Physiological and behavioral thermoregulation in bottlenose dolphins (Tursiops truncatus) in Sarasota, Florida

The temperature differential (? T) between a body surface and the ambient environment is one factor that influences heat loss. Organisms can affect ? T physiologically, by controlling body surface temperature, and behaviorally, by choosing the ambient temperature to which they are exposed. These physiological and behavioral mechanisms of thermoregulation were investigated across seasons in a resident community of bottlenose dolphins (Tursiops truncatus) in the Sarasota Bay, Florida region, where water temperatures range annually from 11 to 33oC. Because the dorsal fin is a highly dynamic thermal window, temperatures of this surface were measured on wild, free-swimming dolphins using infrared thermography. Distribution of these yearround resident dolphins was compared across seasons to assess whether or not local changes in distribution reflect seasonal use of microclimates. Independent, continuous measurements of water temperature at eight locations throughout the region were used to describe the annual thermal profile of Sarasota Bay. To calculate ? T, water temperatures measured during thermal imaging were subtracted from dorsal fin surface temperatures. There was a positive, linear relationship between dorsal fin surface temperature and water temperature, as mean ? T across all seasons was similar. Dorsal fin surface temperatures appear to be modulated in response to environmental temperature to maintain a steady ? T at the dorsal fin skin surface across seasons. In winter, increases in insulation, both integumentary (i.e. blubber) and vascular (via reduced perfusion and utilization of countercurrent heat exchangers) must account for the protection of core temperature and stability of ? T. Water temperature throughout the Sarasota Bay region changed dramatically across seasons and, overall, these patterns were similar throughout the study area. Temperatures tended to plateau in both winter and summer, and change continuously in spring and fall. Overlaid on this annual pattern of temperature change were short-term, cyclical variations in water temperature, with peak frequencies at 11 and 19 days. The amplitudes of these cyclical changes could vary between sites, creating regional heterogeneity in water temperatures across the study area. The amplitudes of these cyclical changes were more pronounced in winter than in summer at all sites; thus, temperatures within the summer were the most stable of any season. To assess dolphin distribution, individuals were classified based on age, sex and reproductive status. Within a season, distribution patterns appeared to be specific to particular dolphin classes. Though not always significant, differences between these distribution patterns were more apparent in summer, spring, and fall, and less so in winter. Water temperatures during summer were, overall, the least variable, and in the transitional spring and fall seasons, water temperatures across the seven measurement sites were most similar to each other. In contrast, during winter, when water temperature oscillations could vary by up to 6°C over a period of 10-11 days, dolphin classes were more similar in their distributions. The relationship between dolphin distribution and water temperature was investigated to assess whether or not a particular dolphin class was consistently observed in warmer or cooler water temperatures, relative to any other dolphin class. In only one dolphin class, and in only one season, was there a significant statistical relationship. Adult males in summer were found to be distributed in significantly cooler water temperatures than all other classes. This result is interesting, as adult males have the smallest surface area to volume ratios across which heat dissipation may occur, in comparison to other dolphins, and it was found in summer, when water temperatures are warmest and most stable. Thus, water temperature may be an important factor influencing the distribution of adult male dolphins, but other biotic and abiotic factors likely play an important role in dolphin distribution throughout the Sarasota Bay region.