We tend to associate cold-blooded animals with cruel, slow, emotionless creatures. Animals who can’t feel pain or fear. Animals who have been sitting patiently in ambush since the Late Ordovician Period and have survived all five mass extinction events…
Animals like sharks.
But of course, now scientists know that sharks aren’t cruel or slow. They show emotions, and they feel pain and fear. So there must be more to it than that. And in fact, not all sharks are cold-blooded.
The Lamnidae sharks, or Mackerel sharks, are able to regulate their body temperature and elevate it higher than the surrounding water. In fact, the great white can maintain internal temperatures 14ºC above the ambient temperature, making it warm-blooded.
But what does it really mean to be warm-blooded? And what perks and drawbacks are there to it?
Warm Blood vs. Cold Blood
Homeostasis is the body’s state of equilibrium. This includes the optimal water content, salinity, and temperature of all the tissues and organs.
When it comes to regulating temperature or thermoregulation, different strategies are used, depending on the environment.
Ectothermy and Endothermy are the smarty-pants terms for cold-blooded and warm-blooded animals.
Both of these are thermoregulation systems, but it’s better not to think of them as binary. Rather, all animals fall somewhere along a spectrum between ectothermy and endothermy.
Among both endotherms and ectotherms, there are animals whose internal temperature varies widely and those whose temperature remains within tight boundaries.
Humans, for example, have a very small range of internal temperature, usually somewhere between 97 and 100F (36°C and 38°C). We call these homeotherms. Animals with a wide range of body temperatures are called poikilotherms.
Then you’ve got some really strange animals who are ectothermic homeotherms. These animals must be really frustrated because they have to chase the shifts in their ambient temperature to maintain a narrow body temperature range.
Animals like desert lizards are examples of this. They will constantly move in and out of the sun to regulate temperature.
Animals towards the endothermy end of the spectrum generate heat through metabolic processes and have to either hold onto it or lose it.
When you sweat, you’re trying to lose it; when your hairs stand on end, you’re trying to hold onto it.
These physiological reactions are how you are able to maintain such a narrow range of body temperature.
Ectotherms don’t sweat or shiver and usually have to come out and sit on a rock in the sun in the morning to get going.
On the plus side, they also don’t metabolize quickly, so they hardly ever have to eat or drink, compared with endotherms who use energy so much they need to eat almost every day.
A handful of shark species have evolved to increase their body temperature.
This typically gives them a speed advantage in colder water over their cold-blooded counterparts.
Most of these species are in the Mackerel shark family, the Lamnids. This group includes the salon shark, the mako, and the great white.
Their warmer blood means they don’t slow down as much when they’re in colder waters, so they can maintain their effective hunting reflexes and travel farther afield, away from the warm, tropical waters of the equator.
It was once thought that warm-blooded sharks would be able to occupy a wider range of habitats, but this has turned out to be false.
The major advantage of warm blood is in the swimming speed and, potentially, better recovery from exertion (such as hunting).
So, if great whites are warm-blooded, does this mean they shiver and sweat?
Well, not exactly. Not all endotherms use the same methods to regulate heat.
Dogs pant because they can’t sweat, and hippos spend the hottest part of the day in the water to stay cool.
Sharks have their own methods for staying warm, and it involves specialized muscles and blood vessels.
So how do endothermic sharks regulate their temperature? Let’s look at some of the endothermic and ectothermic sharks and how they do what they do.
- White sharks use what is called a countercurrent exchanger. This specialized blood vessel system heats deoxygenated blood via the swimming muscles and exchanges that heat with the cold arterial blood from the gills. This way, they can maintain an internal temperature above the surrounding water. With this, they can dive into deep, cold water searching for prey.
- Basking sharks aren’t as effective as whites at raising their temperature, but it’s enough to stay a few degrees above the chilly arctic temperature of their northern-most feeding grounds, where they vacuum up plankton.
- Salmon sharks are in the same family as great whites. They can increase their temperature to around 50°F (10°C) higher than the surrounding water, and as with all the Laminiformes, they are fast sharks that can handle hunting cold waters without slowing down.
These sharks use their endothermy to survive in cold water and maintain their speed advantages.
But what happens to sharks that can’t increase their body temperature? You might think that they’ll stay away from the arctic, but maybe you’ll be surprised.
Most sharks are ectothermic or cold-blooded. This allows them to metabolize slowly, eating less frequently than they would have to if they were endothermic.
That’s a huge benefit to animals living in areas where there isn’t a lot of food, such as migratory sharks who may not come across anything tasty for weeks on end.
But the downside to this is that they can get slow and weak if they get too cold, so they need to stay in warmer waters if they’re going to stay sharp.
Cold-blooded sharks may migrate to follow the warm weather patterns and currents, traveling away from the equator during warm months and returning closer to it during colder periods.
There are some exceptions to this, however. Cold-blooded arctic sharks do exist, and they have some fascinating adaptations to handle the environment!
- Greenland sharks live in some of the coldest water there is. Their internal temperature can be as low as 34°F (1°C)! For a cold-blooded fish, this means moving extremely slowly. And they do! These sharks swim around at less than one mph in ice-cold water, relying on high urea contents to work as anti-freeze in their tissues. They even age slowly, possibly reaching ages of up to 400 years. This is one perk of a slow metabolism!
- Whale sharks really put the definition of ectothermy to the test. When measured, whale shark muscle dropped to only 66°F (19°C) after an hour in 39°F (4°C) water. This fish is so large that its ‘thermal inertia’ protects it. It’s thought that maybe they can use this to dive into the cold depths to remove parasites.
- Scalloped hammerhead sharks are cold-blooded, but it’s suspected that they hold their breath when going on cold dives to maintain their body temperature! This cute behavioral modification would allow them to prevent the cold water from drawing heat out of their gills.
Which is Better – Cold-Blood or Warm-Blood?
So, to summarize the pros and cons of cold vs. warm blood:
- Can swim faster than cold-blooded relatives
- Have a high metabolic requirement
- Can recover from bouts of exertion faster
- Don’t lose speed or agility when they’re in cold water
- Can live for a really long time!
- Don’t need to eat very often
- Can lose heat through their gills in cold water
- Have some incredible adaptations for living in cooler water
Do sharks need sunlight?
All these animals, whether endotherms or ectotherms, have body temperatures that vary wildly.
Thus, they’re poikilotherms too. While the endothermic sharks can maintain a temperature above ambient, they will still warm up and cool down with changes in water temperatures.
Ectotherms might spend longer in the balmy tropics than endotherms, in general, to stay warm, but as we can see, they can live pretty much everywhere too. So, do they need sunlight?
Sharks like the Greenland shark clearly don’t really need sunlight to survive. At least, not directly.
These ancient fish spend days or months slowly traversing the sea bed under the ice. But some tropical sharks need the warm waters of the sunny shallows to maintain an effective body temperature.
Scalloped hammerheads have even been known to tan in the sun, protecting them as they leave the murky bay they grow up in.
Tropical sharks like the blacktip reef shark may be found basking lazily in the sun-warmed shallow waters, but not all sharks need to be solar-heated.
As mentioned, whale sharks have a unique ability to hold onto their body temperature through sheer size. This concept has been termed gigantothermy and relies on the surface area to volume ratio of this enormous fish to maintain a thermic inertia while diving into chilly depths to remove parasites.
White sharks have been spotted using the sun’s angle to their advantage when hunting. This ambush predator makes use of camouflage and angles its attack to come out of the glare of the sun, so its prey never sees it coming.
Many sharks use lighting to their advantage.
Ever wonder why sharks are dark on top and light underneath?
This is called countershading and is a really common visual adaptation of the coloration of marine animals. Basically, when looking from above, the shark is camouflaged against the deep blue beneath it, and when looking from below, it’s as pale as the sunny sky through the ripples.
There seems to be a pattern emerging.
Whenever there’s a question of ‘are sharks one way or another,’ the answer is usually: well, they’re both. And this is for a good reason. These animals have been around for 450 million years! They’ve had plenty of time for evolutionary experiments, and they’ve occupied niches in almost all salty bodies of water on earth.
However, when it comes to the great white shark, it’s safe to say that they are endothermic, or warm-blooded. They have special blood vessel systems that sun through heating muscles to exchange warmth from the body into the incoming arterial blood from the gills.
There are perks and handicaps to being either warm or cold-blooded. Warm blood uses more energy, so there needs to be a richer food supply, but it also allows a shark to have a wider habitat range. Cold-blooded sharks live more slowly, live longer, and eat less, but can be a bit limited in the areas they can inhabit.