Scientists are discovering a new way climate change is threatening cold-blooded animals

Young animals use energy to grow and reproduce later in life. Photo credit: Shutterstock

All animals need energy to live. They use it to breathe, circulate blood, digest food, and move. Young animals use energy to grow and reproduce later in life.

Elevated body temperature increases the rate at which an animal uses energy. Because cold-blooded animals rely on the thermal conditions of their environment to regulate their body temperature, they are expected to require more energy as the planet warms.

Our new study, published today in nature climate change, suggests that temperature is not the only environmental factor influencing the future energy needs of cold-blooded animals. How they interact with other species will also play a role.

Our results suggest that cold-blooded animals need even more energy in a warmer world than previously thought. This can increase their risk of extinction.

What we already know

The amount of energy that animals use in a given amount of time is called the metabolic rate.

Metabolic rate is affected by a variety of factors, including body size and activity level. Larger animals have higher metabolic rates than smaller animals, and active animals have higher metabolic rates than inactive animals.

Metabolic rate also depends on body temperature. This is because temperature affects the speed at which the biochemical reactions involved in energy metabolism occur. Generally, when an animal’s body temperature increases, its metabolic rate accelerates exponentially.

Most animals alive today are cold-blooded or “ectotherms”. Insects, worms, fish, crustaceans, amphibians, and reptiles—basically all living things except mammals and birds—are alternating heat.

As human-caused climate change increases global temperatures, the body temperatures of cold-blooded animals are also expected to rise.

Researchers say the metabolic rate of some land-based ectotherms may already have increased by 3.5% to 12% due to global warming that has already occurred. But this prediction doesn’t account for the animals’ ability to physiologically “acclimate” to warmer temperatures.

Acclimatization refers to an animal’s ability to remodel its physiology to cope with a change in its environment.

But rarely, acclimatization can completely negate the influence of temperature on metabolic processes. For this reason, by the end of the century, land-based ectotherms are still expected to have a metabolic rate about 20% to 30% higher than today.

A higher metabolic rate means animals need more food. This means that if more food isn’t available, they could starve, leaving them less energy to find a mate and reproduce.

Our research

Previous research attempts to understand the energetic costs of global warming for ectotherms have been limited in one important respect. They predominantly used animals studied in relatively simple laboratory settings where the only challenge they faced was a change in temperature.

However, animals face many other challenges in nature. This includes interactions with other species, such as competition for food and predator-prey relationships.

Although species interact constantly in nature, we rarely study how this affects metabolic rates.

We aimed to investigate how species interactions might alter predictions about the energetic costs of climate warming for cold-blooded animals. To do this, we turned to the fruit fly (from the genus Drosophila).

Species of fruit flies lay their eggs in decaying plant matter. The larvae that hatch from these eggs interact and compete for food.

Our study involved rearing fruit fly species alone or together at different temperatures. We found that when competing for food in warmer temperatures, two species of fruit fly larvae were more active as adults than adults who were not competing with other species as larvae. As a result, they also used more energy.

From this, we derived, using models, that with warmer global temperatures, species interactions increase future energy demands of fruit flies by 3% to 16%.

These results suggest that previous studies have underestimated the energetic costs of global warming for ectotherms. Purely physiological approaches to understanding the consequences of climate change for cold-blooded animals are therefore unlikely to suffice.

Let’s get real

Understanding the energy needs of animals is important to understanding how they survive, reproduce and develop in challenging environments.

In a warmer world, hotter ectotherms require more energy to survive and reproduce. When there isn’t enough food to meet their body’s energy needs, their risk of extinction can increase.

Of course, we need to be more accurate in predicting how global warming will threaten biodiversity. This means studying animal responses to temperature changes under more realistic conditions.

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