Poikilothermia is the condition where an organism’s internal temperature fluctuates considerably, closely mirroring the ambient environmental temperature. Maintaining a stable internal temperature requires a constant expenditure of energy, which poikilotherms avoid by allowing their temperature to vary. This ability to function across a range of internal temperatures requires specialized physiological and biochemical adaptations.
Defining Poikilothermia and Thermal Classification
Poikilothermia describes an organism’s internal thermal stability, characterized by a variable internal temperature that generally mirrors the surrounding environment. This contrasts directly with homeothermia, where an organism actively maintains a relatively stable internal body temperature despite external fluctuations. Poikilotherms include most invertebrates, fish, amphibians, and reptiles, which tolerate a wide range of body core temperatures.
Thermal classification also uses the terms ectothermy and endothermy, which refer to the source of the body heat. Ectotherms rely on external sources, like sunlight, to warm themselves. Endotherms generate most of their heat internally through metabolic processes. Although most poikilotherms are ectotherms, the terms are not interchangeable.
Physiological Mechanisms for Coping with Temperature Change
Poikilotherms employ physiological and behavioral strategies to manage their fluctuating temperatures. Behavioral thermoregulation is the most immediate mechanism, involving conscious movement to warmer or cooler areas. For example, a lizard may bask in the sun to raise its body temperature quickly or seek shade to prevent overheating. Changing body orientation or posture can also modulate heat absorption.
The metabolic rate of a poikilotherm adjusts dramatically in response to temperature changes. When temperatures drop, the metabolic rate slows considerably, conserving energy and allowing the organism to survive periods of cold or resource scarcity. This dependence of metabolic function on external temperature is a defining feature of the poikilothermic strategy. At the cellular level, organisms modify the lipid composition of cell membranes to maintain fluidity across different temperatures.
In extreme cold, some amphibians and insects utilize cryoprotectants, like glycerol, to prevent cellular damage. These compounds act as biological antifreeze, lowering the freezing point of body fluids and protecting tissues from ice crystal formation. This cellular tolerance allows freeze-tolerant frogs to survive with much of their body water frozen during winter. These mechanisms ensure that enzyme systems can function over a wider thermal range than is possible for homeotherms.
Ecological Advantages and Limitations
The poikilothermic strategy offers a substantial advantage in energy efficiency. Since they do not rely on constant internal heat generation, poikilotherms require significantly less food energy compared to an endotherm of comparable size. For the same body mass, the energy requirement of a poikilotherm can be as low as 5 to 10% of what a homeotherm needs. This low resting metabolic rate allows poikilotherms to thrive in environments with limited or unpredictable food resources.
This reliance on the external environment introduces significant limitations concerning activity and geographic distribution. Poikilotherms become sluggish and their activity levels drop when the ambient temperature is too low. This dependence constrains their active periods, often limiting them to daytime or warmer seasons, and restricts their geographic range to areas that meet their thermal minimums. To survive prolonged periods of cold or drought, many poikilotherms enter a state of reduced metabolic activity, such as brumation or hibernation.
Poikilothermia in Human Health
The term poikilothermia also applies to a pathological condition in human medicine, describing the loss of the body’s ability to regulate core temperature effectively. This condition is most frequently observed in patients who have sustained severe spinal cord injuries, particularly those at the cervical or high thoracic level (T6 and above). The damage interrupts communication pathways between the central nervous system’s thermoregulatory center and the body’s effector mechanisms.
Consequently, the body loses its ability to initiate peripheral responses like vasoconstriction, shivering, or sweating below the level of injury. The patient’s core body temperature fluctuates according to the external air temperature. They are highly susceptible to both hypothermia in cold environments and hyperthermia in warm environments, requiring careful environmental control to maintain a stable internal temperature.