1. Why do turtles stay in the sun in the morning before looking for food?
Turtles typically stay in the sun until their body temperature rises enough to be active, then go to find food. If they spend too long in the sun, they have less time to search for food, so balancing these needs is difficult [3]. Animals like turtles, which rely on external sources for most of their body heat, are called cold-blooded (ectothermic) animals. They adjust their temperature somewhat by moving between sun and shade. Some insects also raise their body temperature by vibrating their wings [2].
In contrast, mammals and birds are often considered warm-blooded (endothermic) animals because they keep their deep body temperature nearly constant regardless of environmental temperature. They have a heat source inside their bodies [1]. But what is needed to produce heat inside the body?
2. How does keeping a warm body mean needing more food?
A stove uses up fuel quickly if left on. Warm-blooded animals, which constantly produce heat inside their bodies, face a similar situation. To maintain body temperature, warm-blooded animals need to consume large amounts of food energy. Per unit of body weight, this can be tens to hundreds of times more than cold-blooded animals [1]. It is easy to miss the phrase "per unit of body weight." The total amount of food eaten does not necessarily become tens of times larger.
Another source offers a different comparison. The oxygen used by a resting warm-blooded animal is nearly equal to the oxygen used by a cold-blooded animal moving at full speed. Furthermore, when a warm-blooded animal exerts maximum effort, its oxygen use can be 5 to 10 times its resting rate [4]. The different multipliers in these sources are due to different comparison conditions, so they cannot be combined into a single number. What is gained by paying this high cost to keep warm?
3. What are the advantages of keeping warm compared with saving fuel?
The advantage of keeping warm is the ability to move in cold places. In low temperatures, cold-blooded animals become sluggish, and their habitats are often limited. In contrast, mammals and birds have expanded their habitats to environments with various temperatures [1].
However, cold-blooded animals do not only lose out. According to the abstract of a 1980 paper, amphibians and reptiles are said to be better suited than birds and mammals for environments where food, water, or oxygen become scarce at times. This is because they use little fuel [5]. A figure from a source introducing this paper states that about 80% of lizard species remain lighter in weight than small birds or mammals even as adults. Living with a small body using little fuel is considered one reason for this [5]. But how accurate is the term "cold-blooded"?
4. How can tuna and bonito keep their muscles more than 10°C warmer than the water?
Fish are often considered cold-blooded, yet in migratory fish like tuna and bonito, the temperature of the central muscles can be more than 10°C higher than the body surface [2]. Because the body temperature of cold-blooded animals can sometimes be high, encyclopedias state that the term "cold-blooded" is not accurate [2].
According to a review of Pacific bluefin tuna, the mechanism involves a structure called the "rete mirabile," where arteries and veins are arranged alternately. This prevents heat produced by muscles from escaping via blood flow. In adult fish, body temperatures more than 10°C above water temperature have been reported. However, in juvenile fish, the difference is smaller as they are smaller; for fish 25 to 35 cm long, the difference is about 0.25 to 1°C [6].
Fish that warm only part of their body are a very small minority among all fish. They are limited to fast-swimming predators such as tunas, swordfish, and mackerel sharks (including great white sharks). They do not keep their whole body temperature constant [7].
5. Why can body temperature change even in mammals and birds like echidnas and cuckoos?
Even among mammals and birds, some animals, like echidnas and cuckoos, have deep body temperatures that fluctuate somewhat due to environmental temperature. Therefore, it is explained that simply and clearly separating warm-blooded and cold-blooded animals is difficult [1].
One species of Andean hummingbird is reported to lower its body temperature to the 3°C range at night, reducing energy consumption by about 95% while waiting for morning. This is a bird that lowers its body temperature only at night, despite being warm-blooded. For more details, another note on the same site covers this topic [8].
Considering the partial warmth of tuna and the low nighttime temperature of hummingbirds, the boundary between cold-blooded and warm-blooded may not always be a clear division into two groups.
6. How can you investigate sun and shade temperatures yourself?
On a sunny day, compare the temperature of similar stones or ground in the sun and shade at the same location using a thermometer. If you do not have a thermometer, If you do not have a thermometer, compare them from a short distance without touching them.. Do not touch hot stones; they can burn you. Cold-blooded animals use this difference in temperature through their behavior.
When you go out, look for turtles or lizards sunbathing at ponds or zoos. Observe what time they come out into the sun. Watch them from a distance; do not touch or catch them.
SAFETY NOTE: Do not touch hot surfaces with your hands for a long time.