1. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA.
2. Max Planck Institute for Physics of Complex Systems, Nöthnitzer Strasse, 01187 Dresden, Germany.
3. School of Life Sciences, John Maynard Smith Building, University of Sussex, Falmer, Brighton BN1 9QG, UK.
4. Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA.