Use of a GnRH agonist to suppress testosterone in wild male Hawaiian monk seals (Monachus schauinslandi).

A gonadotropin-releasing hormone (GnRH) agonist, d-Trp6-LHRH, was evaluated for its effectiveness in reducing circulating testosterone concentrations in wild Hawaiian monk seals (Monachus schauinslandi). Twenty-eight adult male seals were randomly divided into three groups: 10 were captured and treated with 7.5 mg of the GnRH agonist, 9 were captured but did not receive the agonist, and 9 were captured near the end of the study to serve as handling controls. Blood samples were taken from all 28 seals. From 14 to 58 days after initial capture, 8 of the treated seals and 8 of the untreated seals were recaptured and a second sample of blood was taken. For comparison, blood was also collected from 4 captive adult male seals during the same months as the field study. In the treated group, the agonist induced a significant decline in mean circulating testosterone concentrations, from 1.01 ng/ml (first sample) to 0.21 ng/ml (second sample, taken approximately 38 days later). In the untreated group, mean testosterone concentrations of the first and second samples were statistically indistinguishable (1.11 vs 1.16 ng/ml). The mean concentration of the untreated group (second sample) was also indistinguishable from the mean concentration of seals in the control group (1.16 vs 0.82 ng/ml). Also, mean testosterone concentration in the initial samples from the four captive seals was not statistically different from that of untreated wild seals (1.38 vs 1.11 ng/ml). These results suggest that (1) the GnRH agonist suppresses the production of testosterone in wild adult male Hawaiian monk seals, (2) a single handling of adult male seals does not affect their testosterone levels, and (3) testosterone concentrations in captive male seals appear to be consistent with concentrations in wild seals. Further evaluation of this GnRH agonist is necessary before it is used in the management of this endangered species, but these results suggest it may be a useful tool for reducing mortality of monk seals from adult male aggression related to reproduction and mating behavior.