Reproductive Success of Free-Ranging Bottlenose Dolphins: Experience, Size, and Who You Know Make a Difference
By Randall Wells, Deborah Duffield (Portland State University), Michael Scott, Blair Irvine, Sue Hofmann, Stephanie Nowacek, Edward Owen, Caryn Owen, and Aleta Hohn (NOAA Fisheries)

Knowledge of the factors underlying bottlenose dolphin reproductive success is important to conservation for evaluating and projecting the dynamics of populations, as well as for understanding the evolution of their complex social structure. Collecting such information in the wild is challenging because female reproduction spans more than four decades, and behavioral cues of paternity are largely absent. In Sarasota Bay, Florida, > 30 years of observations and sample collection for genetic tests and age determination have provided an opportunity to evaluate reproductive success within a resident bottlenose dolphin community.

Calf survivorship is related to mother’s age and experience. Calf success was measured for 62 mothers (172 calves), relative to first year survival (74%), survival to normal separation age (3 yrs, 60%), and survival post-separation (47%). Mothers < 10 years old had the lowest calf survivorship, mothers 11-40 years old were intermediate, and mothers > 40 years old were most successful. Primiparous (first-time) mothers experienced poor success, with only 40% of calves surviving the first year, and 12% surviving three years or after separation. In contrast, > 70% of calves of multiparous (experienced) mothers survived their first year, > 60% survived three years; half were identified post-separation.

Male breeding success varies with age, size, and male pair bonding. Blood samples from 62 mother/calf pairs and 47 potential sires were used for paternity exclusion tests. Genetic exchange between communities is significant. Monogamy is not a feature of the Sarasota Bay dolphin community reproductive strategy. Resident males 14 - 41 years old were identified as sires. Body size correlates with breeding success for young males. Both unpaired and paired males bred, and within pairs both males can be sires. Paired males sire disproportionately more calves than unpaired males, suggesting one advantage to this unusual social formation. Many resident males apparently have not bred within the community, so the effective population size is considerably less than the number of residents, a difference of importance when considering the consequences of natural or anthropogenic impacts on resident dolphin communities. Paternity testing is continuing as more matched samples become available and can be linked with behavioral observations (see E. Owens, below). This work was funded by the Disney Wildlife Conservation Fund, the Harbor Branch OI Protect Wild Dolphins Program, and by the Chicago Zoological Society.