Warm-blooded vs. Cold-blooded—What’s the Difference?

Learn the difference between ectothermic and endothermic animals—and why it matters

The year 2023 is on track to be the hottest year on record, and many people are feeling the heat. And humans aren’t the only ones that are affected. Animals around the world and throughout our ocean are impacted by changing temperatures. One particular animal that’s been in the news is coral; coral reefs throughout the northern hemisphere are bleaching at alarming rates and causing great alarm among scientists. 

Predicting how animals will respond to climate change is complicated because there are so many factors at play. The first step is understanding the biology of the animals and then assessing how the animals interact with their environments. Let’s dive into the science behind how animals control their own temperatures, and then see how those strategies affect their response to climate change. 

Thermoregulation 101 

How animals maintain their internal body temperatures (also known as thermoregulation) is critically important to their survival. You might not realize it, but you’ve definitely seen thermoregulation in action: From lizards lying in the sun to dogs panting in the heat, animals employ all kinds of methods to warm up or cool down. Just like humans, if their body temperatures get too hot or too cold, it can cause sickness or even death. 

When it comes to how animals keep their body temperature in check, there are two main categories: ectotherms and endotherms. Let’s break it down. 

What is an ectotherm? 

Ectotherms are animals that rely on external sources for heat. You might have also heard these animals referred to as “cold-blooded”. Think fish, reptiles, amphibians and many invertebrates: these animals’ body temperatures will generally match the environmental temperature around them. That’s why you will see turtles and alligators basking in the sun to warm up or fish burrowing in the bottom of a lake during freezing months. 

Ectotherms live in all kinds of environments all over the planet—even extremely cold or hot ones. These animals have special adaptations that allow them to survive, like these fish that have anti-freeze proteins in their bodies. Some land invertebrates like insects and nematodes have even been known to survive in lab studies in temperatures as cold as -112 °F! 

What is an endotherm? 

On the flip side, endotherms, also known as warm-blooded animals, keep their body temperature consistent. You might be more familiar with endotherms because, well, humans are endotherms! Our bodies stay at about 98 °F, although this can fluctuate a few degrees above or below. Other endotherms include most mammals and birds—mammals tend to keep their body temperatures between 96.8 and 100.4 °F, and, depending on the species, birds can range between 93.2 and 111.2 °F. Unlike ectotherms, endotherms spend a lot of energy keeping their bodies at a consistent temperature. As a result, they tend to consume many more calories than ectotherms. 

As with everything in nature, there are a few cool exceptions. Although most fish are ectothermic, there are a few that are regionally endothermic, meaning they can keep some parts of their bodies at higher temperatures than the surrounding waters. These fish, including bluefin tuna, white sharks, mako sharks and salmon sharks, use special strategies like counter-current blood circulation to raise their body temperatures. This allows these animals to swim faster than they would otherwise—pretty handy for a predator.  

How does this apply to climate change? 

Every animal has a preferred range of temperatures—that’s why you find some animals that are adapted to the tropics and some that are adapted to colder climates, for example. And since more than 90% of all animals are ectothermic, most animals directly depend on the environment around them to provide ideal temperature conditions. When temperatures get out of their comfort zones, animals have three choices: adapt, move or die. Mobile animals that can walk, swim or fly away might be able to move to change their environments—we’re already seeing some fish stocks moving towards the poles and deeper as water temperatures rise. Others can adapt to the new temperatures by adopting new cooling strategies or even changing their bodies to better dissipate heat. For animals that can’t move or adapt, however, the path is bleak: Heat stress could wipe certain species off the planet.

Climate change is affecting temperature and climate patterns all over the planet, and we will see more extreme temperatures, like our extremely hot summer in 2023. Predicting exactly how animals will respond is very challenging for a few reasons. First, we need to understand more about how individual species react to different temperatures, which we try to do through field or lab experiments. That’s where what we learned today comes into play; whether or not an animal is an ecto- or endo- therm will affect how it adjusts to changing temperatures. Even if we understand how individuals will react, we still need to predict how the ecosystem will respond. For example, perhaps one type of fish will be able to adapt well to warmer waters, but its prey will not. Not being able to find food will also affect the fish’s survival. 

We need much more research to understand exactly how animals will respond to climate change. But research is not enough—we know it will cause massive upheavals in our ocean and throughout the planet. We have the opportunity to change the course of climate change and prevent the worst outcomes from happening, but we need to act fast and act now. At Ocean Conservancy, we are working with partners around the world to confront climate change and develop sustainable, ocean-based solutions. Take action for our ocean with us now, and call on the United States to deliver on our commitments to address the climate crisis. 

Our work is focused on solving some of the greatest threats facing our ocean today. We bring people, science and policy together to champion innovative solutions and fight for a sustainable ocean.
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