
Turtles are ectothermic, or cold-blooded, meaning they rely on their external environment to regulate their body temperature. They are able to do this by moving between areas of different temperatures, such as basking in the sun or in warmer waters. Leatherback sea turtles, for example, can maintain a body temperature well above the surrounding sea temperature. They do this by using a vascular lining in their trachea to warm cold air and retain heat on expiration. They also have a dense, intertwining network of arteries and veins in their flippers, which act as counter-current heat exchangers to control heat loss.
What You'll Learn
Basking in the sun
Turtles bask by lying on a rock, dock, log, or another surface to dry off and absorb heat from the sun and the substrate on which they bask. This can be a sandy shore, a floating log, or an exposed rock. By basking, turtles can also kill fungal growth on their shells and encourage attached leeches to drop off.
Basking is also necessary for turtles to produce vitamin D3, which helps with calcium absorption and results in solid shells and good bones. The ultraviolet B (UVB) radiation from the sun assists with vitamin D3 production. Without enough exposure to UVB radiation, turtles can suffer from improper growth and metabolic bone disease.
The duration of basking depends on the temperature variation between water and air and can range from a couple of hours to eight hours or more. Turtles will typically bask for a few hours every day, but this can vary depending on the species and individual turtle. For example, aquatic species like sliders and painted turtles may bask for longer periods, while mud turtles prefer to bask close to the water's surface rather than out of the water.
It is important to note that excessive basking can lead to dehydration in turtles and may indicate health problems such as respiratory infections, low water temperature, or poor water quality. If a turtle that usually basks on land or logs refrains from basking, it could be a sign of illness.
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Moving to warmer waters
Sea turtles are cold-blooded reptiles that depend on the temperature of their surroundings to maintain their body temperature. They can control their body temperature by moving between areas of water with different temperatures or basking in the sun at the water's surface or on the beach.
However, when temperatures rapidly decline and sea turtles are cut off from moving to warmer waters, they can suffer from a form of hypothermia called "cold stunning." Cold stunning can be fatal as sea turtles become lethargic, experience decreased circulation, and a slowing of other body functions.
The leatherback sea turtle is considered to be cold-adapted. They can maintain a body temperature well above the surrounding sea temperature using many of the same adaptations that marine mammals use.
Leatherback sea turtles are large animals (300–500 kg), overlapping in size with marine pinnipeds and cetacean species. Unlike marine mammals, they start their aquatic life as 40–50 g hatchlings, so they undergo a 10,000-fold increase in body mass during their independent existence. Hatchlings are limited to the tropics and near-surface water.
Adult leatherbacks, on the other hand, are obligate predators of gelatinous plankton and encounter cold water at depth (<1280 m) or high latitude. They are gigantotherms that maintain elevated core body temperatures in cold water. The trachea of adult leatherback sea turtles is lined with an extensive, dense, erectile vascular plexus that warms and humidifies cold inspired air and possibly retains heat on expiration.
The vascular lining in the trachea of adult leatherback sea turtles helps them maintain body temperature while foraging in cold water via counter-current exchange. The vascular plexus contains a high proportion of longitudinally arranged, large-diameter blood vessels lying mainly in the deeper two-thirds of the mucosa, with prominent cross-connections between them. The arrangement is consistent with their functioning as a counter-current arrangement, retaining heat and maintaining body temperature.
In summary, leatherback sea turtles are able to maintain elevated core body temperatures in cold water through a combination of behavioural and physiological adaptations. They move to warmer waters, utilise their vascular lining, and adjust their activity levels to regulate their body temperature effectively.
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Physiological and behavioural adjustments
One key behavioural adaptation is their ability to move between different temperature zones. In the wild, turtles move between areas of water with varying temperatures or bask in the sun on the beach or rocks to warm themselves. In aquatic environments, they may utilise warm water currents or migrate with the seasons to maintain optimal body heat. Similarly, in their captive habitats, turtles benefit from a heat gradient in their tanks, with basking areas several degrees warmer than the surrounding air temperature, allowing them to raise their internal body temperature.
Turtles also exhibit physiological adjustments to control their body temperature. For example, leatherback sea turtles have a vascular lining in their trachea, which helps them maintain body heat while foraging in cold water through a counter-current exchange system. This mechanism is similar to the nasal turbinates found in endothermic mammals and birds, making leatherback sea turtles the first known living reptiles with a comparable respiratory thermocontrol function.
Additionally, leatherback sea turtles can regulate their heat gain and loss behaviourally by adjusting their swimming activity. When in cold water, they increase their flipper stroke frequency, thereby generating more heat through metabolic processes. They also minimise heat loss by reducing blood flow to their flippers and utilising their peripheral fat layers, including brown adipose tissue, for insulation.
Furthermore, the size of the turtle appears to play a role in temperature regulation. Larger turtles tend to maintain higher thermal gradients than smaller ones, possibly due to thicker plastrons and carapaces providing better insulation.
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Harnessing physiology and behaviour
Leatherback sea turtles are able to maintain their body temperature through a combination of physiological and behavioural adaptations.
Leatherback sea turtles are able to maintain their body temperature through a combination of physiological and behavioural adaptations.
Physiologically, leatherbacks have a dense, intertwining network of arteries and veins in their anterior and posterior flippers, which act as counter-current heat exchangers. This allows them to control heat flux and regulate their body temperature. Additionally, they have thick layers of adipose tissue, including brown adipose tissue, which helps to prevent heat loss. The large body size of leatherbacks also contributes to their ability to maintain body temperature, as it provides more surface area for heat exchange.
Behaviourally, leatherback sea turtles adjust their swimming activity to regulate their body temperature. In cold water, they increase their flipper stroke frequency, which generates heat through metabolic processes. This allows them to maintain a positive thermal gradient between their body and the environment. In warm water, they reduce their activity levels and increase heat loss through their plastron, carapace, and flippers.
The combination of these physiological and behavioural adaptations allows leatherback sea turtles to maintain elevated body temperatures in cold waters and prevent overheating in warmer environments.
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Vascular lining in the trachea
The vascular lining in the trachea of adult leatherback sea turtles helps them maintain body temperature while foraging in cold water via counter-current exchange. The trachea is lined throughout by a continuous vascular plexus, which contains a high proportion of longitudinally arranged, large-diameter blood vessels lying mainly in the deeper two-thirds of the mucosa, with prominent cross-connections between them. The arrangement is consistent with their functioning as a counter-current arrangement, retaining heat and maintaining body temperature.
The leatherback sea turtle is a large oceanic pelagic reptile that nests in the tropics where the water temperature can be as high as 30°C and spends extended periods of time foraging in cold northern waters that approach 0°C. No other known reptile inhabits such a large ambient temperature range.
Leatherback sea turtles are ectotherms, meaning they don't produce their own body heat. They burn through energy much more slowly than "warm-blooded" creatures of the same size. It also means that they need to keep warm to keep active, which is why they spend a seemingly inordinate amount of time simply basking in the sun.
The adult leatherback trachea is an almost continuous ellipsoidal cartilaginous tube composed of interlocking plates, and will collapse easily in the upper part of the water column during dives, thus avoiding pressure-related structural and physiological problems. It is lined with an extensive, dense erectile vascular plexus that will warm and humidify cold inspired air and possibly retain heat on expiration. A sub-luminal lymphatic plexus is also present.
The vascular lining of the long adult leatherback trachea functions in an analogous fashion to nasal turbinates, which mammals and birds have independently evolved to fulfil a respiratory thermocontrol function. This is the first demonstration of a turbinate equivalent in a living reptile.
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