All animals require a continuous supply of energy to power the complex machinery of life. This energy is extracted from food and chemically transformed into a universal molecule known as Adenosine Triphosphate, or ATP. ATP is often called the cell’s energy currency because its chemical bonds store energy that can be instantly released to fuel nearly every cellular process. The process of cellular respiration, which converts fuel molecules like glucose into usable ATP, is fundamental to survival.
Energy for Essential Bodily Maintenance
The most significant portion of an animal’s daily energy budget is dedicated to simply staying alive at rest. This foundational expenditure is measured as the Basal Metabolic Rate (BMR) in endotherms (such as mammals and birds) or the Standard Metabolic Rate (SMR) in ectotherms. BMR measures the minimum energy an animal needs when it is physically and psychologically undisturbed, not actively digesting food, and in a temperature-neutral environment. For many species, this baseline metabolism can account for approximately 70% of their total daily energy use. A large part of this cost comes from maintaining a stable internal body temperature, a process called thermoregulation. Continuous function of vital organs, including the heart’s rhythmic beating and the kidneys’ constant filtration work, also represents a substantial drain.
The nervous system also represents a major, unseen energy consumer, even during sleep. Nerve impulses rely on the continuous action of specialized protein pumps embedded in cell membranes. These pumps use ATP to move ions like sodium and potassium across the membrane, generating the electrical potential necessary for transmitting signals throughout the body.
Energy for Movement and Active Behavior
Energy expenditure rises dramatically above the basal rate when an animal engages in physical activity, which is powered by the contraction of muscle fibers. Muscle contraction requires ATP to fuel the interaction between the protein filaments actin and myosin, a process known as cross-bridge cycling. The energy required for movement depends highly on the type of locomotion and the animal’s size. Locomotion on land, in water, or through the air involves different energetic challenges, such as overcoming gravity or fluid drag. For instance, the energy cost of transport—the energy spent to move a given distance—is typically higher in smaller animals than in larger ones. High-energy behaviors like hunting, escaping a predator, or long-distance migration require sustained, elevated metabolic rates.
The total energy cost of movement is often separated into the energy needed to move the body’s center of mass and the energy required to accelerate the limbs. Animals utilize strategies, such as elastic strain energy in tendons, to reduce the metabolic cost of repetitive movements like running. However, activities requiring quick changes in speed or direction, such as maneuvering during a chase, significantly increase the overall energy demand.
Energy for Growth and Tissue Repair
The process of building and replacing body structures, known as anabolism, demands a considerable energy investment. This energy synthesizes complex molecules, such as proteins, nucleic acids, and lipids, from simpler dietary components. Growth in juveniles, involving the formation of new bone, muscle, and organs, requires a sustained, high-energy budget to support rapid cell division and tissue expansion. In adult animals, this anabolic energy is continuously dedicated to tissue repair and maintenance. Damaged tissues, such as a wound, trigger a localized, energy-intensive healing response. This process requires a steady supply of energy and building blocks to create new cells and structural components. Without sufficient energy reserves, the body may enter a catabolic state, breaking down its own protein to fuel the repair, which slows the healing process.
Energy for Producing Offspring
Reproduction is one of the most variable and energetically demanding phases in an animal’s life cycle. Energy expenditure includes both direct costs (energy contained within the gametes, eggs, or offspring) and indirect costs. Indirect costs represent the metabolic load carried by the parent, often far exceeding the direct costs. For many mammals, the metabolic expense of gestation can account for up to 90% of the total reproductive energy cost. Following birth, parental care, such as lactation or feeding the young, further escalates the energy requirement. Lactation in small mammals, for example, can increase the female’s energy expenditure to twice her non-lactating rate. Other investments include energy spent building nests, defending territories, or migrating to breeding grounds.