No CrossRef data available.
Published online by Cambridge University Press: 11 May 2010
Authors' note At the time this paper was presented at the Quito symposium, it was already accepted for publication in American Naturalist, and consequently it has not been possible to reproduce it in full here. To retain it in some form for this volume, at the request of the editors F.G.S. drafted the version below, which represents an extended Spanish summary; however, for the full text, data and complete analysis it is necessary to consult the original article (Levey and Stiles 1992).