A poikilotherm is an animal whose internal body temperature varies significantly, often fluctuating with the temperature of its surrounding environment. This contrasts with animals that maintain a relatively constant internal temperature regardless of external conditions.
Understanding Poikilothermy
The term "poikilotherm" describes a broad category of animals that do not possess the physiological mechanisms to maintain a stable core body temperature through internal metabolic heat production. Instead, their body temperature is largely determined by external factors such as air temperature, water temperature, or direct sunlight.
Key characteristics of poikilotherms include:
- Variable Body Temperature: Their internal temperature fluctuates widely, mirroring the ambient temperature.
- Reliance on External Heat: They primarily depend on external heat sources (like the sun or warm rocks) to warm their bodies.
- Lower Metabolic Rates: Generally, poikilotherms have lower metabolic rates compared to animals that regulate their own body temperature, leading to lower energy requirements.
Poikilotherms vs. Homeotherms: A Comparison
The biological world classifies animals based on how they regulate their body temperature. Poikilotherms are often contrasted with homeotherms, which are animals that maintain a stable internal body temperature.
| Feature | Poikilotherm (Ectotherm) | Homeotherm (Endotherm) |
|---|---|---|
| Temperature Regulation | Varies with external environment | Relatively constant, internally regulated |
| Primary Heat Source | External (e.g., sun, environment) | Internal (metabolic processes) |
| Metabolic Rate | Generally lower | Generally higher |
| Energy Consumption | Lower | Higher |
| Examples | Most fish, amphibians, reptiles, invertebrates | Birds, mammals |
Synonyms and Examples
The term ectotherm is often used interchangeably with poikilotherm. While "poikilotherm" emphasizes the variability of body temperature, "ectotherm" highlights that the animal derives its heat from external sources. Most animals, with the notable exceptions of birds and mammals, fall under the category of poikilotherms/ectotherms.
Common examples of poikilotherms include:
- Reptiles: Snakes, lizards, crocodiles, alligators, and turtles. These animals are famous for basking in the sun to warm up.
- Amphibians: Frogs, salamanders, and newts. Their moist skin makes them particularly susceptible to environmental temperature changes.
- Fish: The vast majority of fish species are poikilothermic, with their body temperature closely matching the water they inhabit.
- Invertebrates: A vast array of creatures, including insects, spiders, worms, and crustaceans, are poikilotherms.
Adaptations and Survival Strategies
Despite their temperature variability, poikilotherms have developed diverse and effective strategies to survive and thrive across various environments:
- Behavioral Thermoregulation:
- Basking: Positioning themselves in direct sunlight to absorb heat.
- Seeking Shade/Burrowing: Moving to cooler areas to avoid overheating.
- Changing Orientation: Adjusting their body angle to maximize or minimize sun exposure.
- Migration/Hibernation: Moving to more favorable climates or entering a state of dormancy during extreme temperatures.
- Physiological Adaptations:
- Tolerance to Wide Temperature Ranges: Many can withstand a broader range of internal temperatures than homeotherms.
- Metabolic Depression: Ability to slow down their metabolism during cold periods (e.g., brumation in reptiles) to conserve energy.
- Countercurrent Heat Exchange: In some larger fish (like tuna) and insects, specialized blood vessel arrangements can help retain metabolic heat, allowing them to be "regional homeotherms" or maintain slightly warmer internal temperatures than ambient.
The ability to rely on external heat sources provides a significant energy advantage, as less metabolic energy is expended on maintaining a constant body temperature. This allows poikilotherms to survive on less food compared to homeotherms of similar size.
