Behavioral and metabolic contributions to thermoregulation in freely swimming leatherback turtles at high latitudes

Abstract

Leatherback turtles in the Northwest Atlantic Ocean have a broad geographic range that extends from nesting beaches near the equator to seasonal foraging grounds as far north as Canada. The ability of leatherbacks to maintain core body temperature (T b ) higher than that of the surrounding water is thought to be a key element of their biology that permits them to exploit productive waters at high latitudes. We provide the first recordings of T b from freely swimming leatherbacks at a northern foraging ground, and use these data to assess the importance of behavioral adjustments and metabolic sources of heat for maintenance of the thermal gradient (T g ). The mean T b for individual leatherbacks ranged from 25.4±1.7 to 27.3±0.3°C, and T g ranged from 10.7±2.4 to 12.1±1.7°C. Variation in mean T b was best explained by the amount of time that turtles spent in the relatively warm surface waters. A diel trend in T b was apparent, with daytime cooling suggestive of prey ingestion and night-time warming attributable to endogenous heat production. We estimate that metabolic rates necessary to support the observed T g are ~3 times higher than resting metabolic rate, and that specific dynamic action is an important source of heat for foraging leatherbacks.

Comparative analyses of animal-tracking data reveal ecological significance of endothermy in fishes

“…The speeds of sea turtles (ectotherms, except for the leatherback turtle; ref. 22) were close to those of fishes without RM endothermy.…”

Fine-scale foraging ecology of leatherback turtles

“…In addition, more frequent diving and essentially continuous swimming generates endogenous heat (Bostrom et al, 2010). Casey et al (2014) reported that variation in leatherback body temperatures off Nova Scotia is best explained by time spent in surface waters, and that swimming activity also plays a role in maintaining a significant thermal gradient between leatherback body temperatures and ambient water temperatures. Therefore, although leatherbacks increase diving activity to achieve higher prey capture rates, which, in turn, increases the time they spend handling prey, this increased activity does not impose physiological costs that restricts their foraging effort, in contrast to theoretical predictions for diving predators (Thompson and Fedak, 2001).…”

“…Such analyses are rare for marine migratory species in general (e.g., Seminoff et al, 2006;Moll et al, 2007). Leatherback dives are shorter and shallower while foraging off Nova Scotia than during migration or while in lower latitudes, and feeding is restricted to daylight hours (James et al, 2005a;Casey et al, 2014;Hamelin et al, 2014). Hamelin et al (2014) used high-resolution dive data from three leatherbacks to show that turtles foraging in Atlantic Canada remain at or above the main thermocline, suggesting that prey resource distribution is associated with water mass.…”

Crying a river: how much salt-laden jelly can a leatherback turtle really eat?

“…A complicating factor in choosing a representative metabolic rate is that adult leatherbacks have elevated core temperatures (25-27°C) in cold (10.9-16.7°C) surface seawater (Frair et al, 1972;James and Mrosovsky, 2004;Casey et al, 2014) and regularly dive into nearfreezing water when foraging in Canadian waters (James et al, 2006). Current understanding is that an elevated core temperature is maintained by a combination of gigantothermy, insulation, plus muscular and visceral thermogenesis (Casey et al, 2014;Davenport et al, 2015).…”

“…Current understanding is that an elevated core temperature is maintained by a combination of gigantothermy, insulation, plus muscular and visceral thermogenesis (Casey et al, 2014;Davenport et al, 2015). As leatherbacks swim just as quickly in cold water as they do in the tropics (Davenport et al, 2015), it is likely that their FMRs are elevated at high latitude.…”