An investigation of the possible role of kin selection in the evolution of sociality in Agelena Consociata denis: a west African social spider

While most spiders are territorial and very competitive, Agelena consociata Denis, a West African social spider found in rainforest habitats in Gabon, is highly cooperative. Agelena consociata live together in nests which are often found clustered into colonies. This study involves an investigation of the possible role of kin selection in the evolution of social behavior in this spider through an examination of its dispersal ability and the genetic relatedness of nest and colony members.

The dispersal of Agelena consociata was investigated during three two month field seasons at the Institut de Recherche en Ecologie Tropicale (I.R.E.T.) near Makakou, Gabon. Observations of marked individuals indicate that there is free movement of individuals between nests of the same colony. Long distance dispersal of A. consociata was examined using sticky traps to assess the extent of aerial dispersal and release and observation of marked spiders to determine their probability of successful ground dispersal. Results of the sticky trap study indicate that these spiders do not disperse aerially. Results of the ground release experiment demonstrate a low probability of successful ground dispersal due to invertebrate predators. The possibility of passive long distance dispersal via animal carriers was examined by pushing an artificial mammal and bird through A. consociata nests, carrying the artificial animals for 60 meters, and daily censusing of the 60 meter transects for the formation of new A. consociata nests. The results indicate that it is 11 possible for this spider to be successfully dispersed by animal carriers.

Genetic relatedness of Agelena consociata nest and colony mates was analyzed using horizontal starch gel electrophoresis. A total of over 1200 spiders from 67 nests were assayed at 22 loci. The proportion of loci found to be polymorphic was 0.381, while the mean heterozygosity per locus was 0.018. Wright's F-statistics, Nei's (1978) genetic distance coefficient, and the G-test of genetic heterogeneity all indicate that there is a high degree of genetic difference between A. consociata nests representing different colonies, while there is little genetic difference between nests of the same colony. The degree of inbreeding within nests remains unclear due to the wide discrepancy in the values of Wright's F IS statistic for the different polymorphic loci. The mean genetic relatedness of individuals within the same colony is 0.5226.

The lack of long distance dispersal probably accounts for the observed high degree of relatedness between Agelena consociata colony members. Kin selection, which requires genetic relatedness between interacting individuals is, therefore, a plausible selective mechanism for the evolution of cooperative behavior in this species. Agelena consociata appears to best fit the description of the demography of populations in which the 'new group selection' (family-structured populations) may play an evolutionary role.