The Use of Progestins to Control Unwanted Male Behavior in Bottlenose Dolphins (Tursiops truncatus)
IAAAM 2019
Thomas H. Reidarson1*; Mauricio Francia Rivero2
1Reidarson Group: Marine Animal Specialists NV, Netherlands Antilles and San Diego, CA, USA; 2Dolphin Explorer, Punta Cana, Dominican Republic


For as long as male bottlenose dolphins have been under the human care, controlling male aggression including fighting, raking, and uncooperative behaviors has been difficult, especially in the presence of females. Solutions include behavioral modification, separation, and the use of the synthetic progestin, Megace (Megesterol acetate).

Common side effects Megace in humans are weight gain, weakness, adrenal suppression, diabetes, carcinogenesis, and immune suppression causing urinary tract infections and pneumonia.1 Many of these side effects have also been observed in dolphins, but when used in low doses for short periods of time, the side effects are minimal. Unfortunately, since the effect is dose dependent, so is the benefit of Megace.

Another synthetic progestin used extensively in cetaceans to control estrous is Regu-Mate (Altrenogest, Merck Animal Health). In most instances dolphins, mares, and pigs have been treated for months to even years without any apparent or reported side effects. With the understanding that progestins are commonly used with transgender human males2 with minor risks, we undertook a preliminary clinical trial with the use of Regu-Mate in nine immature and four mature male bottlenose dolphins.

Within a week of administering Regu-Mate at the standard dose used to contracept female dolphins (0.044 mg/kg PO SID), all males appeared much calmer, while raking and fighting diminished significantly. Each were treated for three months during the early winter breeding season and the only physiologic change was a 10–20% reduction in the mid diameter of only the mature males’ testicles, with no alteration of testosterone blood levels.

None of the dolphins became ill and the BBIs (body blubber indexes) of each individual (including females) increased by an average of 8%. Since the physiques of each dolphin improved (with some well above minimum standards), one cannot conclude it was due to Regu-Mate, however, the fact that heavier mature males were more cooperative lends more credence to the positive effect of this progestin on behavioral modification.

Although the study group and period of treatment was small Regu-Mate unequivocally reduced aggression without any untoward effects. Future studies will include effects on sperm production, libido, and even perhaps the effect of known androgen blockers such as Spironolactone and Finsteride.3 Although it would be interesting to study the effect of estrogenic compounds on these males we believe the number and severity of side effects makes these hormones as risky as the chronic use of high dose Megace.


The authors are grateful to the owner, Mr. Regino Alvarez, and the CEO, Mr. Luis Jose Mendez, of Dolphin Explorer, Punta Cana, Dominican Republic for support in this project. We are also grateful to lead trainers, Isreal Gonzolez Correa and Nikola Roxanna Muzika and staff trainers at Dolphin Explorer.

*Presenting author

Literature Cited

1.  Megestrol—Oral (Megace) side effects, medical uses, and drug interactions.

2.  Primary symptomatic adrenal insufficiency induced by megestrol acetate. 2013. Delitala, AP, Franciulli G, Maioli, M, Piga, G, and Delitala, G. Neth J Med;71(1):17–21.


4.  Hormone therapy for transgender patients. Unger, CA. 2016. Transl Androl Urol; 5(6): 877–884.

5.  Guidelines for the primary and gender affirming care of transgender and gender nonconforming people: Overview of feminizing hormone therapy. Deutsch, MB. 2018.


Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Thomas H. Reidarson
Reidarson Group: Marine Animal Specialists NV
Netherlands Antilles and San Diego, CA, USA

MAIN : Session 5: Innovations in Marine Mammal Medicine : Progestins for Unwanted Male Behavior in Dolphins
Powered By VIN