Troy Day1, Peter A Abrams2. 1. Department of Mathematics and Statistics, Queen's University, Kingston, ON, K7L 3N6, Canada; Department of Biology, Queen's University, Kingston, ON, K7L 3N6, Canada. Electronic address: day@queensu.ca. 2. Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, M5S 3B2, Canada.