Effects of GnRH agonists on testosterone and testosterone‐stimulated parameters for contraception and aggression reduction in male lion‐tailed Macaques (Macaca silenus)

Managing social groups in zoos requires controlling reproduction in individuals that do not have a current breeding recommendation, while simultaneously maintaining social harmony and animal well‐being. Contraceptives, such as gonadotropin releasing‐hormone (GnRH) agonists, that suppress testosterone production, offer a potential solution. They achieve infertility by interrupting spermatogenesis and may ameliorate androgen‐induced aggression. This study investigated the effects of two GnRH agonists, histrelin and deslorelin, on testosterone, testis size, body weight and sperm production in male lion‐tailed macaques, along with subjective observations of aggressive behavior. Five trials at three institutions with 14 males demonstrated that 100 mg histrelin or 9 to 12 mg of deslorelin could at least temporarily reduce testosterone, but a lower 6 mg dose was ineffective. However, ability of deslorelin to produce azoospermia varied among males, even at the highest dose. In general, a higher dose was needed (1) to achieve than to maintain suppression of any measured parameter and (2) to suppress sperm production than testosterone concentration. Testosterone production was also more likely than sperm production to recover, suggesting possible damage to seminiferous tubules but not to Leydig cells. Aggressive behavior was reduced in all but the group receiving the lowest dose. This allowed social groups to be maintained for many years despite recovery of testosterone in some males, suggesting that new social roles had been learned and become independent of androgen influence. Gonadotropin releasing‐hormone agonists' ability to reduce testosterone, spermatogenesis and associated physical parameters was dose‐dependent and not reversible in all individuals. Even transitory suppression of testosterone was accompanied by sustained reductions in aggression that allowed all‐male social groups to be maintained. RESEARCH HIGHLIGHTS Gonadotropin releasing‐hormone agonists histrelin and deslorelin reduced testosterone, sperm production and aggression in lion‐tailed macaques in a dose‐dependent manner. Testosterone was more likely than spermatogenesis to recover, making return to fertility uncertain.