Why can crocodiles and snakes go for months without eating?
― Just lying in bed, a person burns about 10 times the fuel of a crocodile of the same weight
A person who skips food for a few days starts to feel shaky. Yet crocodiles and big snakes can go for months without eating. The odd one out isn't the crocodile. It's us: even at rest, we keep burning a huge amount of fuel just to stay warm.
A crocodile pond at a zoo. The crocodile lies at the water's edge with its mouth open, motionless for tens of minutes. The keeper says it is fed about once a week, and even less in winter.
Meanwhile, you skip breakfast, and by late morning your stomach is growling and your head feels foggy.
The two of you are about the same size. Why such a difference?
There are just two reasons
People burn the food they eat to hold the body at 37°C. Crocodiles and snakes leave the job of warming up to the sun and the surrounding temperature.
As body temperature falls, chemical reactions in the body slow. The energy they use drops even further.
So a crocodile isn't especially "patient". It simply uses far, far less fuel each day.
Where does a person's fuel go?
Even lying still all day, an adult uses about 1,500 kilocalories. This is called the basal metabolic rate. It is the energy that keeps the heart and brain running and rebuilds cells.
All the energy used ends up as heat. People hold on to this heat and use it to stay at 37°C even when it is cold outside. It's as if there were a stove inside the body, left on all day long.
Animals such as crocodiles, snakes, lizards and frogs are called cold-blooded animals (ectotherms). The stove inside them is tiny. Compared at the same body weight, the energy they use at rest is said to be about one fifth to one tenth that of animals like us.
Look at Figure 1. The tall bar on the left is a person, and the short bar on the right is a crocodile of the same weight, 60 kilograms. Use the slider below to change the crocodile's body temperature. The height of the right bar and the number of days its stored fat lasts will change.
Animals that "borrow" their body heat
When a cold-blooded animal wants to warm up, it steps into the sun. When it wants to cool down, it moves into shade or water. It gets its body heat from sunlight rather than from food. Compare the left and right sides of Figure 2.
On top of that, when body temperature falls, chemical reactions in the body slow down. As a rule of thumb, every 10°C drop in body temperature cuts the energy used to about one half to one third. This is why crocodiles and snakes in the cold season hardly move and don't eat.
As a result, there are records of large crocodiles and pythons going without food for several months, and sometimes more than a year. A person can survive on water alone for, depending on conditions, at most about one to two months.
In exchange for using lots of fuel, people and birds can move quickly even on a cold morning or at night. A cold-blooded animal can't move fast when its body is chilled. Warm-blooded animals like us pay for "full power at any time" by eating a lot.
Summary
Crocodiles and snakes can go without food because they leave the job of warming up to the sun and the surrounding temperature, and burn almost no fuel themselves. When they cool down, they save even more. A person, even asleep, burns about 10 times the fuel of a crocodile of the same weight to stay at 37°C.
It isn't that crocodiles eat little.
It's that people keep the stove on all day long.
The link between body size and fuel is covered in "Why do smaller animals have to keep eating to stay alive?", and the strategy of lowering body temperature itself in winter is in "Why can hibernating animals go for months without eating?". An example of body temperature setting the speed of reactions also appears in the story of "the cricket's chirp".
- At a zoo or aquarium, compare whether crocodiles or turtles are in the sun or the shade in the morning and at midday. In the morning they are often gathered in the sun.
- On a sunny day at noon, touch a stone in the sun and a stone in the shade. You'll feel why cold-blooded animals warm up on sunny rocks.
- When you wake up in the morning, notice that your body is warm under the covers. It's proof that the stove inside you kept running through the night while you hadn't eaten anything.
Do not approach or touch wild snakes, crocodiles or turtles. Watch them from a distance.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school biology
- High school+Advanced high school material, or textbook sidebars
- UniversityUniversity-level courses (animal physiology, ecology) not taught in high school
- ResearchTopics that researchers are still investigating, not yet settled even at university level
Middle schoolTerms: this phenomenon has names
- Cold-blooded animal (ectotherm): an animal whose body temperature changes with the surrounding temperature. Reptiles, amphibians and most fish.
- Warm-blooded animal (endotherm): an animal that keeps its body temperature nearly constant even when the surrounding temperature changes. Birds and mammals.
- Basal metabolic rate: the energy used just to stay alive, even when doing nothing and resting.
Middle schoolHigh schoolChecking with a formula: how many days does the same fat last?
Suppose a 60 kg person and a 60 kg crocodile each store the same 10 kg of fat. We calculate how many days each lasts without eating. In reality other parts of the body get used too, so this is only a rough estimate.
| In symbols | D ＝ E ÷ P |
| In words | Days it lasts ＝ stored energy ÷ energy used per day |
| Where it comes from | Conservation of energy. If nothing comes in, the store shrinks by the daily amount used, and the number of days until it reaches zero is D |
| Symbol | Meaning and unit |
| D | Days it lasts without eating (days) |
| E | Stored energy (kilocalories) |
| P | Energy used per day (kilocalories/day) |
| Stored fat | 10 kg (10000 g) |
| Energy in 1 gram of fat | about 9 kilocalories |
| Basal metabolic rate of a 60 kg person | about 1500 kilocalories/day |
| A cold-blooded animal of the same weight (body temp. 30°C) | taken as about one tenth of a person's |
| Drop when body temperature falls by 10°C | taken as about one 2.5th |
| Stored energy E | 10000 × 9 ＝ 90000 kilocalories |
| Days a person lasts | 90000 ÷ 1500 ＝ 60 days |
| Daily energy of a crocodile (30°C) | 1500 ÷ 10 ＝ 150 kilocalories |
| Days a crocodile (30°C) lasts | 90000 ÷ 150 ＝ 600 days |
| Daily energy of a crocodile (20°C) | 150 ÷ 2.5 ＝ 60 kilocalories |
| Days a crocodile (20°C) lasts | 90000 ÷ 60 ＝ 1500 days |
With the same fuel tank, a person lasts about two months, while a crocodile lasts over a year and a half by this calculation, and longer still if it gets colder. The person's 60 days or so roughly matches the one to two months thought to be the limit for surviving on water alone.
High schoolHigh school+Body temperature and the speed of reactions
High schoolChemical reactions in the body are driven by proteins called enzymes. Reactions speed up when the temperature rises and slow down when it falls. This is why the metabolism of cold-blooded animals changes so much with body temperature.
High school+The number of times faster a reaction becomes when the temperature rises by 10°C is called the temperature coefficient (Q10). For the metabolism of living things it is about 2 to 3. The slider in Figure 1 uses a value of 2.5.
UniversityThe law linking body size and metabolism
It is known that the energy an animal uses rises in proportion to about the three-quarters power of its body weight. This is called Kleiber's law. The relationship has nearly the same form for warm-blooded and cold-blooded animals, and only the coefficient differs. The coefficient for warm-blooded animals is said to be about 5 to 10 times larger than for cold-blooded ones. When the amount used per day in the wild (field metabolic rate) is compared, the gap is reported to widen even further.
📖 Derivation of the formula and further reading: Poikilotherm (Wikipedia, in Japanese) ／ Basal metabolism (Wikipedia, in Japanese)
ResearchWhat is still not clear
- Why did warm-bloodedness evolve? To keep running for a long time, to raise young, or to live in cold places? There are many theories, and none has won out.
- What was the body temperature of dinosaurs? It is being studied from how their bones grew and from the chemistry of fossils, but some think it differed between kinds.
- Why the three-quarters power? There is a theory that explains it from the shape of branching networks like blood vessels, but it has its critics.
So what this article says is also "an explanation within what is known today". The numbers vary greatly with species, body size and season.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Classification of animals" | Cold-blooded and warm-blooded animals |
| High school | Biology: "Metabolism and enzymes" | Body temperature and the speed of reactions |
| High school+ | Biology (advanced) | Temperature coefficient |
| University | Animal physiology, ecology | Kleiber's law, field metabolic rate |
| Research | Evolutionary physiology | Origin of warm-bloodedness |
| ― | Link to daily life | Why people have to eat every day |
- Wikipedia, "Poikilotherm" (Japanese)
- Wikipedia, "Basal metabolism" (Japanese)
- Nagy, K. A. (2005) Field metabolic rate and body size. Journal of Experimental Biology 208: 1621–1625.
- Schmidt-Nielsen, K. Animal Physiology: Adaptation and Environment. Cambridge University Press.
- McCue, M. D. (2010) Starvation physiology: Reviewing the different strategies animals use to survive a common challenge. Comparative Biochemistry and Physiology A 156: 1–18.
*This article is a popular-science explanation. The numbers given are rough guides to help you understand how things work. Going without food is dangerous to your life, so please do not try it.