“…For this reason, the existence and uniqueness of positive cycles and limit cycles of system (1) has attracted much interest in recent years. For a sample of these studies, see Cheng [1], Cheng et al [2], Kuang and Freedman [12], Huang and Merrill [8], Ding [3], Kuang [11], Kooij and Zegeling [10], Sugie et al [19], Sugie [17], Moghadas [14], and Xiao and Zhang [21]. Most of the recent work has employed technical methods, such as transforming the system to an equivalent generalized Liénard system or trying directly to exploit the special structure of the limit cycle and the prey isocline.…”