Abstract
Many animals form aggregations with individuals of the same species (single-species aggregations, SSA). Less frequently, individuals may also aggregate with individuals of other species (mixed-species aggregations, MSA). Although the benefits and costs of SSA have been intensively studied, the same is not true for MSA. Here, we first review the cases of MSA in harvestmen, an arachnid order in which the records of MSA are more frequent than other arthropod orders. We then propose several benefits and costs of MSA in harvestmen, and contrast them with those of SSA. Second, using field-gathered data we describe gregariousness in seven species ofPrionostemmaharvestmen from Costa Rica. These species form MSA, but individuals are also found solitarily or in SSA. We tested one possible benefit and one possible cost of gregariousness inPrionostemmaharvestmen. Regarding the benefit, we hypothesized that individuals missing legs would be more exposed to predation than eight-legged individuals and thus they should be found preferentially in aggregations, where they would be more protected from predators. Our data, however, do not support this hypothesis. Regarding the cost, we hypothesized that gregariousness increases the chances of parasitism. We found no support for this hypothesis either because both mite prevalence and infestation intensity did not differ between solitary or aggregated individuals. Additionally, the type of aggregation (SSA or MSA) was not associated with the benefit or the cost we explored. This lack of effect may be explained by the fluid membership of the aggregations, as we found high turnover over time in the number of individuals and species composition of the aggregations. In conclusion, we hope our review and empirical data stimulate further studies on MSA, which remains one of the most elusive forms of group living in animals.
Highlights
One of the simplest forms of group living in animals is gregariousness, defined as “the tendency of an animal to aggregate with others such that the animals are in contact with one another, or are nearly so, and that the distribution of the animals in the local environment is extremely patchy” (Vulinec, 1990)
Regarding the costs associated with mixed-species aggregations (MSA), we provide for the first time a set of hypotheses that may guide future empirical studies on the subject
Physiological Benefits Given the weak support for two simple explanations that require no individual benefit for the existence of MSA, we argue that this unusual form of group living evolved and is maintained in harvestmen because it provides benefits
Summary
One of the simplest forms of group living in animals is gregariousness, defined as “the tendency of an animal to aggregate with others such that the animals are in contact with one another, or are nearly so, and that the distribution of the animals in the local environment is extremely patchy” (Vulinec, 1990). Mixed-Species Aggregations in Harvestmen on groups formed by individuals of the same species (reviewed in Ward and Webster, 2016), there is increasing evidence that aggregations composed of two or more species are common across the animal kingdom, including birds, mammals, fish, and arthropods (Rasa, 1990; Heymann and Buchanan-Smith, 2000; Zamon, 2003; Boulay et al, 2019; Goodale et al, 2019, 2020). We explore the less-studied question of whether the benefits and costs experienced by individuals in single-species aggregations (SSA) apply to mixed-species aggregations (MSA). We use our empirical findings to propose future directions to study group living among heterospecific individuals
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