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Most studies of seasonal reproduction in odontocetes have considered bottom-up forcing through seasonal changes in the physical environment and/or prey availability (Wells et al. 1987, Read 1990, Urian et al. 1996, Mann et al. 2000, McGuire and Aliaga-Rossel 2007,Westgate and Read 2007), which are typically more pronounced in high latitudes. In contrast, there has been little consideration of top-down selection through seasonal changes in predation pressure. Predation pressure by sharks has been shown to affect delphinid habitat use and group size (Norris and Dohl 1980, Wells et al. 1987, Heithaus 2001, Heithaus and Dill 2002), but the influence of predation pressure on reproductive patterns of dolphin populations has rarely been assessed. Here, we report on a distinct seasonal peak in calving for bottlenose dolphins (Tursiops truncatus) using Little Bahama Bank, despite its tropical latitude and describe an inverse seasonal peak in the risk of predation by sharks, inferred by the incidence of fresh shark-bite wounds (Heithaus 2001). We suggest seasonal changes in the environment that might mediate increased predation risk and hypothesize that this may be a selective force for calving seasonality.
Little Bahama Bank (∼26°N, ∼77°W) comprises part of the northern Bahamas and encompasses the two main islands of Great Abaco and Grand Bahama (Fig. 1). This carbonate bank is approximately 17,000 km2 in size (approximately 65% covered by water), characterized by shallow waters (average <7 m deep) and chains of small islands, and is bounded on all sides by the deep (>500 m) waters of the subtropical NW Atlantic Ocean. Approximately 1,000 bottlenose dolphins inhabit the bank (Durban 2002), and individuals from this inshore dolphin population have not been sighted in the surrounding pelagic waters, despite extensive survey effort over more than 16 yr, and therefore, this population appears to be restricted to this shallow bank system (Parsons et al. 2006). Seasonal variability in surface water temperature on Little Bahama Bank is limited, but temperatures are moderately elevated in summer compared to winter (Table 1). This increase in summer temperatures is coincident with tropical cyclones in the NWAtlantic (Vecchi and Soden 2007), which typically occur between June and October (Landsea 1993). To examine seasonal differences, we defined a 6 mo “summer” period (May–October), characterized by increased water temperatures and encompassing the entire season of tropical cyclones, and a 6 mo “winter” period (November–April), characterized by cooler water temperatures and a complete absence of cyclonic activity (Table 1).