George W. Uetz Professor Ph.D., University of Illinois Animal Behavior, Behavioral Ecology, & Arachnology

Colonial web-building spider project.

             A central question in the study of animal behavior concerns why some species live in groups while others remain solitary.  The study of animal sociobiology is an enormous field, and integrates theory and research on animal social groups as diverse as ant colonies, bird flocks and herds of antelope.  The goal of this project is to understand the ecological and behavioral mechanisms underlying the evolution of social behavior in an atypical social animal – spiders that join webs together in colonies.

             While most of the world’s 37,000 spider species live solitarily, a small minority (about 50 species from a dozen or so families) live in groups and sometimes even cooperate in web-building, prey capture and/or brood care. These spiders are "an exception to the rule" among most (typically asocial) spiders, and provide a unique model system for testing hypotheses about the evolution of sociality (see reviews in Uetz & Hieber 1997; Uetz 2002). Our work is primarily with spiders in several species in the genus Metepeira (Araneidae), which exhibit varying degrees of social living. Colonial web-building in Metepeira incorporates both social behavior (web attachment, shared construction and maintenance of the silk colony framework) and solitary behavior (individual orb web construction, individual prey capture and feeding, territorial defense of web space).

            Different species of Metepeira are found in temporary or permanent colonies, depending on annual climatic factors and prey availability of their habitats. 

Metepeira incrassata - Tropical rainforest/agricultural
Metepeira spinipes - Riparian open field, woodland and coastal shrubs
Metepeira atascadero - Desert grassland

            Variation in grouping tendency in these Metepeira species is an example of risk-sensitivity in foraging (Uetz 1988; 1996; Caraco et al.1995).  Colonial web-building in orb weavers like Metepeira may be advantageous because of the "Ricochet effect" (Uetz 1989), where prey escaping from one web are deflected onto others, resulting in higher rates of prey capture.  Because foraging in groups also decreases prey capture variance, spiders in prey-rich habitats tend to forage in groups (risk-averse behavior) to take advantage of a steady food supply.  Foraging in groups is less advantageous in prey-poor habitats, however, as reduced variance with low mean prey availability would result in starvation.  In habitats where prey are scarce, Metepeira tend to forage solitarily (risk-prone behavior).

            Much of our previous work has been focused on Metepeira incrassata from tropical Veracruz, and the balance between ecological costs and benefits - including prey resource variance (Uetz 1988; 1996; Caraco et al.1995), predators and parasitoids (Rayor & Uetz 1990, 1993; Hieber & Uetz 1990; Uetz & Hieber 1994) and web-invading spider species (Hodge & Uetz 1996). Recent work (with Dr. Linda Rayor of Cornell University and Dr. Elizabeth Jakob of the University of Massachusetts) has addressed the influence of social and ecological factors on the web-placement decisions of individuals that create the structure of colonies (Jakob et al. 1998; Rayor & Uetz 2000). We are also involved in using these data for development and testing of dynamic optimization models (with Dr. Jakob)

 A special note:  Dr. Craig Hieber of Saint Anselm College, long-time collaborator on this project, and dear friend, passed away suddenly and unexpectedly on July 18, 2002. His contribution to this work was substantial, but immeasurable, as he was involved in every aspect – the genesis of ideas, the design of unique experiments, construction of field cages, observation, data collection, and manuscript preparation. Our work continues in his absence, but his participation is irreplaceable. 

Work currently in progress includes a finer-grained analysis of behavioral interactions between spiders and their predators (with the late Dr. Hieber, Dr.Stim Wilcox of Binghamton University, and former student Jay Boyle). Colonial web-building spiders are vulnerable to predation from several species of predators (wasps and flies). Selection pressure from these natural enemies has resulted in evolution of a fascinating array of anti-predator behaviors.

Spider defenses against wasps – the “Early Warning Effect”
The spider and fly revisited – ploy-counterploy behavior in a unique predator-prey system.

Supported by the the National Science Foundation and the National Geographic Society

             Note: While we are currently still analyzing data collected in previous studies, we have not returned to Mexico for several years.  Sadly, we lost our lease on the coffee plantation that served as our study site, as agricultural “terraforming” for new coffee varieties has eliminated the existing habitat (one more reason to support shade-grown organic coffee!). The good news is that we have moved our sphere of operations to California, where populations of colonial web-building M. spinipes are currently under study (see El Niño Spider Project).

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 Research: 1. Wolf Spider Research | 2. Colonial Web-Building Spiders Research | 3. El Niño Influence on Colonial Web-Building Spiders. |  4. Impact of a Ecosystem Disturbance on Spider Populations and Communities