Blubber deposition during development in free-ranging bottlenose dolphins: balancing disparate roles of insulation and locomotion
By Shawn Noren, PhD, Institute of Marine Science, University of California Santa Cruz and Randall Wells, PhD

        Insulation from blubber is a critical component of marine mammal body temperature regulation (thermoregulation). This may be particularly important in dolphins because they spend their entire lives in water, which conducts heat away from a body 25 times faster than air at the same temperature. For young dolphins, heat loss is exacerbated by their relatively small body size, resulting in larger surface area relative to their volume as compared to adults, theoretically promoting heat loss to the environment. The greater propensity for heat loss in the smallest, youngest dolphins could be circumvented by maintaining thicker blubber layers than larger, older members of the same species (conspecifics). However, blubber also contributes to buoyancy, and individuals who are positively buoyant must expend more energy to dive than individuals who are neutrally buoyant. Therefore, the cost of descent to overcome blubber’s buoyant force while diving could constrain blubber deposition in dolphins.
        The results from our study using the long-term data set across age class and season from dolphins in Sarasota Bay provided interesting insights into dolphin thermoregulation. For animals measured during summer, yearlings had significantly thicker blubber than 2-12 year-olds; this was expected. However, this difference diminished by winter because blubber deposition in response to the colder water temperature was smaller in yearlings (2 mm increase) compared to 2-12 year-olds (3-6 mm increase). This may be explained by the buoyant force of blubber and the associated locomotor costs (increased energy devoted to swimming) to overcome buoyancy while diving. During summer, yearlings had positive buoyancy compared to neutral buoyancy for 12 year-olds. As a result, the calculated mass-specific cost of descent to overcome buoyancy on a theoretical 10 m (32.5 ft) dive was greatest in yearlings. Limits to costs of diving may be set by the inability of yearlings to produce large amounts of thrust for swimming due to diminutive body size and underdeveloped locomotor muscles. Thus yearlings may have difficulty balancing the energetic demands of thermoregulation and locomotion. Ultimately, this trade-off could constrain the lower temperature limits yearlings are able to withstand.