Use of Depot Leuprolide, Cyproterone, and Deslorelin to Control Aggression in an All Male California Sea Otter (Enhydra lutris nereis) Colony
IAAAM 1999
Paul P. Calle1; Bonnie L. Raphael1; Robert A. Cook1; Catherine McClave2; Jo Anne Basinger2; Hans Walters2
1Wildlife Health Sciences, Wildlife Conservation Society, Bronx, NY, USA; 2Aquarium for Wildlife Conservation, Wildlife Conservation Society, Brooklyn, NY, USA


Male marine mammals in zoos and aquariums are sometimes maintained alone or in same sex groups for behavioral demonstrations, to prevent breeding, control aggressive behavior, or retain animals for future reproduction. These management schemes may be complicated by undesirable behaviors (aggression, anorexia, and loss of operant training). While castration minimizes these problems, anesthesia and surgery are required and it eliminates future reproduction.

Leuprolide acetate is an injectable, long acting, reversible, gonadotropin-releasing hormone (GnRH) agonist. Administration results in an initial increase in follicle stimulating hormone (FSH) and luteinizing hormone (LH), causing transiently elevated testosterone. A progressive suppression then occurs, reducing circulating testosterone levels to those of a castrate. Leuprolide acetate has been demonstrated to suppress testicular function in Atlantic bottlenose dolphins (Tursiops truncatus).2,3 Cyproterone is an orally administered testosterone receptor blocking agent. In addition to blocking androgen receptors, it exerts a negative feedback on the hypothalamic pituitary axis, inhibiting LH secretion. Consequently, it suppresses testicular testosterone production. Deslorelin is a GnRH agonist in a long acting sustained release implant.

Four male California sea otters (Enhydra lutris nereis) were exhibited together at the Aquarium for Wildlife Conservation. As they matured, intraspecific aggression developed until eventually they could no longer be maintained as a compatible colony. During this study (July 1996 to December 1997, and March to December 1998) parenteral leuprolide acetate in either a 1 month (phase 1) 1,4 or 4 month (phase 2) 5 depot suspension, or oral cyproterone acetate (phase 3) 5, were administered in succession to control aggression. In phase 4, deslorelin implants were inserted subcutaneously (March 1998) 3 months after completion of phase 3.

Four otters were included in phase 1. 1,4 One otter died of causes unrelated to the study and the remaining three otters were included in phases 2 5, 3 5, and 4. Ages and weights of otters ranged from 3.5 to 7 years, and 20 to 34 kg, over the course of the study. Otters were manually confined in an otter restraint device for venipuncture and all injections.6

During phase 1 1,4 otters received 3.75 mg (0.11 to 0.19 mg/kg) of a 1 month depot formulation of leuprolide acetate IM in a rear limb at monthly intervals. Initial drug administration was staggered, with otters receiving 3 to 7 doses in 6 months. In phase 2 5 otters were given 30 mg (0.9 to 1.1 mg/kg) of a 4 month depot formulation of leuprolide acetate IM or SQ in the dorsal surface of a pelvic limb twice at intervals of 3.5 to 4 months. In phase 3 5 otters were given 50 or 75 mg (1.5 to 2.3 mg/kg) cyproterone acetate PO SID for 84 days. In phase 4 otters received a single 6 mg (0.18 to 0.23 mg/kg) deslorelin implant subcutaneously over the hip region.

Testicular lengths of 2 otters were measured before the initial treatment; at the time of deslorelin implant insertion; and at the end of each phase.1,4,5 Marked testicular atrophy occurred and was maintained in all phases. Pretreatment testicular length (mean ± SD) (53 ± 3.5 mm) was significantly greater (Paired t-test; p<0.05) than at the end of each phase (35 ± 2.5 mm, 27.5 ± 2.5 mm, 26.8 ± 2.6 mm, and 26 ± 1 mm for phases 1, 2, 3, and 4 respectively). Mean testicular length in phase 1 was significantly greater (Tukey Honest Test, p<0.05) than in phases 2, 3, and 4; there was no significant difference in testicular length between phases 2 and 3. Mean testicular length in phases 2 and 3 were significantly greater than in phase 4. There was no significant difference (Paired t-test, p>0.05) in testicular length (mean ± SD) pretreatment (53 ± 3.5 mm) compared to length at the initiation of deslorelin treatment (47.3 ± 2.75 mm), indicating that testicular atrophy induced by treatment with depot leuprolide and cyproterone was reversible.

Testosterone levels were determined for all otters by radioimmunoassay before treatment.

Determinations were made monthly in phase 1; 1,4 3 to 5 times at 4 to 6 week intervals in phase 2;5 2 to 3 times 4 to 12 weeks after initiation of treatment in phase 3; 5 at the beginning of phase 4; and every 3 months in phase 4. Pretreatment testosterone levels of the youngest otter were below the assay detection limit (<0.05 ng/ml), while values for the other two ranged from 0.31 to 2.28 ng/ml. Testosterone levels in phase 1 decreased after one month of treatment (<0.05 to 0.19 ng/ml), and after two months of treatment all were undetectable. In phase 2, all testosterone levels except one (0.7 ng/ml) were below the assay detection limit. In phase 3 testosterone was below the assay detection limit on all sampling dates. Levels at the beginning of phase 4 were below the detection limit in one otter, and values for the other two were 0.69 (the youngest otter) and 2.52 ng/ml. At all subsequent sampling dates in phase 4 all values were below the assay detection limit.

Routine hematologic and biochemical assays were performed pretreatment, at the time of deslorelin implantation, and at the end of each phase. There were statistically significant differences (Paired t-test; p<0.05) between pretreatment and post-treatment mean values in each phase. Interpretation of the importance of these results was complicated by the limited number of otters in the study, artifacts, which may have resulted from varying degrees of sample hemolysis, and interlaboratory variation. Additional complicating factors included changes resulting from growth and maturation of the otters throughout the study, consecutive treatment phases, and possible effects of physical restraint. At the end of phase 1, white blood cell, monocyte, lymphocyte, and band neutrophil counts were higher than pretreatment values. At the end of phase 2, globulin values were higher, and BUN and chloride values were lower, than pretreatment values. At the end of phase 3, total protein and albumin values were higher than pretreatment values. At the end of phase 4, monocyte and lymphocyte counts were lower than pretreatment values. In addition, total protein, globulin, total bilirubin, and sodium values were all higher than pretreatment values. Values which may have resulted from acute and chronic reactions to leuprolide injections (total and differential WBC counts, total protein, albumin, globulin) were consistent with inflammation. The majority of the changes were not clinically relevant, despite the statistically significant differences, and most were within the range of values reported for clinically normal, immobilized, free ranging California sea otters.7

Despite the effectiveness of leuprolide at inducing testicular atrophy and testosterone suppression, with consequent reduction in intraspecific aggression, treatment was discontinued due to adverse injection site reactions.1,4,5 These consisted of anorexia and depression with moderate to marked injection site lameness, swelling, or sterile abscesses. Preventing or treating these reactions with either diphenhydramine (1.5 mg/kg PO BID for 7 days) or flunixin meglumine (0.9 mg/kg PO SID for 5 days) was not successful while carprofen (1.5-2 mg/kg PO BID for 5 to 10 days) was a more effective treatment. Some otters were also treated with trimethoprim sulfa (33.6 mg/kg PO SID for 10 days).

Administration of depot leuprolide acetate, cyproterone acetate, or deslorelin was successful in suppressing testosterone and controlling aggression in an all-male sea otter colony. This enabled otters to be maintained in a more compatible social group. Leuprolide and deslorelin were equally effective, and more effective than cyproterone, in controlling aggression. Compared to castration, advantages of these medications include no requirement for anesthesia or surgery and potential reversibility. Disadvantages of leuprolide are the drug cost, injection site reactions, and the necessity for frequent animal handling. Cyproterone acetate is currently not commercially available in the United States but offers the advantage of oral administration. Deslorelin implants, while currently not commercially available in the United States, are undergoing clinical trials and may be more widely available in the future. These treatments have potential application for testosterone suppression and control of male associated undesirable behaviors in sea otters in zoos and aquariums.


The authors are grateful to TAP Pharmaceuticals for generous donation of depot leuprolide acetate; Dr. Wolfgang Jochle, Feracon LLC and Dr. T.E. Trigg, Peptech Animal Health Pty Ltd for provision of deslorelin implants; Karen Drayer and the PKD Trust for financial support; Dr. Sharon Deem for assistance with statistical analyses; and veterinary technicians and animal keepers for their expert technical assistance.


1.  Basinger JA, H Walters, PP Calle, DA Thoney, C McClave, M Hall. 1997. Efforts to reduce dominant and aggressive behavior in an all male California sea otter (Enhydra lutris nereis) colony by the use of testosterone suppressing drug therapy. Proceeding of the International Marine Animal Trainers Association, P. 24.

2.  Briggs MB, W Van Bonn, RM Linnehan, D Messinger, C Messinger, S Ridgway. 1995. Effects of leuprolide acetate in depot suspension on testosterone levels testicular size and semen production in male Atlantic bottlenose dolphins (Tursiops truncatus). Proceedings of the International Association of Aquatic Animal Medicine, Pp. 112-114.

3.  Briggs MB, D Messinger, C Messinger, RM Linnehan, W Van Bonn, S Ridgway, G Miller. 1996. Effects of leuprolide acetate in depot suspension on testosterone levels testicular size and semen production in male Atlantic bottlenose dolphins (Tursiops truncatus). Proceedings of the American Association of Zoo Veterinarians, Pp. 330-333.

4.  Calle PP, MD Stetter, BL Raphael, RA Cook, C McClave, JA Basinger, Walters H, K Walsh. 1997. Use of depot leuprolide acetate to control undesirable male associated behaviors in the California sea lion (Zalophus californianus) and California sea otter (Enhydra lutris). Proceedings of the International Association of Aquatic Animal Medicine, Pp. 6-7.

5.  Calle PP, C McClave, JA Basinger, H Walters, BL Raphael, RA. Cook. 1998. Use of depot leuprolide and cyproterone to control aggression in an all male California sea otter (Enhydra lutris neries) colony. Proceedings of the American Association of Zoo Veterinarians and the American Association of Wildlife Veterinarians Joint Conference, Pp. 375-377.

6.  Williams TD, DM Baylis, SH Downey, RO Clark. 1990 . A physical restraint device for sea otters. Journal of Zoo and Wildlife Medicine 21(1):105-107.

7.  Williams TD, LT Pulley. 1983. Hematology and blood chemistry in the sea otter (Enhydra lutris). Journal of Zoo and Wildlife Medicine 19(1): 44-47.

Speaker Information
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Paul P. Calle, VMD, DACZM
Wildlife Health Sciences, Wildlife Conservation Society
Bronx, NY, USA

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