Meeting Abstract
Animal colonies range from loose aggregations of largely unrelated individuals to tightly organized, closely related groups with reproductive division of labor. A key characteristic influencing this variation is the number of individuals colonies contain. Although factors affecting colony size have been explored, to date there have been relatively few studies exploring how colony size affects the temperature and performance of colonies as a whole, with performance gauged by growth and mortality rates. Using the western tent caterpillar, Malacosoma californicum pluviale, I examined the effects that different colony sizes had on tent temperatures and larval growth and mortality rates. Tent caterpillars build communal silk tents, which act as an extended phenotype and that may allow them to modify their local microclimates. I predicted that large colonies would build larger, warmer tents more rapidly, allowing larvae to grow faster in cool spring conditions. To test this prediction, I manipulated colonies of the western tent caterpillar (10, 50, 100, 150 individuals) at a site in Condon, MT, and examined the differences in growth rates, mortality rates, and tent temperatures. Larvae from larger colonies grew to their final instar 30% faster than did those from smaller colonies, most likely because their tents reached temperatures up to 22C higher than air temperatures, 15C higher than the smaller colonies. Also, no larvae from smaller colonies but 16% of larvae from larger colonies survived to pupation. Together, these results indicate positive selection on colony size in the wild. I discuss several life-history constraints, and potential opposing selective factors, that may prevent the evolution of larger size.
