[1] Univ Kent, Inst Math Stat & Actuarial Sci, Kent CT2 7NF, England
[2] CNRS, Ctr Ecol Fonct & Evolut, F-34293 Montpellier 5, France
[3] Johns Hopkins Univ, Dept Biostat, Baltimore, MD 21205 USA
[4] CNRS, UPR 1934, Ctr Etud Biol Chize, F-79360 Villiers En Bois, France