11.4 Heat Conservation and Dissipation
Learning Objectives
- Describe how animals conserve and dissipate heat through structural, physiological, and behavioral adaptations.
- Explain how the thermal properties of water contribute to temperature regulation in organisms and environments.
Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment and yet maintain a constant, warm, body temperature. The arctic fox, for example, uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect from shivering and increased muscle activity: arrector pili muscles cause “goose bumps,” causing small hairs to stand up when the individual is cold; this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end. Loss of significant amounts of body fat will compromise an individual’s ability to conserve heat.
Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body. Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals have adaptations to their circulatory system that enable them to transfer heat from arteries to veins, warming blood returning to the heart. This is called a countercurrent heat exchange; it prevents the cold venous blood from cooling the heart and other internal organs. This adaptation can be shut down in some animals to prevent overheating the internal organs. The countercurrent adaptation is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations can help cool endotherms when needed, such as dolphin flukes and elephant ears.
Water itself plays a vital role in thermoregulation due to its unique physical properties. Water has a high specific heat capacity, meaning it can absorb or release large amounts of heat with little change in temperature. This helps stabilize both internal body temperatures and external aquatic environments, providing thermal buffering for aquatic animals. Water also has a high heat of vaporization, making evaporation an efficient way to cool the body. For example, sweating in mammals or evaporative cooling in reptiles uses water loss to remove excess heat. These properties make water essential to both active and passive cooling strategies. In addition, because ice is less dense than liquid water, it floats, creating insulating layers in aquatic habitats and protecting organisms below from freezing.
Ground squirrels offer a compelling example of how animals adjust their activities to regulate temperature and maintain thermal balance. During the colder months, many species are known for their remarkable ability to enter hibernation, a prolonged state of dormancy where their metabolic rate can plummet by up to 98%, drastically reducing internal heat production and conserving vital energy reserves. To avoid the intense heat of the summer months, ground squirrels may burrow deeper into the ground, seeking the cooler and more stable subterranean temperatures which allow for effective heat dissipation through conduction to the soil, minimizing heat gain from solar radiation and convection at the surface. Furthermore, ground squirrels exhibit torpor, a state of significantly reduced physiological activity characterized by a dramatic drop in temperature. These shorter bouts of torpor, which can occur daily or seasonally, allow them to conserve energy and reduce heat loss rapidly in response to acute cold conditions or periods of food scarcity.
Some ectothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from getting too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity such as the activity of bees to warm a hive to survive winter.
Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. Severe cold elicits a shivering reflex that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat.
Figure 4 (Variation in Thermoregulatory Strategies – building)