Benefits of Group Living Include Increased Feeding Efficiency and Lower Mass Loss during Desiccation in the Social and Inbreeding Spider Stegodyphus dumicola

Abstract

Group living carries a price: it inherently entails increased competition for resources and reproduction, and may also be associated with mating among relatives, which carries costs of inbreeding. Nonetheless, group living and sociality is found in many animals, and understanding the direct and indirect benefits of cooperation that override the inherent costs remains a challenge in evolutionary ecology. Individuals in groups may benefit from more efficient management of energy or water reserves, for example in the form of reduced water or heat loss from groups of animals huddling, or through reduced energy demands afforded by shared participation in tasks. We investigated the putative benefits of group living in the permanently social spider Stegodyphus dumicola by comparing the effect of group size on standard metabolic rate, lipid/protein content as a body condition measure, feeding efficiency, per capita web investment, and weight/water loss and survival during desiccation. Because energetic expenditure is temperature sensitive, some assays were performed under varying temperature conditions. We found that feeding efficiency increased with group size, and the rate of weight loss was higher in solitary individuals than in animals in groups of various sizes during desiccation. Interestingly, this was not translated into differences in survival or in standard metabolic rate. We did not detect any group size effects for other parameters, and group size effects did not co-vary with experimental temperature in a predictive manner. Both feeding efficiency and mass loss during desiccation are relevant ecological factors as the former results in lowered predator exposure time, and the latter benefits social spiders which occupy arid, hot environments.

Isolated individuals and groups show opposite preferences toward humidity

“…According to our hypothesis, for identical population sizes under WS or DS, this decrease of interactions leads to a higher probability of leaving from WS. Stated differently, as the population under DS increases, the probability of leaving decreases and the aggregate under this shelter compensates water losses caused by dryness (Vanthournout et al, 2016;Yoder and Grojean, 1997). In fact, the probabilities associated to the sheltering behaviour depend not only on the different humidity levels but also on the already sheltered population.…”

“…Most arthropods are highly sensitive to water losses and possess physiological (dryer feces, cuticular hydrocarbon) and behavioural (hygrotaxis, clustering) mechanisms to counterbalance them (Edney, 1951;Gibbs et al, 1997;Leather et al, 1995;Reynolds and Bellward, 1989;Vanthournout et al, 2016;Yoder and Grojean, 1997). Here, we test the preference between a dry shelter (DS) with a relative humidity (RH) of 40% and a wet shelter (WS) (RH 90%) offered to isolated individuals and groups of the American cockroach Periplaneta americana.…”

“…Most arthropods are highly sensitive to water losses and possess physiological (dryer feces, cuticular hydrocar-19 bon) and behavioural (hygrotaxis, clustering) mechanisms to counterbalance them [Edney, 1951, Gibbs et al, 20 1997, Leather et al, 1995, Reynolds and Bellward, 1989, Vanthournout et al, 2016, Yoder and Grojean, 1997. 21…”

Age and level of self‐organization affect the small‐scale distribution of springtails (Collembola)

“…, Vanthournout et al. ). The degree to which self‐organized behavior influences the spatial distribution of individuals depends on its relative dominance over sensitivity to environmental factors.…”

“…Species may also show "self-organized" spatial structuring (sensu Parrish and Edelstein-Keshet 1999), that is, a behavior not driven by specific habitat or environmental factors, but caused by internal factors within a population, such as responses on odor (pheromones), vocal or visual stimuli. Reasons for aggregation, not directly related to environmental variables, include reducing the risk for predation (Hamilton 1971), increasing reproductive success (Levitan and Young 1995), or improved ability to detect favorable patches (Leinaas et al 2015), as well as other benefits of social group living (Benoit et al 2009, Vanthournout et al 2016. The degree to which self-organized behavior influences the spatial distribution of individuals depends on its relative dominance over sensitivity to environmental factors.…”