(d) Sigmoid function c=a5/(1+ea6 ((N/A)−a7)) with fitting parameters a5, a6 and a7. This function has a minimum, constant number of contacts at low densities, after which contact rates increase with density until reaching a plateau when reaching a maximum number of contacts. This shape has been observed for multimammate mice contacts , and has been proposed previously as a biologically plausible function .