Concept 29.1   Animals Eat to Obtain Energy and Chemical Building Blocks

• Animals build new molecules and cells from ingested organic compounds. A growing animal adds new cells to its body, and all animals are constantly replacing worn-out molecules and cells.
• An animal is characterized by the organization of its body’s cells, tissues, and organs. Energy can be seen as the capacity to create or maintain this organization. Review Figure 29.1

Concept 29.2   An Animal’s Energy Needs Depend on Physical Activity and Body Size

• An animal’s metabolic rate is defined as its rate of energy consumption—the rate at which the animal uses chemical energy, turning it into heat. Metabolic rate can be quantified by measuring the rate of O₂ consumption or (far less commonly) by measuring the rate of heat production.
• Physical activity increases an animal’s metabolic rate, but the exact relationship between activity level and metabolic rate varies. Review Figure 29.2
• Basal metabolic rate (BMR) is the metabolic rate of a resting, fasting individual in a comfortable thermal environment. When BMR is expressed per unit of body weight in grams, it is called BMR/g.
• Smaller mammals consistently have higher BMR/g than larger mammals do. This is an example of a scaling relationship. An animal with a high BMR/g must consume more food relative to its body weight than an animal with a lower BMR/g. Review Figure 29.3

Concept 29.3   Metabolic Rates Are Affected by Homeostasis and by Regulation and Conformity

• Within the animal body, cells exist in an internal environment of tissue fluids. Conditions outside the body are the external environment.
• Animals that maintain a consistent internal environment despite variation in the external environment are called regulators. Conformers are animals in which the internal environment varies to match the external environment. Review Figure 29.4
• Regulation of the internal environment is more energetically expensive than conformity.
• Homeostasis refers to the stability of an animal’s internal environment and the mechanisms that maintain this stability.
• Homeotherms exhibit thermoregulation, maintaining a constant internal temperature by varying metabolic rate and insulation with external temperature. Above or below a specific temperature range called the thermoneutral zone (TNZ), metabolic rate increases to compensate for increased heat input or to offset heat loss, respectively. Review Figure 29.5 and ACTIVITY 29.1
• Most animals are poikilotherms (ectotherms), with body temperatures that vary with, and match, external temperature. Metabolic rates decrease as temperature drops and increase as temperature rises. Review Figure 29.6 and Figure 29.7
• The sensitivity of a reaction or process to changes of internal (tissue) temperature can be expressed as the numerical factor Q₁₀, which usually has a value of 2 to 3. Review Figure 29.6B and Figure 29.7
• A poikilotherm can exert some control over its tissue temperature through behavior, by moving to a more favorable location. Review Figure 29.8
• Homeothermy expends more energy than poikilothermy. Review Figure 29.9
• Homeothermic heat-producing mechanisms used by mammals include shivering, the subtle contraction of the skeletal muscles to convert ATP to heat; and nonshivering thermogenesis (NST) in brown adipose tissue, which produces heat directly in the mitochondria by short-circuiting oxidative phosphorylation. Insects sometimes regulate their thoracic temperature using heat generated from ATP by the flight muscles in their thorax Review Figure 29.10 and Figure 29.11
• In hot environments, homeothermic animals increase their rate of heat loss by sweating or panting, processes that increase heat transfer to the environment by evaporation of water.
• Mammalian hibernation refers to a strategy of reducing energy (and thus food) needs by entering a state of thermoconformity during extended cold periods. Review Figure 29.12

Concept 29.4   Animals Exhibit Division of Labor, but Each Cell Must Make Its Own ATP

• The fluids in an animal’s body are distributed among various body compartments. Intracellular fluid is inside the body’s cells and is said to be in the intracellular compartment. Extracellular fluid, which includes blood plasma and interstitial fluid, is outside the cells and is said to be in the extracellular compartment.
• Different extracellular fluids are separated by epithelia, sheets of cells that cover a body surface or organ or line a body cavity. Review Figure 29.13
• The boundary between intracellular fluids and extracellular fluids is formed by cell membranes.
• Animals exhibit a high °ree of division of labor. This division of labor typically necessitates a rapid transport system to distribute materials among parts of the body.
• Cells, tissues, organs, and multi-organ systems represent ever-more-complex levels in the hierarchy of organization of the animal body. Review Figure 29.14 and ACTIVITY 29.2
• Each cell must make its own ATP. Cells may use aerobic (with oxygen) or anaerobic (without oxygen) processes to produce ATP.

Concept 29.5   The Phenotypes of Individual Animals Can Change during Their Lifetimes

• Phenotypic plasticity refers to an individual’s ability to display two or more different phenotypes at different times during its life. Phenotypic plasticity is a way for an animal to acclimate or acclimatize to changes in its environment. Review Figure 29.15
• At a biochemical level, phenotypic plasticity often involves inducible enzymes, enzymes that change in abundance or type in response to an animal’s environment.
• Phenotypic plasticity also occurs at the level of tissues and organs.
• The capacity for phenotypic plasticity is genetically based and is subject to natural selection.

Concept 29.6   Animal Function Requires Control Mechanisms

• The four essential elements of a control system are the controlled variable, sensors, effectors, and control mechanism. The controlled variable is the property or characteristic of an animal that is being controlled. Sensors detect the current level of the controlled variable. Effectors are tissues or organs that can change the level of the controlled variable. The control mechanism uses information from the sensors to determine which effectors to activate—and how intensely—to modify the controlled variable.
• A control mechanism may involve negative feedback, in which the control mechanism activates effectors in ways that reduce or negate any difference that exists between the controlled variable’s actual state and its set point. Negative feedback is stabilizing. Review Figure 29.16 and ANIMATED TUTORIAL 29.1
• Positive feedback occurs when deviations of a controlled variable from its existing level are increased or amplified by the action of the control mechanism. Positive feedback is destabilizing but still can be advantageous. Review Figure 29.17
• Animals have self-contained mechanisms of keeping track of time, known as biological clocks, that allow the animals to anticipate future events. Biological clocks that have free-running timing cycles of about 24 hours are called circadian clocks. Some animals also have circannual or circatidal clocks. Review Figure 29.18 and ANIMATED TUTORIAL 29.2