Fostering model explains variation in levels of sociality in a spider system

We developed an 'assured fitness returns' model for the evolution of sociality independent of high relatedness within colonies. We first developed this model based on parameters from the northern social spider Anelosimus studiosus, which shows a higher incidence of multiple-female colonies caring for a common brood at higher latitudes and colder conditions within latitudes. The mathematical model developed predicts that multiple females will cooperatively care for a brood in those environments in which a single female has a high probability of dying before her offspring are able to care for themselves. The frequency of multiple-female nests observed in variable temperature sites in eastern Tennessee, U.S.A., is consistent with that found in our latitudinal censuses: a higher frequency of multiple-female colonies was present at cold-water sites than at warm-water sites. A test of a critical model assumption found a positive correlation of temperature and juvenile development rate in both the field and the laboratory. Colony success in field studies was also consistent with model predictions.