The common term “cold-blooded animal” refers to ectothermy, a biological strategy where an organism’s internal temperature is governed primarily by its surroundings. Ectotherms (including reptiles, amphibians, fish, and most invertebrates) cannot generate enough internal heat to maintain a stable body temperature. Their physiological activity is thus directly linked to the temperature of the air, water, or ground around them.
Defining External Temperature Regulation
Ectothermy is the practice of regulating body temperature by relying almost entirely on external heat sources. Animals employing this strategy absorb thermal energy from the environment to reach their optimal operational temperature. For a lizard, this might mean absorbing solar radiation directly or lying on a sun-warmed rock to soak up conductive heat.
This reliance on the external world means that the animal’s internal temperature often fluctuates with the ambient conditions. This characteristic is known as poikilothermy, describing an organism whose body temperature varies widely. Ectothermy defines the source of heat, while poikilothermy describes the variability of the resulting temperature.
A fish, for instance, is an ectotherm whose body temperature remains nearly identical to the water it swims in. Terrestrial ectotherms, such as snakes and turtles, actively manage their internal heat by moving between warm and cool microclimates. Without external heat access, their body temperature remains low, limiting physical functions.
How Ectotherms Differ from Endotherms
The fundamental difference between ectotherms and endotherms (like mammals and birds) lies in the source of their body heat and metabolic cost. Endotherms produce heat internally through metabolic processes, a strategy that allows them to maintain a near-constant, high body temperature (homeothermy) regardless of the external environment. This internal regulation is energetically expensive.
Ectotherms, by contrast, have significantly lower metabolic rates and use far less energy. Their energy expenditure can be as little as 10% of what an endotherm of comparable size requires. This low energy demand means ectotherms can survive on much less food, allowing them to thrive in environments where resources are scarce or unpredictable.
The trade-off is a dependence on the environment that limits activity. When temperatures are low, an ectotherm’s physiological functions slow dramatically, making movement, digestion, and reaction time sluggish. Endotherms can remain active over a wide range of temperatures, but ectotherms are restricted to operating within a narrower, environmentally defined thermal window.
Survival Strategies for Temperature Extremes
Ectotherms are not merely passive recipients of environmental heat; they employ sophisticated behavioral thermoregulation to survive temperature extremes. To warm up, a reptile may flatten its body to maximize surface area exposure while basking in the sun. To cool down, it will seek shade, enter a burrow, or move into water to take advantage of evaporative cooling.
In response to sustained cold, many ectotherms enter states of dormancy to conserve energy. This includes torpor, a short-term reduction in metabolic rate, or brumation (seasonal hibernation in reptiles and amphibians). During brumation, the animal’s metabolism slows to a fraction of its normal rate, sometimes dropping to only one to two percent of its resting level.
Some species possess physiological defenses against freezing temperatures. The wood frog, for example, can survive having up to 65% of its body water turn to ice by flooding its cells with cryoprotectants, such as glucose. This adaptation lowers the freezing point of the cell fluid, preventing destructive ice crystal formation and protecting tissues until warmer conditions return.