1. Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4K1 Canada, Departamento de Ciências Fisiológicas, Fundação Universidade Federal do Rio Grande, Rua Eng. Alfredo Huch 475, Rio Grande, Rio Grande do Sul, 96201-900 Brazil, and Department of Zoophysiology, August Krogh Institute, University of Copenhagen, Universitetsparken 13, DK-2100 Copenhagen, Denmark