Peter Ashwin1, Alexey Zaikin2. 1. Centre for Systems, Dynamics and Control, University of Exeter, Exeter, United Kingdom. 2. Institute for Women's Heaalth, University College London, London, United Kingdom; Department of Mathematics, University College London, London, United Kingdom; Lobachevsky State University of Nizhniy Novgorod, Nizhniy Novgorod, Russia. Electronic address: rmjbale@live.ucl.ac.uk.