Investigating the Thermal Response of Sarasota Bay Dolphins to Changing Environmental Temperatures
By “Team Thermal” - Ann Pabst, PhD, Bill McLellan, PhD, Andrew Westgate, PhD, Erin Meagher, PhD candidate,  Michelle Barbieri, MSc student, and Ari Friedlaender, PhD candidate

The goal of our work with the Sarasota Dolphin Research Program is to better understand reproductive and whole-body thermoregulatory (body temperature regulation) function in bottlenose dolphins.  The long-term, health-monitoring program for Sarasota Bay dolphins offers us a unique opportunity to study thermoregulation in wild cetaceans.  Our current project is aimed at understanding how Sarasota Bay dolphins thermally adapt to seasonal changes in environmental temperatures.  Their year-round residency exposes these dolphins to water temperatures that can drop below 10^oC (50^oF) in the winter and exceed 32^oC (90^oF) in the summer.

                Bottlenose dolphins in Sarasota Bay may invoke a suite of physiological modifications to cope with their changing thermal environment.  The goal of our current study is to describe seasonal variation in the thermal responses of bottlenose dolphins in Sarasota Bay.  We investigate thermal function in dolphins using multiple measurement techniques, which include skin surface temperatures and heat flux values, measured at multiple positions on the dolphin’s body.  Heat flux is the rate of energy transfer per unit area measured in Watts/m².  Deep core temperatures, measured with a specialized colonic probe, and blubber thicknesses, measured using ultrasound are also recorded.  A dorsal fin “Trac Pac” is deployed on a subset of dolphins, recording skin surface temperatures and heat flux values, as well as velocity and time-depth records.  These Trac Pacs are attached to the fin’s surface using suction cups, and are deployed for periods lasting up to 8 hours.  Infrared thermal imaging is used to measure skin surface temperatures of wild dolphins, both during temporary restraint and while they are free-swimming.

Our research team has collected this suite of physiological data on Sarasota dolphins during summer health-monitoring studies over the past three years.  These data suggest that dolphins must actively dissipate body heat during the summer to maintain constant body temperatures.  Our current study has permitted us the first opportunity to investigate the mechanisms used by wild dolphins to maintain homeothermy over the course of a year, as they experience a wider range of environmental conditions. 

               Dolphins use their appendages (dorsal fin, pectoral flippers and flukes) to either conserve or dissipate body heat, thus, these body sites are considered thermal windows. This study examines the roles of thermal windows and other body surfaces in regulating the body temperature of dolphins. Heat flux provides a real-time, dynamic method of assessing the thermal status of an individual animal. Thus, we are mapping heat flux patterns over multiple body surfaces, including the appendages, tailstock and lateral body wall, in wild bottlenose dolphins. Assessing heat flux at multiple body sites simultaneously will elucidate whether dolphins prioritize one body surface or thermal window over another when dissipating excess body heat.

These heat flux measurements are taken seasonally on individual, wild bottlenose dolphins to examine how they vary their whole body conductance to adjust to changing environmental temperatures. Heat flux values will be related to seasonal changes in blubber thickness within individual bottlenose dolphins. To date, heat flux data have been collected during six separate capture-release events in three seasons (June 2002, 2003 and 2004, Fall 2002, Winter 2003 and 2004). There are a total of 38 animals included in the summer data set (20 males, 18 females), 3 animals in the fall data set (2 males, 1 female), and 13 animals in the winter data set (4 males, 9 females).

Overall, mean heat flux in winter was significantly higher than that in summer. Generally, the highest mean heat flux values in winter were found at the base of the dorsal fin and at the tailstock. In summer, mean heat flux values were generally lower and showed less variation than those measured in winter. Mean blubber thickness measurements at the lateral body wall and at the tailstock for dolphins in this study were significantly higher in the winter than in the summer.

Preliminary results suggest that bottlenose dolphins in Sarasota Bay do not necessarily respond to decreased ambient temperatures by decreasing heat loss across the body surface. Rather, heat flux values across the lateral body wall and tailstock were significantly higher in the winter, relative to those measured in the summer. These winter increases in heat flux occurred despite significantly thicker blubber layers at these sites in winter.

Although we expected heat flux values across the dolphin’s thermal windows (dorsal fin, pectoral flippers and flukes) to be higher in the summer, due to an increased need to dissipate body heat in tropical water temperatures, surprisingly there was no significant difference between summer and winter heat flux values at most of these sites. These results suggest that bottlenose dolphins resident to Sarasota Bay, FL may be using alternative mechanisms to dissipate excess body heat in the summer, such as enhanced respiratory evaporative heat loss or spending more time in cooler microclimates. These alternative mechanisms are currently under investigation.

We deployed our first thermal Trac Pac on a Sarasota bottlenose dolphin during the summer of 1999, and have deployed the thermal Trac Pac an additional 51 times.  These deployments have provided us with over 100 hours of unique data on the thermal biology of free swimming bottlenose dolphins.  We have gained many insights into the thermal behavior of wild dolphins from the data we have collected.  For example, we have learned that dolphins are profoundly affected be small differences in water temperature.  Differences as little as 1^o C can bring about significant changes in the amount of heat an animal loses to the water.  These experiments have reinforced the idea that bottlenose dolphin thermoregulatory function is much more complex than we had previously imagined.  The data we have been fortunate to collect will allow us to investigate this complexity in detail.

The temperature difference between a body surface and the environment is one factor that contributes to heat loss from an organism.  The goal of this study was to investigate how dolphins may use this temperature differential to thermoregulate across large seasonal changes in water temperature.  We chose to study the dorsal fin because it is a poorly insulated and dynamic thermal exchange surface that exits the water upon surfacing.  Infrared thermography was used to non-invasively measure dorsal fin surface temperatures of bottlenose dolphins (n=551 images) encountered during synoptic surveys of the Sarasota study area in the summer, fall, and winter from 2002-2004.  There is a significant positive, linear relationship between dorsal fin surface temperature and water temperature, as mean temperature differential (0.9°C) was similar across all seasons.  Thus, dorsal fin surface temperatures appear to be modulated in response to water temperature to maintain a steady dorsal fin temperature differential across seasons.  This implies that there is a much larger temperature gradient between the dolphin deep body core and the dorsal fin surface in winter than in summer.  Thus, in winter, increases in insulation, both integumentary (i.e. blubber) and vascular (via reduced perfusion and utilization of heat exchangers) must account for the protection of core temperature and stability of the temperature differential. 

Preliminary results suggest that bottlenose dolphins in Sarasota Bay do not necessarily respond to decreased ambient temperatures by decreasing heat loss across the body surface. Rather, heat flux values across the body wall are significantly higher in the winter, relative to those measured in the summer. These winter increases in heat flux occurred despite significantly thicker blubber layers at these sites in winter, and despite relatively stable temperature differentials.

These combined data suggest that the responses of wild bottlenose dolphins to changing environmental temperatures are complex.  We are, however, beginning to gain a better understanding of the physiological responses of wild Florida dolphins to changing environmental temperatures.              This study has also offered two of our current graduate students, Ms. Erin Meagher (PhD) and Ms. Michelle Barbieri (MS), the opportunity to gather data critical to their thesis research on dolphin thermoregulation.  Support for this project has been provided by an HBOI Protect Wild Dolphins grant, by Dolphin Quest, by the Disney Wildlife Conservation Fund, and by NOAA Fisheries through the Chicago Zoological Society.