Metabolism represents the chemical processes within a living organism that maintain life, and its rate determines how quickly an animal consumes energy to fuel these processes. This energy consumption rate is highly variable, changing with an animal’s activity level, body size, and whether it is actively regulating its internal temperature. Therefore, the “slowest” animal can be defined by its minimum rate during normal function or by its temporarily induced state of extreme energy conservation.
Defining Metabolic Rate in Animals
Scientists quantify an animal’s energy expenditure using standardized measurements to allow for meaningful comparisons across species. The Basal Metabolic Rate (BMR) is the standard measurement for endotherms, which are animals like mammals and birds that generate internal heat to maintain a constant body temperature. BMR is determined under a specific set of conditions: the animal must be physically and psychologically at rest, in a non-stressful environment, and in a post-absorptive state, meaning it is not actively digesting food. This measurement represents the minimum energy required for basic life functions, such as breathing and circulation.
A related but less stringent measure is the Resting Metabolic Rate (RMR), which relaxes the strict fasting and thermal requirements of BMR. For ectotherms, such as reptiles, amphibians, and fish, the comparable metric is the Standard Metabolic Rate (SMR). Unlike BMR, SMR must specify the environmental temperature at which the measurement was taken, as an ectotherm’s metabolic rate rises and falls with the ambient temperature. Ectotherms rely on external heat sources, and their internal functions slow down when the environment cools.
The Slowest Standard Metabolic Rates
Among endotherms, the three-toed sloth holds the record for the lowest sustained Basal Metabolic Rate (BMR). Studies show that the BMR of three-toed sloths is remarkably low, often falling between 40 and 60% of the predicted rate for a mammal of similar body mass. This extreme reduction in energy expenditure allows the sloth to subsist on a diet of low-nutrient leaves. The field metabolic rate, which measures daily energy use, for the three-toed sloth has been recorded as the lowest of any non-hibernating mammal.
Another contender in the mammalian group is the koala, which also exhibits an exceptionally low BMR, although typically slightly higher than that of the three-toed sloth. These low rates are directly tied to their sedentary lifestyles and the poor energy content of their eucalyptus leaf diet.
However, the lowest standard rates overall belong to ectotherms, which do not need to expend energy to maintain a constant, high body temperature. A large ectotherm like a giant tortoise, for example, can have an SMR that is less than 10% of the BMR of a similarly sized mammal, though this rate will fluctuate with the outside temperature. Their overall energy needs for survival are fundamentally lower than those of endotherms.
Metabolic Extremes: The Role of Torpor and Hibernation
The most dramatic metabolic slowdowns occur when animals enter temporary, induced states of energy conservation, known as hypometabolism. Torpor is a state of decreased physiological activity marked by a reduced body temperature and metabolic rate, often lasting less than 24 hours to conserve energy during cold nights or food shortages. Many small endotherms, such as hummingbirds and some bats, use daily torpor to survive periods when they cannot forage.
Hibernation is a prolonged form of torpor that can last for weeks or months, typically occurring during winter to survive harsh conditions. During true hibernation, the metabolic rate drops drastically, sometimes to just 2 to 3% of the animal’s normal active rate. For example, the thirteen-lined ground squirrel can lower its core body temperature to between 4 and 6 degrees Celsius, and its heart rate can drop from over 200 beats per minute to as few as 3 to 10.
Evolutionary Reasons for Slow Metabolism
The evolution of a slow metabolism is an adaptation driven by specific environmental and dietary pressures. Many animals that exhibit slow metabolism, like certain island species, have evolved this trait due to limited food supplies and reduced predation pressure in their isolated habitats.
Slower metabolism is also fundamentally linked to body size, following a concept known as hypometric scaling. Larger animals tend to have a lower mass-specific metabolic rate than smaller ones. This reduced energy cost is partly due to a lower proportion of highly active tissues, such as the liver and brain, relative to their total body mass. The slow metabolic rate represents an energy-saving strategy that has allowed these species to thrive in environments with limited resources.