# Effect of Long-Term Contraception with Altrenogest in Dolphins (Tursiops truncatus)

## TL;DR

This study examines the long-term effects of using Altrenogest as a contraceptive in bottlenose dolphins, finding it effective but linked to reproductive health issues.

## Contribution

The study provides new insights into the long-term effects of Altrenogest in dolphins, highlighting its contraceptive efficacy and associated health risks.

## Key findings

- Altrenogest effectively suppresses reproduction in dolphins by lowering progesterone levels.
- Prolonged use of Altrenogest is associated with reproductive disorders like pyometra and follicular cysts.
- The drug inhibits LH secretion but allows some follicular activity to persist.

## Abstract

In zoological facilities, controlling reproduction in bottlenose dolphins is essential to ensure animal welfare and appropriate population management. Altrenogest is a synthetic hormone widely used to prevent pregnancy in female dolphins, but information on its long-term effects is still limited. In this retrospective study, we evaluated the impact of prolonged Altrenogest administration in 18 female bottlenose dolphins housed in four zoological parks. Hormonal monitoring, ultrasound examinations of the reproductive system, and behavioral observations were analyzed over several years. The treatment effectively suppressed reproductive behavior and ovulation, as indicated by consistently low progesterone levels. However, dolphins receiving long-term Altrenogest treatment showed a higher occurrence of reproductive disorders, including uterine infections (pyometra) and ovarian follicular cysts, compared with untreated animals. These findings suggest that although Altrenogest is an effective contraceptive method, its prolonged use may increase the risk of reproductive health problems. Careful evaluation of treatment duration and regular clinical monitoring are, therefore, crucial to ensure the long-term health and welfare of dolphins under human care.

Altrenogest is a synthetic progestin used as a contraceptive method in various animal species, including bottlenose dolphins. This is a retrospective study based on the analysis of data collected between 2020 and 2025. Eighteen female dolphins (Tursiops truncatus) from four zoological parks on the Iberian Peninsula were included, treated with the progestin for variable periods. The animals were monitored through ultrasound examinations, behavioral observations, and hormonal assays for progesterone and estrogen. The results showed statistically significant changes in hormone levels, with a significant decrease in serum progesterone and a significant increase in estrogen levels in treated dolphins compared to untreated ones. These findings suggest that Altrenogest inhibits LH secretion, while it does not appear to directly affect FSH, allowing some degree of follicular activity to persist. Despite its demonstrated contraceptive efficacy, prolonged use of the drug was associated with the development of pyometra in four individuals and follicular cysts in three individuals. These results highlight the effectiveness of Altrenogest in reproductive management of dolphins, while also emphasizing the need for careful evaluation of treatment duration and the potential risk of long-term adverse effects.