[1] Univ Penn, Dept Math, Philadelphia, PA 19104 USA
[2] Univ Penn, Ctr Math Biol, Philadelphia, PA 19104 USA
[3] Harvard Univ, Dept Phys, Cambridge, MA USA
[4] IST Austria, Klosterneuburg, Austria
[5] Max Planck Inst Evolutionary Biol, Max Planck Res Grp Dynam Social Behav, Plon, Germany