The term poikilotherm describes an animal whose internal body temperature naturally varies, fluctuating significantly with the external environment. This word is derived from the Greek poikilos, meaning “varied,” and therme, meaning “heat.” Scientists prefer poikilotherm over the common phrase “cold-blooded” because the animal’s blood can be quite warm when exposed to heat. This variability in temperature is a defining characteristic that shapes the animal’s survival.
Metabolic Consequences of Temperature Variability
The internal world of a poikilotherm is directly tied to the surrounding temperature, which profoundly dictates its physiological capabilities. Unlike organisms that generate internal heat, a poikilotherm’s body temperature mirrors the ambient conditions, which controls the speed of all biochemical reactions. This relationship is often described by the Q10 effect, where a 10° Celsius rise in temperature can double or even triple the rate of metabolism.
When external temperatures drop, enzyme activity slows significantly, leading to a reduction in processes like digestion and oxygen consumption. The animal becomes sluggish and unable to move or react effectively because its metabolic machinery operates at a fraction of its capacity. Conversely, warmer temperatures accelerate these processes, enabling rapid movement, feeding, and growth. This reliance on external heat means poikilotherms require only about 5 to 10% of the energy needed by organisms that maintain a constant internal temperature.
To function effectively across a temperature range, some poikilotherms have evolved multiple versions of the same enzymes, known as isozymes, each optimized for different temperatures. This physiological adaptation allows certain species to maintain a minimal level of metabolic activity even when conditions are less than ideal. However, this flexibility comes with the trade-off of having a slower, lower maximum output compared to their constantly warm counterparts.
Behavioral Strategies for Thermal Regulation
Since poikilotherms cannot internally generate sufficient heat to maintain a set temperature, their primary method for survival is behavioral control. These actions maintain the body within a narrow, species-specific optimal temperature range necessary for peak performance. A lizard, for instance, must reach this preferred temperature before it can move quickly enough to hunt or escape predators.
A common strategy is heliothermy, which involves basking in direct sunlight to absorb thermal energy. The animal may change its body posture, such as flattening itself or orienting perpendicular to the sun’s rays, to maximize the exposed surface area. When the body temperature approaches its upper limit, the poikilotherm shifts its behavior, seeking thermal refugia like shade, cool water, or underground burrows.
Movement between sun and shade creates a dynamic temperature profile, allowing the animal to operate at temperatures much higher than the average ambient air temperature. Some species will also press their bodies against warm substrates, like sun-baked rocks, a behavior known as thigmothermy, to absorb conducted heat. This constant adjustment of location and posture allows these animals to regulate their temperature accurately.
Long-Term Adaptations for Survival
Poikilotherms have developed long-term strategies to survive prolonged periods of environmental stress. When temperatures fall below freezing or soar to unsustainable levels, many species enter a state of dormancy. For cold survival, reptiles enter brumation, a period of metabolic slowdown distinct from the deep, continuous sleep of mammalian hibernation.
During brumation, a reptile’s metabolism decreases, but the animal remains somewhat responsive and may periodically wake up to drink water or move to a slightly warmer location. In contrast, estivation is dormancy triggered by extreme heat or drought conditions, often seen in desert amphibians and lungfish. To estivate, a lungfish may burrow deep into mud and secrete a mucous cocoon to prevent desiccation, drastically lowering its metabolic rate until rain returns.
A specialized adaptation for cold climates is freeze tolerance, exemplified by the wood frog. When ice forms in its body tissues, the frog mobilizes glucose from its liver glycogen stores into its cells. This glucose acts as a cryoprotectant, preventing the formation of damaging ice crystals and protecting cellular structures from dehydration and freezing. This physiological preparation allows the animal to survive with up to 65% of its total body water frozen solid.