Thermal Biology 4

Warm-Bodied Fish

In most fish species, body temperature aligns with the surrounding water temperature. However, certain species maintain elevated body temperatures in specific tissues. These warm-bodied fish are typically large, fast-moving predators, including great white sharks, marlin, swordfish, and some tuna species.

Some warm-bodied fish, like certain shark species, maintain red muscle temperatures at a relatively constant level above the water temperature, for example, 5-10 degrees Celsius above the water temperature. Bluefin tuna maintain consistent red muscle temperatures regardless of ambient water temperature.

Countercurrent Heat Exchange

Tunas and certain sharks employ a countercurrent heat exchange mechanism to elevate swimming muscle temperature. Red swimming muscle is located deep within the body. Longitudinal veins and arteries run along the fish's length.

Small arteries branch off, delivering blood to the red muscle. Blood returns via veins connected to longitudinal veins leading back to the heart. Arteries carrying blood inward lie close to veins carrying blood outward. Venous blood loses heat to arterial blood, carrying heat inward to the muscle.

Marlins and Swordfish

Marlins and swordfish selectively warm their brains and retinas. Specialized heater tissues break down ATP to generate heat in these regions.

Survival in Extremely Cold Environments

Adult reindeer can maintain body temperature without expending energy until outside temperatures drop to -30 degrees Celsius or below. This is achieved through a combination of a thick winter coat and regional heterothermy. Reindeer allow their legs and other body parts to cool relative to their body core, minimizing heat loss from appendages.

Regional Heterothermy

This is the process of allowing limbs to cool which can lead to problems with membrane fluidity. To compensate, the lipid composition changes along the leg. Unsaturated fatty acids such as oleic acid concentration increases at the distal end of the leg, while saturated fatty acids decrease.

Reindeer Diet

Reindeer consume lichens to cope with food scarcity in winter. Reindeer can extract nutritional value from lichens, while other ruminants cannot. Lichens form a significant part of their winter diet.

Newborn Reindeer

Newborn reindeer lack adult thermoregulatory abilities. They rely on their thick pelage and thermogenesis mechanisms to survive. Newborns can maintain body temperatures of approximately $39$ degrees Celsius when the outside temperature is $-25$ degrees Celsius.

Reindeer calves grow rapidly, reducing the metabolic energy required to maintain body temperature as they grow. Non-shivering thermogenesis involving brown fat is the primary thermogenesis method for newborns. Brown fat declines rapidly with growth, and shivering becomes the main thermogenesis method by one month.

Hibernation

Another strategy for coping with extreme cold is hibernation. Arctic ground squirrels exhibit periodic arousals during hibernation.

The animal experiences one to three-week periods of hypothermia, with body temperature rapidly rising to approximately $37$ degrees Celsius for $12$ to $24$ hours during arousals. Body temperature can drop below zero, with supercooling preventing freezing.

The squirrel regulates its body temperature to stay above the hibernation chamber's temperature. Metabolic heat production involving brown fat thermogenesis maintains body temperature above ambient when severely cold temperatures occur, from $0$ to $-20$ degrees Celsius.

Diet impacts hibernation efficiency. Fats need to be fluid to be metabolized. Saturated fats are ineffective during hibernation. Chipmunks fed diets rich in polyunsaturated fatty acids exhibited more effective hibernation, experiencing fewer arousals, tolerating lower body temperatures, and having low metabolic rates.

Periodic arousals occur universally among hibernating animals. These arousals are energetically costly, yet they happen and at this point, the function is unknown.