Friday, 31 December 2010

Naked Mole Rat: World's Strangest Rodent

Image showing a naked mole rat
Naked Mole Rat
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Family: Bathyergidae
Genus: Heterocephalus
Species: Heterocephalus glaber
Conservation Status: Least Concern (Not Threatened)
Common Name(s): Naked mole rat, Sand puppy, desert mole rat

The naked mole rat is arguably one of the world's strangest mammals, combining a number of crazy and unique traits that no other mammal has. Some of these traits include a lack of pain sensation, very low metabolic and respiratory rates and an apparent immunity to cancer! Interestingly, they are also one of the handful eusocial mammals we know, meaning that they form societies similar to those of ants and termites!

All these and even more will be covered in full detail later in the article.

If you find this mammal interesting after reading this article, I highly suggest you to have a look at the platypus, a cute little mammal from Australia that also features all kinds of bizarre evolutionary adaptations.

Distribution and Habitat
The species is native to the drier parts of the tropical grasslands of East Africa, occurring throughout most of Somalia, central Ethiopia, and much of northern and eastern Kenya, extending as far south as the eastern border of Tsavo West National Park and the town of Voi.  The species has also been recorded in Djibouti, indicating that it has a wider range than the one currently known. It has an altitudinal range, starting from 400 to 1,500 m.

Naked mole-rats live in arid habitats, characterized by high temperatures and low and irregular rainfall, which usually averages at 200-400 mm per year. They are most commonly found in hard, consolidated, lateritic loams, although they have been sighted in fine sand.

Map showing the distribution of the naked mole rat
Naked Mole Rat Distribution Map

It is a burrowing rodent and the sole representative of the genus Heterocephalus.

Adults are 8 to 10 cm (3-4 in.) long and have a weight that ranges from 30 to 35 g. However, queens tend to be larger and many times weigh well over 50 g, with the heaviest ever recorded queen being 80 g.

The legs are thin and short but still allow them to casually move underground, even backwards; almost at the same speed. They use their protruding teeth to dig, while their lips are sealed just behind them, in order to prevent soil from filling their mouths while doing so. They have very little hair and the body is covered by wrinkled pink or yellowish skin. They have small eyes and poor vision, not that they need it much, as there is little to no light in the tunnels they inhabit.

The naked mole rat has sensory whiskers on its face and tail, along with some hairs between the toes, enabling the feet to function as brooms with which it sweeps the soil.

As aforementioned, these little critters feature a crazy combination of strange physiological traits. Let's see them one by one:

1# Slow Metabolic rate
Oxygen availability in the burrowing nests is limited and the species has adapted adequately. The naked mole rat has very small lungs whereas its blood has a very strong affinity for oxygen, greatly increasing the efficiency of oxygen uptake. Considering their size and rodent nature, the respiration and metabolic rate of the species is extremely low, only 2/3 of what is expected of similarly sized rodents. Additionally, they have the capacity to reduce their metabolic rate by an extra 25 % in periods of famine.

A naked mole rat (in captivity) eating
Naked mole rat eating

2# Living  a Pain-free Life
Their skin lacks a key neurotransmitter known as "substance P". In mammals, this neurotransmitter is responsible for sending pain signals to the central nervous system. As a result, they experience no pain when exposed to acids or capsaicin. This is most probably an evolutionary adaption that allows them to live pain-free in their tunnels, characterized by high levels of carbon dioxide resulting from poor ventilation. It is assumed that substance-P deficiency is also why these strange animals don't exhibit the histamine-induced itching and scratching behavior which is typical of rodents.

3# Unique Thermoregulation 
Contrary to all other mammals - including humans -, the naked mole rat doesn't regulate its body temperature to a fixed temperature (homeostasis). Instead, they are thermoconformer animals, meaning that their body temperature is subject to changes, depending on the environment's temperature. As of 2014, the species remains the only known mammalian thermoconformer.

However, the relationship between oxygen consumption and ambient temperature switches from a typical poikilothermic pattern to a homeothermic mode at temperatures of 28 °C (82.4 F) or higher. At lower temperatures, they use behavioral thermoregulation. When cold, they cuddle together or bask in the shallow, more sun-warmed parts of their tunnels. Conversely, when they get too hot, they retreat to the deeper, cooler parts of their burrow systems.

Researcher Rochelle Buffenstein talks about naked mole rats and how
they are the the "Holy Grail" of ageing

4# Remarkably Long Life Span
Another interesting fact about these weird animals is their extraordinarily long life span. They can live to be at least 28 years old and hold the record for longest living rodents. The exact mechanisms behind their longevity have yet to be unveiled, however their longevity is believed to relate to their low metabolism which in turn prevents oxidative stress and damage.

5# Resistance and possible immunity to cancer
One other strange thing about these rodents is that they seem to have a very high resistance to cancer. Actually cancer has never been observed in them, they may as well be immune to it!

Like most mammals, naked mole rats have a gene (known as p27) which prevents cell division once a group of cells reaches a certain number. Furthermore, they are also protected by the p16 gene, which does much about the same job, although it has a lower cell-number threshold, compared to p27. The combination of these two genes acts as a double barrier to uncontrolled cell proliferation, protecting the species from the onset of cancer.

On June 19, 2013, researchers from the University of Rochester, reported that one other possible reason that naked mole rats are impervious to cancer may be because they produce an "extremely high-molecular-mass hyaluronan" (HMW-HA) (a natural sugary substance), which is over "five times larger" than that in cancer-prone humans and cancer-susceptible laboratory animals. Few months later, the same research team reported that naked mole rats have ribosomes that produce extremely error-free proteins. For these two "achievements", the journal Science named naked mole rat as "Vertebrate of the Year", in 2013.

Naked mole rats huddling together in an underground burrow
Naked mole rats huddling together

Already, the strange things about these little critters seem never ending. But wait, there's more! Naked mole rats are also one of the just two known species of eusocial mammals. The other one is the Damaraland Mole Rat (Fukomys damarensis). Like ants, the species forms social colonies, each with:
  • One queen 
  • Two to three breeding males 
  • Workers 

Colonies range in size from 20 to 300 individuals, with the average number being 75 individuals per colony.

Now let's see each class in more detail:

1# Queen
The relationship between the queen and the breeding males lasts for many years, while the rest of the females are temporarily sterile. A queen's typical life ranges from 13 to at least 18 years. They are very hostile to females that start to behave like queens or produce hormones for becoming queens.

The queen's urine contains certain chemicals (pheromones) which prevent other females in the nest from having pups of their own. She is also very dominant and the stress caused by her "bullying" keeps the other females from breeding.

After the queen dies, another female takes her place, many times following a violent struggle among other female "champions". Once the new queen has established her dominion, she stretches the space between the vertebrae in her backbone and becomes longer and ready for bearing pups.

The queen is able to breed after the first year of her life and breeds once every year. In captivity they breed all year long. Gestation lasts for about 70 days and the litter usually ranges from three to twelve pups, although litters of up to 28 pups have been reported! The newborns have an average weight of 2 grams and are nursed by the queen during the first month. After this period, they are fed feces by the workers, until they are old enough to eat solid food

A newly born naked mole rat
Naked mole rat baby
Click here for more strange animal babies !

2# Workers
As aforementioned, workers are sterile. Similarly to ants, they are further divided into castes. Some are tunnellers, expanding the large network of tunnels within the burrow system and some are "soldiers", protecting the group from outside predators.

Naked mole rats primarily feed on very large tubers they find inside their burrowing tunnels, which commonly weigh as much as 1000 times their average body weight. A single tuber may be enough to feed the colony for months, or even years, as they are smart enough to eat the inside but leave the outside, allowing it to regenerate.

They also eat their feces (coprophagia), most probably because the tubers and roots are hard to ingest. By eating their feces, they get a second chance to absorb the tuber's nutrients. Two meals for the price of one! This behavior serves another purpose as well. The members of the colony routinely roll around the "toilet" chamber, acquiring a distinctive smell that allows them to identify intruding individuals from other colonies.

Surprisingly, they obtain all water through their food, they do not drink a single drop of water.

In captivity, the species consumes sweet potatoes, all kinds of fruits, baby cereals, dog biscuits and other treats.

Great footage by National Geographic

Conservation status
Naked mole rats are currently listed by the IUCN as of Least Concern and this won't probably change anytime soon. The species distribution falls under several protected areas, including Tsavo, Meru, and Samburu National Parks in Kenya and has no known threats that endanger its survival.

In Captivity
Naked Mole-rats are a common exhibit in zoos all over the world. Just do a google search to see if there is one nearby. From personal experience, I know that London zoo has a few. Here's a video I shot when I visited it in June, 2014:

Sorry for the bad quality :)

Interesting Facts about the Naked Mole Rat
- When naked mole rats are injected with substance P, they appear to regain their pain sensation, although it seems to work with capsaicin and not acids.
- Blind mole rats Spalax golani and Spalax judaei are two other rodents with an apparent immunity to cancer, although different mechanisms seem to be involved.

References & Further Reading
- Burland TM, Bennett NC, Jarvis JU, & Faulkes CG (2002). Eusociality in African mole-rats: new insights from patterns of genetic relatedness in the Damaraland mole-rat (Cryptomys damarensis). Proceedings. Biological sciences / The Royal Society, 269 (1495), 1025-30 PMID: 12028759
- M. Justin O'Riain, Chris G. Faulkes (2008). African Mole-Rats: Eusociality, Relatedness and Ecological Constraints Ecology of Social Evolution, 207-223 DOI: 10.1007/978-3-540-75957-7_10
- Daly TJ, Williams LA, & Buffenstein R (1997). Catecholaminergic innervation of interscapular brown adipose tissue in the naked mole-rat (Heterocephalus glaber). Journal of anatomy, 190 ( Pt 3), 321-6 PMID: 9147219
- Park TJ, Lu Y, J├╝ttner R, Smith ES, Hu J, Brand A, Wetzel C, Milenkovic N, Erdmann B, Heppenstall PA, Laurito CE, Wilson SP, & Lewin GR (2008). Selective inflammatory pain insensitivity in the African naked mole-rat (Heterocephalus glaber). PLoS biology, 6 (1) PMID: 18232734
- Azpurua, J., Ke, Z., Chen, I., Zhang, Q., Ermolenko, D., Zhang, Z., Gorbunova, V., & Seluanov, A. (2013). Naked mole-rat has increased translational fidelity compared with the mouse, as well as a unique 28S ribosomal RNA cleavage Proceedings of the National Academy of Sciences, 110 (43), 17350-17355 DOI: 10.1073/pnas.1313473110
- Maree, S. & Faulkes, C. (2008). Heterocephalus glaber. In: IUCN 2008. IUCN Red List of Threatened Species.
- Seluanov A, Hine C, Azpurua J, Feigenson M, Bozzella M, Mao Z, Catania KC, & Gorbunova V (2009). Hypersensitivity to contact inhibition provides a clue to cancer resistance of naked mole-rat. Proceedings of the National Academy of Sciences of the United States of America, 106 (46), 19352-7 PMID: 19858485
- Smith ES, Blass GR, Lewin GR, & Park TJ (2010). Absence of histamine-induced itch in the African naked mole-rat and "rescue" by Substance P. Molecular pain, 6 PMID: 20497578
- Yang, Z., Zhang, Y., & Chen, L. (2013). Investigation of anti-cancer mechanisms by comparative analysis of naked mole rat and rat BMC Systems Biology, 7 (Suppl 2) DOI: 10.1186/1752-0509-7-S2-S5
- Yang, Z., Zhang, Y., & Chen, L. (2013). Investigation of anti-cancer mechanisms by comparative analysis of naked mole rat and rat BMC Systems Biology, 7 (Suppl 2) DOI: 10.1186/1752-0509-7-S2-S5
- Buffenstein, R. (2002). The Naked Mole Rat--A New Record for the Oldest Living Rodent Science of Aging Knowledge Environment, 2002 (21), 7-7 DOI: 10.1126/sageke.2002.21.pe7
- Robinson, M. (1991). The Naked Mole-Rat: A Mammalian Termite? The Biology of the Naked Mole-Rat Paul W. Sherman Jennifer U. M. Jarvis Richard D. Alexander BioScience, 41 (10), 723-724 DOI: 10.2307/1311768
- Vinciguerra, M. (2011). Hormones, reproduction and disease in the longest-lived rodent: the naked mole rat Endocrinology Studies, 1 (1) DOI: 10.4081/es.2011.e4
- Csiszar A, Labinskyy N, Orosz Z, Xiangmin Z, Buffenstein R, & Ungvari Z (2007). Vascular aging in the longest-living rodent, the naked mole rat. American journal of physiology. Heart and circulatory physiology, 293 (2) PMID: 17468332
- Sherman, Paul W.; Jennifer Jarvis, Richard Alexander (1991). The Biology of the Naked Mole-rat. Princeton, N.J.: Princeton University Press. ISBN 0691024480.