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Gisele Montano, Pat Clough, Todd Schmitt, Michelle Davis, Karen Steinman, Justine O’Brien, and Todd Robeck

To gain more knowledge about the influence of hormone regulation on follicle development, ovarian ultrasounds were performed, and urinary hormone profiles were determined in ovulating and non-ovulating female bottlenose dolphins (n  = 15) following estrus synchronization with altrenogest. Ovarian ultrasounds were conducted daily, post-synchronization to describe follicular recruitment in relation to the endocrine profile. Follicle sizes were grouped into very small (VSM), small (SM), medium (MD) and large (LG). In ovulating females, two follicular waves were identified, and follicular deviation toward establishing a dominant follicle only occurred during the second wave. For non-ovulating females, only the first wave was observed. For all urinary hormones, the non-ovulating group presented significantly lower concentrations of follicle-stimulating hormone (uFSH), luteinizing hormone (uLH), estrone conjugates (uE1-C) and estriol (uE3) but similar progestagen and cortisol concentrations compared to the ovulating group. Concentrations of uE1-C and uE3 and the numbers of MD and LG follicles significantly (P < 0.05) increased, while uFSH concentrations significantly (P < 0.05) decreased as ovulation approached. Urinary LH significantly increased concurrently with increasing numbers of LG follicles and decreasing numbers of SM follicles. The characterization of follicular development and its relationship with hormone assessment complements our understanding of follicular recruitment post-synchronization in bottlenose dolphins and provides new information concerning differences between ovulating and non-ovulating females in response to an estrous synchronization protocol.

Lay summary

Many species of fresh and saltwater dolphins and porpoises are facing increasing pressure for food and habitats due to human activities. One of the primary aspects that can be affected by such activities is reproduction. The bottlenose dolphin has been successfully bred in zoos and aquariums for at least 70 years, and they can be trained for several voluntary behaviors that enable medical examinations. Therefore, they represent a unique resource for research and understanding of normal dolphin reproduction. In this research, voluntary ultrasound exams and urine samples from 15 female dolphins were used to describe changes in their ovaries. The resulting descriptions and comparisons provide insight into the unique ovarian physiology of bottlenose dolphins and into the differences between animals after treatment. This information adds to the body of knowledge which one day may be used for developing advanced reproductive techniques to help preserve endangered species of dolphins.