By Blaise Jones
Sharks have a reputation as “cold-blooded killers” and despite how negative that term is, it’s not entirely inaccurate. Sharks are by no means evil, vicious killers like that quote suggests. Nonetheless, sharks are, for the most part anyways, efficient ectothermic predators.
Endo vs Ecto
What does it mean to be warm or cold blooded? Technically, the terms “warm-blooded” and “cold-blooded” are misnomers. The terms that truly describe what is going on are “endothermic” and “ectothermic.” Endothermic animals are animals that can regulate their body temperatures and produce their own heat through their metabolic processes. Because of this, endotherms are able to keep their bodies at a constant temperature. For example, humans are endotherms, and we keep our bodies at 98.6°F (3). Endothermy is one of the key traits in classifying animals, and only mammals and birds are truly endothermic.
Everyone else is ectothermic. This includes reptiles, arthropods, and fish. This means that these animals cannot generate their own heat, nor regulate it, and rely on the temperature of their environment and other outside sources in order to stay warm or cold. This has led to a vast variety of survival strategies across the animal kingdom, many of which sharks use.
Shark Strategy Guide to Staying Warm
Most sharks maintain high body temperatures simply by living in warm water. This is reflected by the preferred habitat of most sharks, the tropics. Most sharks prefer water around 70°F or warmer, and thus their bodies are kept around that same temperature (4).
However, not all sharks stick to the tropics. Quite a few species of shark live in temperate water, where the temperature gets as low as 50°F. Some sharks go even colder, such as the Greenland sleeper shark. These sharks live in icy water that is typically around 35°F. These sharks have extremely low metabolisms, and are very slow and sluggish. However, not all cold-water sharks are as cold as their environments.
Seven members of the Lamnidae or Mackerel sharks have evolved a workaround of sorts to being cold-blooded. Tthe great white, the long fin and short fin makos, the porbeagle, the salmon shark, and the common and big-eye thresher shark all can pseudo-regulate their body temperatures. This is because these sharks have a very unique set of blood vessels.
The main purpose of blood vessels, be they arteries or veins, is to transport blood, and therefore nutrients, throughout the body and to transport waste material out of the body. These seven mackerel sharks have three sets of special arteries known as the “rete mirable.” The retia serve as a sort of insulation system for the blood, putting several layers of countercurrent blood flow between the cold blood being transported from the gills and the warm blood flowing from the muscles.
So while these seven special mackerel sharks cannot actively generate a constant level of body heat like mammals and birds can, their unique circulatory system allows them to stay warmer than their surrounding environment.
- “Sharks: The Animal Answer Guide” by Gene Helfman and George H. Burgess
- “The Encyclopedia of Sharks” by Steve Parker