Naked Mole-Rats: Strange Discoveries About an Unusual Animal
Weird and Fascinating Rodents
Naked mole-rats are burrowing rodents with a strange appearance and even stranger features. A new discovery has added to their weirdness. Scientists already knew that the animals are resistant to some kinds of pain and very rarely get cancer. Now researchers have found that the animals can survive for up to eighteen minutes in an environment without oxygen. Given that rodents are mammals like us, the mole-rat's features are intriguing. Understanding its abilities could be useful with respect to understanding human biology and perhaps even in helping to treat some of our health problems.
Naked mole-rats (Heterocephalus glaber) live in the deserts of East Africa in large, underground colonies. They are neither moles nor rats, but they are rodents. The animals have grey to pink skin that is wrinkled and fits loosely on their body. The skin bears very little hair. The animals aren't completely naked, however. They have sensory bristles on their skin as well as fine hairs.
The body of a naked mole-rat has a tubular shape. Although the eyes are small, the animals aren't blind as is sometimes claimed. They do have poor vision, though. The Cornell University professor in the video below says that the eyes are used only to distinguish light from dark. There is no external ear flap, but there is a hole on the side of the head to allow sound waves to enter the ear.
The animals have two long upper incisors that protrude through their lip just below their nostril. They also have two protruding lower incisors. Since the teeth are outside the mouth, the animal can keep its lips closed while digging a tunnel. This stops it from swallowing soil.
Naked mole-rats are more closely related to guinea pigs and porcupines than to rats and moles.
The mole-rat colony may be very large. Researchers say that there may be from twenty to three hundred animals in the group, although seventy to eighty animals is the most common colony size. The burrow may extend over a large area and is organized into different chambers, or "rooms". These have specific purposes, including serving as a nursery, a food storage area, a sleeping area, or a toilet.
The animals feed on underground roots and tubers and get all the water that they need from their food. They also eat their poop in order to extract extra nutrition from any undigested food. The rodents role in the poop in addition to eating it, which gives them the typical smell of the colony. This enables the animals to identify colony members. The use of smell is important, since the rodents have bad vision and live in a dark area.
Naked mole-rats are vocal animals and have good hearing. Researchers have discovered that they produce at least eighteen distinct vocalizations. Sound is an important method of communication for them. The animals that find food that can't be moved inform the other members of the colony about their discoveries via sound and behaviour. The rodents can run as fast backwards as forwards in their tunnels.
In order to create a new tunnel in their burrow, the mole-rats work cooperatively. They form a chain of animals. The first animal removes soil from an area and the others kick it backwards through the chain until the soil reaches the surface.
Organization of the Colony
The colony is ruled by the dominant female, or the queen. She is the only animal to breed. Her body becomes longer and larger once she becomes the queen, which enables her to contain more pups. If necessary, she fights to maintain her role in the colony.
The queen allows only two or three of the males to mate with her and is often said to give birth to up to twenty-seven pups at a time. The researcher in the video above says that the largest litter at Cornell contained thirty-three pups, however. The usual litter size is twelve to eighteen pups. The gestation period is about seventy days. The queen may breed four to five times a year, which helps the colony to become very large.
The other members of the colony are known as workers or soldiers. They have specific jobs, such as feeding and caring for the queen, taking care of the pups, digging tunnels, searching for food, and protecting the colony from enemies. They work as a group to fight predatory snakes.
It's interesting that the mole-rat colony has a social organization that resembles that of a bee colony. The rodents and the insects are very different animals, yet they have each developed a similar way of life.
Interesting and Surprising Facts About Naked Mole-Rats
- The skin of adult naked mole-rats can't detect pain caused by acid or by capsaicin from hot peppers. The first ability could be very useful in tunnels filled with exhaled carbon dioxide, since the chemical reacts with water to form carbonic acid.
- It's frequently claimed that the animals don't get cancer. It might be more accurate to say that they rarely get the disease because at least two animals in captivity have developed conditions that at least resemble cancer. Whether the animals ever get the disease in the wild is unknown.
- Unlike most other mammals, naked mole-rats don't regulate their internal body temperature very well. They are often said to be "cold-blooded", which means that their body temperature is approximately that of the environment. They need to huddle together on cold nights so that they stay warm.
- Animals kept in captivity have been able to chew through concrete.
- The animals have lived for as long as thirty-two years in captivity. They stay agile as they age. Most rodents of similar size die when they are five years old or younger.
Living Without Oxygen
The tunnels in the naked mole-rat colony have a low oxygen content and a high level of carbon dioxide, but this doesn't seem to bother the animals at all. Hypoxia is a condition in which an inadequate amount of oxygen reaches the tissues. This definition doesn't apply very well to naked mole-rats because even when the amount of oxygen in their bodies is low, they don't appear to experience any ill effects.
In April 2017, an amazing discovery was announced. A group of researchers reported that the naked mole-rats in their study survived for eighteen minutes with no oxygen at all in the environment. The animals lost consciousness and their heart and breathing rate slowed very significantly, but they didn't die. When oxygen was returned to their environment they recovered and behaved normally.
Mice kept in the oxygen-free environment died after a minute. All of the mole-rats survived for eighteen minutes. Three mole-rats left in the environment for longer were dead after thirty minutes. The topic of cruelty to animals could certainly be raised at this point, but if this unpleasant thought is ignored, the results of the experiment are very interesting.
The scientists also discovered that the mole-rats lived with no problems in air with only 5% oxygen. (Air normally contains close to 21% oxygen.) After five hours of watching the animals in the experimental chamber and seeing no effects of the low oxygen content, the scientists stopped the project and returned the animals to their habitat. In contrast, mice died after fifteen minutes in the low-oxygen atmosphere.
These animals (naked mole rats) fuel anaerobic glycolysis with fructose by a rewired pathway that avoids tissue damage.— Park et al, Science journal, AAAS
Humans and other mammals normally get energy from glucose, though fructose is used in some parts of the body. The complete process for producing energy from a simple sugar is called cellular respiration. It involves a chain of ten reactions known as glycolysis as well as other reactions that follow glycolysis. Cellular respiration requires oxygen, which is why we need to inhale the gas. Glycolysis on its own doesn't require oxygen, however.
The energy released by cellular respiration is stored in ATP (adenosine triphosphate) molecules. Glycolysis produces ATP molecules, but far fewer than the rest of the cellular respiration process. ATP can be quickly broken down when energy is needed.
Each purple step in the diagram above represent the breakdown of an ATP molecule. The yellow steps represent the manufacture of ATP molecules. Since steps 7 to 10 occur twice, there is a net manufacture of two ATP molecules per glucose molecule. An arrow pointing in two directions means that a reaction is reversible. In anaerobic conditions, the pyruvate that is made in glycolysis is converted to lactic acid.
Fructose Use in Naked Mole Rats
Glycolysis can and does occur without oxygen. When it takes place without the steps that normally follow it in cellular respiration, however, it produces chemicals that can potentially inhibit an earlier reaction in the glycolysis pathway. An enzyme that is commonly inhibited by chemicals in its environment is phosphofructokinase, which controls reaction three of glycolysis. (The enzyme is represented by PFK in the illustration above.) If the enzyme is inhibited, glycolysis stops, ATP molecules aren't made, cells are deprived of energy and damaged, and the cells eventually die.
We might expect that the same feedback process could occur in naked mole-rats in an environment without oxygen, since they are mammals like us and primarily use glucose as an energy source. They seem to have an advantage compared to us, however. When oxygen is removed from the mole-rat's environment, fructose becomes the animals' primary energy source, glycolysis is not inhibited, and tissue damage is avoided.
Adaptations of the Rodents
The researchers found unexpectedly high concentrations of fructose in the bodies of the animals placed in an environment without oxygen. The site or sites in the body that released this fructose is currently unknown. The scientists also discovered that the animals contained a high level of a molecule called GLUT5, which transports fructose into cells, as well as a high level of an enzyme called ketohexokinase. The enzyme changes fructose into fructose-1-phosphate. This chemical is then converted to fructose-1,6-biphosphate or another molecule that enters glycolysis after the potential bottleneck at reaction three. This process allows glycolysis to continue even if reaction three is blocked.
Scientists say that there may be other factors responsible for the rodents' survival besides their use of fructose. These factors may include a low body temperature compared to that of other mammals and a low metabolic rate. Still, many researchers are interested in the results of the study because they are so unusual for a mammal.
F1P (fructose-1-phosphate) was undetectable in normoxic brains but appeared in significant amounts only in anoxic naked mole-rat brains.— Park et al, Science journal, AAAS
The Pacific Science Center in Seattle has a webcam that lets people look at the naked mole-rat exhibit from their home. The video above was made by the webcam and is slightly sped up. All of the animals that escaped from the habitat were returned safely to their home.
Possible Applications of the Research
The researchers studying fructose use in mole-rats think their discoveries might help humans. People who experience a heart attack or stroke often develop hypoxia immediately after the incident. The brain in particular needs a constant supply of oxygen so that it can produce energy. If it doesn't get this energy, its cells start to die. The production of energy from fructose without the need for oxygen as naked mole-rats do might be helpful for preventing or reducing tissue damage after a stroke or heart attack. It might even save lives while patients are waiting for other treatments to work.
One of the scientists involved in the rodent research wonders whether deep-sea divers who hold their breath for a long time while hunting for pearls or while freediving have unknowingly triggered a system related to that in naked mole-rats. This is pure speculation at the moment, but it's an interesting idea to consider.
It's possible that understanding the healthy aging of mole-rats and their resistance to pain and cancer may also be helpful to humans. Thinking about the features that may or might one day apply to humans is an interesting activity. The animals have some amazing and bizarre characteristics. They are fascinating creatures to study.
- Naked mole-rat information from the San Diego Zoo
- Mammals that can survive without oxygen from CNN (This article includes an interview with a scientist involved in the research.)
- Oxygen deprivation in naked mole-rats (a discussion of the research results from the America Association for the Advancement of Science, or AAAS)
- Fructose-driven glycolysis in naked-mole rats (the original paper from the Science journal, American Association for the Advancement of Science)
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© 2017 Linda Crampton