Wednesday, July 20, 2016

Take Off Your Coat And Stay Awhile

Biology concepts – thermoregulation, ectothermy, endothermy, genetic mutation 

Let me introduce you to the most wondrous animal on the surface of the Earth, or under the surface of the Earth – the naked mole rat, Heterocephalus glaber (hetero  = different and cephalus = headed, refers to the fact that it lives in a colony where different members have different jobs; glaber = smooth skin).

Why, you ask, is this pruny thumb with two eyes the most incredible animal? Its odd looks and cutsie pink color belie the fact that this rodent is the most heinous rule breaker in all the biological world. It hasn’t meant a convention it wouldn't defy or a norm at which it wouldn’t thumb its nose.

Meet H. glaber, the naked mole rat. He has teeth, pink skin, and a probable
inferiority complex. The right image shows that H. glaber is not much bigger
than the thumb he resembles.
Take for instance, its name – NAKED mole rat. It is a mammal, but it’s naked. Mammals are always covered with hair or fur, but not his guy. Even we humans, the most hairless of all the apes (except for Robin Williams, he looks like he wears a sweater into the pool), look like we’re covered in fur compared to this rodent.

Look at yourself in a mirror. There’s hair on top of your head (well at least most of you). There is fine, unpigmented vellus hair (vellus = fleece in latin) that we know as peach fuzz, on your arms and legs when young and more coarse hair when older. You see eyebrows, and nose hairs as well. There is hardly a spot on us that isn’t hairy, save the palms of our hands to increase friction for gripping, and the soles of our feet, probably to keep it from tickling when we walk.

H. glaber eschews all this hair, but even he isn’t completely naked. From the picture, you can see the several sets of whiskers protruding from the wrinkly pink face that only a very devoted mother could love. The whiskers are crucial to helping the mole rat make its way in its surroundings, and therefore have not been lost, but why on Earth is it nearly naked?

The horn of Africa, a great place not to be noticed, and hot enough
to make underground living a plus.
The reason lies in how and where the naked mole rat lives. Found only in the desert of the horn of Africa (Ethiopia, Kenya, Somalia, Eritrea), this rodent that is neither a mole nor a rat lives underground its entire life. It burrows to find roots to nibble on, and they can be few and far between – it’s a desert for crying out loud!

In its tunnels, body hair imparts no advantage, and can contribute to negative outcomes, such as carriage of parasites (this is why scientists believe humans lost most of their hair), overheating, or getting stuck in narrow spaces. The mole rat’s skin helps with this last problem, although it seems counter-intuitive. Defensive lineman in football like to wear very tight uniforms so that the offense has nothing to grab a hold of, and it would follow that a tight skin on the naked mole rat would also help it slide around and not get caught on anything.

But the advantage to big skin is that the rat can turn around almost completely in its uniform, and dig from any direction to move itself along. Like the owl that can turn its head 270˚, the naked mole rat can rotate its whole body to get out of a jam. That loose skin is also helpful in traffic jams; mole rats can slip past one another in a tunnel without even slowing down.

The whiskers serve to guide the mole rat around in its dark environment. It feels its way, it feels for its food, and it feels other mole rats that it may meet in the tunnels. Therefore, the hairs it has kept serve a definite purpose, and one can see why there are whiskers along its entire body, as opposed to just around its nose (see photograph above).

Other mammals might appear to hairless, some even have it in their name, but they don’t match H. glaber for nakedness on an overall basis. Rhinoceroses, elephants, pigs, they all have coarse hair on many parts of their bodies, so they can’t compete for the world hairlessness title. Even marine mammals like whales and dolphins have some hair (mostly when they are younger) and have nose hairs as well (so I’m told – I never looked up a dolphin’s nose). The Sphynx cat is supposedly hairless, but its entire body is covered in vellus hair.

Dolphins have whisker as infants, and the whisker pits help sense electrical fields. The Sphynx cat was revered by the ancient Egyptians, which was fine, because the Egyptians shaved off most of their own hair. On the right, the Xoloitzcuintli was said by the Aztecs to guard human souls in the underworld. It looks intimidating enough to be good at that.

Finally, there is the Mexican hairless breed of dog, properly called the Xoloitzcuintli or Xoloitzcuintle. While some of these dogs are completely hairless, it is a mutation rather than normally occurring. Hairlessness is the dominant form of the mutation, but even most of these animals have hair on their heads and tails. It is less common that the dog is completely hairless.

Powder was a 1995 movie about a young man with alopecia
universalis amidst other issues, like psychokinesis and a lack
of sun exposure.
Humans can also be hairless, called Alopecia universalis (alopecia is Greek for “fox mange” and universalis means everywhere). The condition is an autoimmune disorder, meaning that our own immune system has decided that our hair follicles are no longer part of us and are attacked as being foreign. Many human diseases can be autoimmune in origin, including diabetes and muscular dystrophy.

But of all the animals mentioned, H. glaber takes the crown as hairlessiest! And it serves a good purpose. Along with living underground, living in a community, having smooth skin, living in a desert, and having a limited food source – these features have contributed to another decision nature has thrust on H. glaber, it is ectothermic! It doesn’t warm itself, rather it assumes the temperature of its surroundings. Is that any way for a self-respecting mammal to behave?

In the cold, hair traps air and keeps it close to the body to act as thermal insulation. However, H. glaber is communal, and they have larger chambers in which they all huddle together during sleep. Over the course of the cold desert night, the mole rats will rotate positions, so no one animal is on the outside for too long, much like penguins do in Antarctica. This keeps them warm and negates the need for hair as an insulator.

The communal sleeping is just one aspect of the social life of H. glaber. There are one of only two eusocial mammals. The have a queen and a caste system, like many bees and ants. A recent study shows that the queen is very important to the building of the tunnels, as well as all aspects of H. glaber life. 

The tunnels of each worker may be widened to form sleeping chambers or pup rearing chambers, but which. The 2012 study indicates that the presence of the queen will increase the dirt moving by all castes, while workers will work more than the others if she is not present. What is more, the odor of the queen is enough to increase the dirt moving in a particular area, so her movements do influence the geometry of the nest.

An arrector pili muscle is attached to every hair on your body. You can
see that if it contracts (shortens), the hair will stand up. Thank you,
black cat for the Halloweenish demonstration.
Hair can also act to dissipate heat. In most mammals, each hair is attached to a small muscle (arrector pilori; pili is the plural) that can stand the hair on end and release the trapped warm next to the body, cooler air will then carry the heat away from the skin and the hairs, thereby reducing the temperature of the animal. Interestingly, this same action is seen when we get scared. The fright or flight release of adrenaline causes the arrector pili muscles to contract; think of how a cat’s tail gets bushy and the hair on its back stands up when scared. The arrector pilli muscles will also spasm in an effort to produce added heat when the skin gets cold (goose bumps).

Being underground all the time means that H. glaber is protected from the most intense heat of the desert day and therefore needs fewer thermoregulatory mechanisms.  So, the naked mole rat doesn’t need to dissipate heat via the arrector pilli action.

Finally, by practicing ectothermy, the naked mole rats reduce the amount of food they have to consume; they don’t need all that energy to produce heat and maintain a constant temperature. This works out well for them, since they live in the desert where there isn’t a heck of a lot food for them anyway. Could H. glaber have ended up as anything other than ectothermic? Its design just makes too much sense for its environment. We could learn a thing or three from how nature has tweaked its design.

And we have only scratched the surface of the ways that this rodent refuses to conform to established biological norms. Future posts will introduce more aspects of this amazing animal’s physiology, including longevity, pathology (or lack thereof), social structure, senses, immunity, biochemistry, and reproduction.

But you’ll have to wait for those stories. Next time we will turn our attention to a necessity of all life, sleep. But aren’t we learning that no single characteristic applies to ALL life – there’s always an exception.

Kutsukake, N., Inada, M., Sakamoto, S., & Okanoya, K. (2012). A Distinct Role of the Queen in Coordinated Workload and Soil Distribution in Eusocial Naked Mole-Rats PLoS ONE, 7 (9) DOI: 10.1371/journal.pone.0044584

For additional information, classroom activities or laboratories on H. glaber, animal hair, alopecia universalis, arrector pili:

H. glaber

animal hair –

alopecia universalis –

arrector pili –


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