“…However, recent studies with a synthetic progestin, levonorgestrel, have demonstrated it to be a safe, nonpathologic compound in several species [Baldwin et al, 1994;Bettinger et al, 1997;Looper et al, 2001;Nisker et al, 1988;Tell et al, 1994;Wise et al, 1980]. Levonorgestrel's modes of contraceptive action include anovulation, deficient luteal phases, suppression of follicular growth, and alterations to cervical mucus [Croxatto et al, 1981[Croxatto et al, , 1982[Croxatto et al, , 1988Brache et al, 1985Brache et al, , 1990Brache et al, , 1999Nisker et al, 1988;Ratsula et al, 1989;Faundes et al, 1991;Lemus et al, 1992;Baldwin et al, 1994;Darney, 1994;Telleria et al, 1994;Kuhnz et al, 1995;Shirly and Bundren, 1995;Bettinger et al, 1997;Tell et al, 1999]. Levonorgestrel exhibits strong progestational effects and lacks estrogen-like activity [Lemus et al, 1992] which may promote the occurrence of species-typical behavior [e.g., genital swelling in chimpanzees; see Bettinger et al, 1997 for a complete review].…”