3]. For diurnal invertebrates that heavily rely on vision specialized for daylight conditions, we might expect that nocturnal resting strategies are important due to the sensory disadvantage faced at night. When visual perception of potential threats is limited, we expect adaptive strategies that either allow perception via different or additional sensory modalities and/or selection for specific
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