Poikilothermia describes organisms whose internal temperature varies considerably, often mirroring their immediate surroundings. The term originates from the Greek words poikilos, meaning “varied,” and thermos, meaning “heat.” An animal with this trait, a poikilotherm, cannot maintain a stable internal temperature, so its body temperature fluctuates with the environment. This variability requires specific adaptations to cope with temperature changes.
Distinguishing Key Thermoregulation Terms
The direct opposite of poikilothermy is homeothermy. Homeothermic animals, such as mammals and birds, maintain a stable internal body temperature regardless of the external environment. A human’s body core temperature, for example, stays around 37 degrees Celsius. In contrast, a poikilotherm’s temperature is not constant and fluctuates over a wide range.
Ectothermy and endothermy describe the source of an organism’s body heat. Ectotherms rely on external sources, like the sun, to warm their bodies, while endotherms generate most heat internally through metabolic processes. These terms are related but not interchangeable. Most ectotherms are also poikilotherms because their reliance on external heat leads to a variable internal temperature.
The term “cold-blooded” is often used for poikilotherms but is scientifically inaccurate. A lizard, for example, can have very warm blood after basking in the sun—potentially warmer than a human’s. The defining characteristic is not the coldness of the blood but its variability, as the animal’s internal temperature conforms to its environment.
Examples of Poikilothermic Animals
Poikilothermia is a common strategy across many animal groups. Most vertebrates that are not mammals or birds exhibit this trait. Examples include:
- Reptiles, such as lizards, snakes, turtles, and crocodiles, which depend on environmental conditions for their body temperature.
- Amphibians, like frogs and salamanders, whose moist skin makes them sensitive to temperature and hydration changes.
- The vast majority of fish species, living in aquatic environments with fluctuating temperatures.
- Invertebrates, which are the largest group and include nearly all insects, arachnids, worms, and mollusks.
Behavioral and Physiological Adaptations
Poikilotherms use behavioral and physiological adaptations to manage their body temperature. Behavioral thermoregulation involves actions to control temperature. A common behavior is shuttling between sun and shade; a lizard might bask on a rock to warm up, then retreat to a cooler spot to avoid overheating.
Other behavioral tactics include:
- Changing posture to maximize or minimize sun exposure.
- Burrowing underground to find more stable temperatures.
- Undergoing brumation, a state of dormancy, to survive long periods of cold.
- Warming up by rapidly contracting wing muscles before flight, as seen in some insects.
Physiological adaptations are internal mechanisms for coping with temperature extremes. Some insects and fish in freezing climates produce cryoprotectants, like glycerol or “antifreeze” proteins, in their blood to prevent ice crystals from forming in their cells. The metabolic rate of a poikilotherm also slows in the cold, conserving energy. Some have multiple enzyme systems for the same chemical reaction, each optimized for a different temperature.
The Energetic Trade-Off
The primary benefit of poikilothermy is its low energy cost. Poikilotherms do not expend metabolic energy to maintain a constant internal temperature, so they require far less food than homeotherms of a similar size—needing only 5% to 10% of the energy. This efficiency allows them to thrive where food is scarce or unpredictable. A given habitat can therefore support a much greater density of poikilotherms than homeotherms.
This energy conservation comes at a cost. A poikilotherm’s activity level is highly dependent on the ambient temperature. When it gets too cold, their metabolic processes slow, making them sluggish and vulnerable. This limits their ability to hunt, escape predators, or reproduce during colder periods and restricts their geographical distribution to warmer climates.