Abstract

The Florida manatee, Trichechus manatus latirostris, is an endangered marine mammal inhabiting Florida's marine and fresh waters. There is a great need for data regarding their poorly understood reproductive cycles in order to improve management strategies. Florida manatees are diffuse seasonal breeders, reproducing during warmer months, but rarely in colder months due to low temperatures and decreased food quality. Therefore, manatees should exhibit increased progesterone and estradiol concentrations during warmer months and decreased concentrations during colder months. Cortisol concentrations are predicted to be elevated during reproductive periods due to the intensity of mating herd events. It is also hypothesized that nursing will suppress the estrous cycle and weaning will reactivate it.

A female free-ranging manatee from Crystal River, Florida, was tagged and monitored over a two-year period. Behavioral data and fecal samples were collected, and progesterone and estradiol concentrations were measured using an enzyme immunoassay (EIA); progesterone CL425 (monoclonal UC Davis), intra-assay CV = 7.1%, inter-assay CV = 8.0% and estradiol R0008 (UC Davis), intra-assay CV = 4.9%, inter-assay CV = 6.2%. Average fecal progesterone concentrations during colder months (November - March; 349.4 pg/g, n = 7) were 1.5 fold lower (p = 0.08) than during warmer months (April - October; 537.2 pg/g, n = 15). Average fecal progesterone concentrations while nursing (364.3 pg/g, n = 16) were 2.1 fold lower (p = 0.02) than after weaning (779.3 pg/g, n = 6). Average fecal estradiol concentrations during colder months (71.38 ng/g, n = 7) were 1.4 fold higher (p = 0.19) than during warmer months (53.04 ng/g, n = 15). Average fecal estradiol concentrations while nursing (58.50 ng/g, n = 16) were 1.1 fold lower (p = 0.71) than after weaning (59.90 ng/g, n = 6). Fecal cortisol concentrations were measured using a radioimmunoassay (RIA); cortisol R1222, intra-assay CV = 5.9%, inter-assay CV = 8.5%. Average fecal cortisol concentrations during colder months (298.8 pg/g, n = 7) were 1.8 fold lower (p = 0.05) than during warmer months (549.8 pg/g, n = 15). Average fecal cortisol concentrations while nursing (359.4 pg/g, n = 16) were 2.1 fold lower (p = 0.14) than after weaning (764.5 pg/g, n = 6).

These findings support the hypothesis that a reproducing wild, female Florida manatee exhibits a trend of higher progesterone and cortisol concentrations during warmer months and lower concentrations during colder months; however, the estradiol data were not consistent with this trend. Additionally, the results support our preliminary data and the hypothesis that nursing suppresses reproductive hormones, and the event of weaning reactivates the estrous cycle.

Acknowledgements

Funding is provided through the Florida Fish and Wildlife Conservation Commission, Whitney Marine Laboratory for Marine Bioscience, and the University of Florida Aquatic Animal Health Program at the University of Florida College of Veterinary Medicine. Behavior and fecal samples were collected under UF IACUC number 200802158 and US Fish & Wildlife permit number MA038448-3. We thank USGS - Biological Resources Division, Sirenia Project, especially Dr. Robert Bonde and Susan Butler for their assistance with tagging and tracking these animals. The authors also wish to thank the Crystal River Wildlife Refuge for assisting in the tagging and tracking of the animals, and Allison Gopaul for assisting in fecal sample preparation and hormone extractions.