COLONY SIZE AND INDIVIDUAL FITNESS IN THE SOCIAL SPIDER ANELOSIMUS-EXIMIUS

The effects of colony size on individual fitness and its components we re investigated in artificially established and natural colonies of th e social spider Anelosimus eximius (Araneae: Theridiidae). In the trop ical rain forest understory at a site in eastern Ecuador, females in c olonies containing between 23-107 females had a significantly higher l ifetime reproductive success than females in smaller colonies. Among l arger colonies, this trend apparently reversed, This overall fitness f unction was a result of the conflicting effects of colony-size on diff erent components of fitness. In particular, the probability of offspri ng survival to maturity increased with colony size while the probabili ty of a female reproducing within the colonies decreased with colony s ize. Average clutch size increased with colony size when few or no was p parasitoids were present in the egg sacs. With a high incidence of e gg sac parasitoids, this effect disappeared because larger colonies we re more likely to be infected. The product of the three fitness compon ents measured-probability of female reproduction, average clutch size, and offspring survival-produced a function that is consistent with di rect estimates of the average female lifetime reproductive success obt ained by dividing the total number of offspring maturing in a colony b y the number of females in the parental generation. Selection, therefo re, should favor group living and intermediate colony sizes in this so cial spider.