Abstract
Cooperative behaviors are evolutionary stable if the direct and/or indirect fitness benefits exceed the costs of helping. Here we discuss cooperation and behaviors akin to cooperation in subsocial group-living species of two genera of herbivorous spider mites (Tetranychidae), i.e., the largely polyphagous Tetranychus spp. and the nest-building Stigmaeopsis spp., which are specialized on grasses, such as bamboo. These spider mites are distributed in patches on various spatial scales, that is, within and among leaves of individual host plants and among individual hosts of single or multiple plant species. Group-living of spider mites is brought about by plant-colonizing foundresses ovipositing at local feeding sites and natal site fidelity, and by multiple individuals aggregating in the same site in response to direct and/or indirect cues, many of which are associated with webbing. In the case of the former, emerging patches are often composed of genetically closely related individuals, while in the case of the latter, local patches may consist of kin of various degrees and/or non-kin and even heterospecific spider mites. We describe and discuss ultimate and proximate aspects of cooperation by spider mites in host plant colonization and exploitation, dispersal, anti-predator behavior, and nesting-associated behaviors and conclude with theoretical and practical considerations of future research on cooperation in these highly rewarding model animals.
Highlights
Cooperative behaviors abound in animals but pose a challenge for evolutionary theory because of direct fitness costs to actors
We describe the current state of knowledge of behavioral characteristics and proximate aspects, and we contemplate and discuss whether the described behaviors have evolved for direct and/or indirect fitness benefits, which subtype of cooperation they seem to represent, and whether they require a given degree of genetic relatedness to enhance the fitness of both actors and recipients
In interactions such as host plant exploitation, collective dispersal, and shared nests, closer than average genetic relatedness is a likely consequence of host plant colonization and settling processes inevitably resulting in more frequent and more likely encounters between kin than non-kin
Summary
Cooperative behaviors abound in animals but pose a challenge for evolutionary theory because of direct fitness costs to actors (helpers). Indirect fitness benefits play a role in joint host plant exploitation and grouping because of founder effects, and females depositing and aggregating their eggs at local feeding sites, often result in patches where kin are more likely to interact with each other than with non-kin. Overall, these benefits commonly outweigh the costs of group-living such as intensified local and regional competition for shared resources, food and mates. Cooperation in nest building and social immunity activities have clear direct benefits, so arise from byproducts, but it is more than plausible to assume a role of kin selection in these behaviors and indirect fitness benefits since it is usually and predominantly kin that live together and enlarge nests (kin fidelity sensu Sachs et al, 2004)
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