Endothermy and ectothermy relate to a widely accepted classification scheme based on the source of heat stored in the body.
- Endotherms: animals that can generate their own heat
- Ectotherms: animals that rely on environmental sources of heat
Ectotherms, sometimes called 'cold blooded' animals, are poorly insulated and produce metabolic heat at a low rate compared with endotherms. They also lose heat to cooler surroundings quickly. High thermal conductance, however, allows ectotherms to absorb heat readily from the environment. It is mostly through behavioural mechanisms that ectotherms regulate body temperature, for example, by basking in the sun or burrowing under ground.
Ectothermic Animals And Their Ability To Live In Cold Environments
Ectotherms living in cold environments manage to survive because they can generate adequate levels of metabolism at the lowest level of enzyme activity. Enzymes in these animals show most activity at much lower temperatures then animals living in warmer climates.
When ectotherms live in areas where temperatures can be freezing, it is a great risk for them due to the formation of ice crystals in cells. These ice crystals can grow larger, rupture and destroy cells. Ice crystals that form outside of the cells, however, do little damage, so ectotherms living in these environments have adapted to prevent intracellular ice crystal formation. The three ways they do this are:
- Preemptive ice crystal formation
- Antifreezes
- Supercooling
How Ectotherms Use Preemptive Ice Crystal Formation, Antifreezes And Supercooling To Deal With Cold Environments
Certain beetles can withstand freezing temperatures because they form ice crystals in their extracellular fluid. The extracellular fluid (but not intracellular) contains a substance that accelerates nucleation.
Nucleation in the process that begins ice crystal formation. As the temperature drops below freezing, extracellular fluid freezes, forming ice crystals around nucleating agents. Solutes (substance dissolved in a given solution) are excluded from these ice crystals, which raises extracellular solute concentrations. This increase in solute concentration then draws water out of surrounding cells, which in turn, raises the solute concentration inside the cell, lowering its freezing point and preventing ice crystals from forming.
Some ectotherms’ body fluids contain antifreeze substances. Many arthropods, including insects, have glycerol in their body fluids that often increases during winter. Glycerol lowers the freezing point of body fluids and this prevents ice crystal formation.
The Arctic fish Trematomus, has glycoprotein antifreeze in its blood which delays ice crystal formation. Glycoprotein doesn’t lower the temperature at which ice crystals are formed, but lowers the temperature at which they would enlarge and begin to destroy cells.
Supercooling is a state where an ectotherm’s body fluids are cooled to below freezing without forming ice crystals, because they have no nuclei to initiate ice crystals. There are some fishes that live in Arctic fjords in a constant supercooled state and do not freeze. Many insects have this adaptation also.
Ectothermic Animals and Their Ability to Survive in Hot Environments
Ectotherms have a critical thermal maximum (CMT) temperature threshold. If their temperature goes above their CMT for a length of time, it could prove fatal. Most ectotherms don’t allow long-term temperatures to come close to their CMT. To avoid unacceptably high body temperatures, ectotherms move to reduce heat gain. Once their body temperature drops, they may move back into the sun.
Physiological mechanisms also play a part in controlling an ectotherms’ temperature. For example the marine iguana, Amblyrhynchus cristatus’ body temperature rises at almost twice the rate at which it drops. To regulate its temperature, the iguana moves between land and water and adjusts its heart rate and the rate of blood flow to its surface tissue.
While basking in the sun, the iguana will decrease its heart rate and diverts cooler core blood to its body surface by vasodilatation (dilation of blood vessels). When in the water, the iguana’s heat loss is retarded by its slowed heart rate and vasoconstriction (constriction of blood vessels), which minimize blood flow to the skin.
Resource and further reading:
Silverthorn, D.U, 2007, ‘Energetic Costs of Meeting Environmental challenges’, in Human Physiology, ed. Benjamin Cummings, San Francisco, chap. 17.
 |
