When ‘Cold-Blooded’ Animals Produce Their Own Body Heat
While the terms “cold-blooded” and “warm-blooded” might seem straight-forward enough, there’s actually a large grey area that those two terms don’t capture. Many animals produce their own body heat at some times and rely on external heat sources other times. Insects in particular have adapted to regulate their own body temperatures in surprising and fascinating ways, from moths warming themselves up on cold nights to honey bees turning into living radiators to help their colony survive.
What is a ‘Cold-Blooded’ Animal?
While it’s true that cold-blooded animals usually have a significantly lower body temperature than warm-blooded animals, that’s not all that makes this group of animals unique. “Cold-blooded” animals tend to share three different characteristics — getting their body heat from their surroundings, having a variable body temperature, and having a lower metabolic rate.
However, being cold-blooded or warm-blooded is far more of a spectrum than a binary. Many animals will have one or two “cold-blooded” traits without sharing the others. Additionally, animals that are normally considered cold-blooded might be able to regulate their own temperature in certain situations, and vice versa. There are even some insects and reptiles that have an average body temperature of 40–44°C (104–111°F) — definitely not cold. This is why biologists use more precise terminology, using the term ectothermic for an animal that gets most of its body heat from an external source, the defining feature of what are commonly called “cold-blooded” animals.
All animals need heat to live. The molecular processes that keep an animal’s body running can only function in a certain window of temperature. Too hot or too cold, and an organism will starts to shut down. While being too hot is definitely a concern for animals, the much bigger concern is getting too cold. Heat dissipates constantly — if you leave a mug of hot tea out, the tea will quickly become room temperature as the heat energy leaves, seeking the state of lowest entropy with temperature evenly distributed everywhere.
It’s a big challenge for animals to maintain their body temperature, and different animals solve this problem in different ways. Endotherms, or “warm-blooded” animals, accomplish this by creating their own body heat, while ectotherms get their body heat from sources like the sun. Ectotherms will commonly bask in the sun to get their body temperature to the range where they can be active and look for food.
The smallest animals — like honey bees — have the biggest challenge staying warm. Due to the square-cube law, their surface area to volume ratio is much smaller than that of larger animals, meaning that they have an exceptionally large surface area for their body heat to radiate from. Their small bodies and large thin wings mean that insects warm up quickly but also cool down quickly.
For most insects, this trade-off works. But for some insects, this lack of thermoregulation presents a challenge. Insects are found in all biomes and climates, often performing energetically costly activities like flying long distances. They might be active during inclement weather or at night. So it makes sense that some insects have developed unique strategies to regulate their body temperature in the absence of external heat sources.
Beyond ‘Cold-Blooded’ & ‘Warm-Blooded’
There are pros and cons to each thermoregulatory strategy. Endothermy is a convenient adaptation in many ways, allowing mammals and birds to live in cold places more easily and be active during the night when it’s cooler out, for one example. The obvious downside of endothermy is that it requires a lot of energy in the form of food to maintain. In comparison to cold-blooded animals, warm-blooded animals seem to spend almost all their time looking for food. A warm-blooded animal is always in very real danger of their body temperature leaving the narrow healthy range, and need massive amounts of energy to prevent this.
Another major problem is that most endotherms can’t tolerate fluctuations in body temperature very well, which is called homeothermy. For humans, a change of just a few degrees in either direction can be enough to land you in the hospital. Ectotherms, by contrast, can usually tolerate much larger fluctuations in temperature. As their body heat dissipates throughout the day and their body temperature begins to lower, they don’t suffer from the same negative effects as a warm-blooded animal would — though they do start to run out of the energy needed for being active and need to rest, or find another external heat source.
The term for an animal that has a variable internal body temperature is poikilothermic, and it is a trait found in most animals that are referred to as “cold-blooded”. However, some ectotherms, such as those that live in a consistently hot environment, don’t exhibit poikilothermy, while some endotherms, such as naked mole rats, do seem to exhibit poikilothermy.
It’s clear that animals employ a wide variety of strategies when it comes to thermoregulation, and often mix-and-match these strategies in unexpected combinations. This patchwork approach is known as heterothermy, where an animal exhibits both homeothermy and poikilothermy. This could be a normally homeothermic animal that is able switch to a poikilothermic state during hibernation or torpor, or, on the other side of the coin, a normally ectothermic animal that has evolved a way to regulate their own temperature.
Moths Shiver to Stay Warm
Often, heterothermic ectotherms warm only one part of their body, leaving the rest of their body to match the temperature of the environment. For insects, the part of their body that they prioritize is their thorax, specifically their flight muscles.
Moths generate heat in their thorax before flight by contracting their flight muscles in a way that does not lead to wing flapping. Biologists call this physiological thermoregulation — but it’s better known as shivering.
Shivering is an endothermic process that allows an organism to turn energy into body heat very efficiently, without losing precious energy to making large muscle movements. Using shivering, moths are able to raise the temperature of their thorax substantially, and maintain it while flying. Researchers have pointed to the way a moth’s circulatory system works as an important factor in their ability to maintain their body temperature while flying, with their countercurrent of heat exchange making sure that their thorax always stays warm enough to fly.
Countercurrent heat exchange is a common strategy among endothermic animals and is especially used to prevent extremities from freezing, so this, along with research into how moths use their fat reserves for thermoregulation, has led to biologists referring to many moth species as endothermic.
But if there’s one insect that has truly mastered endothermy, it would have to be the honey bee.
‘Warm-Blooded’ Honey Bees Help Hives Survive
Like moths, honey bees focus on keeping their thorax at a constant temperature and rely on exciting their flight muscles to do so, decoupling their wings to prevent flight.
But honey bee thermoregulation goes beyond just their biology and includes a remarkable level of social cooperation to ensure the optimal temperature not just for themselves, but for the hive’s eggs and larva as well:
These social bees are endothermic at both the individual and colony level, expending considerable energy to maintain brood temperatures within a remarkably narrow, 34–36 °C range close to mammalian body temperature. Honey bee physiology is also highly temperature-sensitive, with peak function achieved within a narrow range of high temperatures. […] The immature life stages, which develop entirely within the thermoregulated colony core, are even more temperature-sensitive than are adults.
Bees are experts at thermoregulation, from building comb in configurations that allow for optimal ventilation to leaving open gaps besides brood cells where a bee can fan cool air or shiver warm air as needed for the optimal development of the eggs and larvae. In the winter, honey bees cluster in the center of the nest surrounding their queen. A combination of insulation by the outer layer of bees and endothermic heat production allows the bees to survive harsh winters.
Honey bees can also use their thermoregulation for less benevolent purposes. A defensive tactic known as balling behavior, a group of workers will surround a target in a tight ball and vibrate their flight muscles, producing enough heat that, along with the low levels of CO₂ at the center of the sphere, will kill the unfortunate target. Most often, this is done when a hive rejects their queen. Famously, Asian honey bees (Apis cerana) will also use this to kill northern giant hornets. Despite this behavior not being as common in the European honey bee species (Apis mellifera), there have also been reports of defensive balling behavior in A. mellifera against both native and introduced hornet species.
It is this social organization and adaptability that makes honey bee thermoregulation so unique. A 2010 paper proposed that both the endothermic and ectothermic honey bees in a hive played an important role in thermoregulation of the colony, forming a thermoregulatory superorganism.
It’s not only individual honey bees that are “warm-blooded” organisms— the entire colony is one as well.
