As Many Exceptions As Rules: ectotherm

Showing posts with label ectotherm. Show all posts

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 –

http://www.digimorph.org/specimens/Heterocephalus_glaber/

http://animals.nationalgeographic.com/animals/mammals/naked-mole-rat/

http://nationalzoo.si.edu/publications/zoogoer/2002/3/nakedmolerats.cfm

http://www.npr.org/2011/10/13/141313080/-re-exposed-naked-mole-rat-genome-sequenced

http://www.bristolzoo.org.uk/naked-mole-rat

http://www.pbs.org/wnet/nature/lessons/ugly-says-who/lesson/422/

www.brookfieldzoo.org/pagegen/inc/whattodo.pdf

http://www.torontozoo.com/ExploretheZoo/AnimalDetails.asp?pg=622

http://www.sciencedaily.com/releases/2009/03/090305171903.htm

http://www.youtube.com/watch?v=AX94UtcRu14

http://www.youtube.com/watch?v=fj48ewrkSFQ

animal hair –

http://newswatch.nationalgeographic.com/2011/04/27/weird-wild-hairless-animals%E2%80%94the-naked-truth/

http://www.iamaweb.com/Animal_Hair/animal_hair_images.html

www.usitc.gov/publications/332/PUB3145.pd

http://www.ehow.com/about_5317291_human-hair-vs-animal-hair.html

http://sciencespot.net/Pages/classforsci.html

http://www.vetmed.vt.edu/education/curriculum/vm8054/Labs/Lab15/Lab15.htm

http://www.furcommission.com/Biology/furtypes.html

http://www.cashmere.org/cm/facts.php

alopecia universalis –

http://rarediseases.info.nih.gov/GARD/Disease.aspx?diseaseID=614

http://barryyeoman.com/articles/christiano.html

http://www.mc.vanderbilt.edu/reporter/index.html?ID=3542

arrector pili –

http://openlearn.open.ac.uk/mod/oucontent/view.php?id=398668&section=6

http://whyevolutionistrue.wordpress.com/2011/02/22/vestigial-trait-the-arrector-pili/

http://www.vetmed.vt.edu/education/curriculum/vm8054/Labs/Lab15/Lab15.htm

http://www.bio.davidson.edu/Courses/anphys/2000/Hatfield/Hatfield3.htm

http://emedicine.medscape.com/article/1294744-overview

http://www.bioclix.org/default.asp?pg=217

Wednesday, June 29, 2016

Why Can’t We Just Go With the Flow? – The High Cost of Heating

Biology concepts – endothermy, ectothermy, poikilothermy, thermoregulation

So you are lying in bed, cold and hungry, contemplating living like a monk just so you can have more time to live like a monk (see this post on increasing life span via reduced core temperature and caloric restriction). Your temperature is going down after bedtime and coming back up in the early morning. This implies that you can control and maintain a constant temperature - pretty impressive. But don’t get a big head, most mammals can do it, even your pet hamster.

The hypothalamus is located in the lower center of the

brain. Different parts are involved in sensing and

regulating temperature, but also for blood pressure,

circadian rhythms, and feeling full after eating.

To be able to control your temperature (thermoregulate), you must know what your temperature is in the first place. Mammals have sensors in their skin and organs which relay information about temperature to the hypothalamus of the brain. The neural sensors in skin (peripheral thermoreceptors) sense the temperature just under the surface. This can be quite different from the core temperature. Central thermoreceptors sense the temperature in the brain, bladder and muscles. Your hypothalamus sets your skin thermostat at about 72˚C, so you still feel hot when the ambient temperature is >75˚F even though your core temperature averages 98.6˚F (37˚C).

Heat is constantly being generated by your metabolism (The breaking down and building up of molecules in your cells). Burning ATP to produce work also produces heat as a byproduct, and this goes a long way to keeping our temperature around 98.6˚F. Generating internal heat to maintain a body temperature is called endothermy (endo = within, therm = heat). Mammals are endotherms, and we hold a constant temperature, so we are also homeotherms (homeo= same). However, we have seen that constant temperature is a relative term, since our circadian rhythm cycles our core temperature up and down as the day goes on.

This is an infrared image of the human body.

Temperatures increase from blue to green to

yellow to orange to red. The head is often one

of the warmer parts of the body since the brain

uses so much energy, while the testicles are

housed outside the body to keep them cooler.
image source: http://www.medicalir.com/

In addition, holding a constant temperature doesn’t mean that all parts of the organism are the same temperature. Just like your skin is cooler than your core body temperature, parts of your internal body can be warmer than your average core temperature. During intense exercise, your muscle temperature can go to 107˚F or higher! On the other hand, sperm is damaged by high temperature, so the testicles are usually housed in an external pouch in order to keep their temperature one or two degrees below body temperature.

The higher than average temperature is O.K. for a short while or in a small part of the body, but if it involves too much volume or stays high for too long, then your core temperature can rise to dangerous levels (104˚F). On the other hand, having too low a temperature in any part of the human body can be dangerous. If ice crystals form in the cell, the jagged edges will cut the cell to ribbons and kill it; this is frostbite.

Your body thermoregulates to maintain a healthy temperature range. It finds ways to dissipate heat when the core temperature rises, such as sweating in humans, panting in dogs, or pushing more blood through the large ears of rabbits. If your core temperature is too low, you can generate heat by shivering (small muscle spasms that mean more ATP burned and more heat). Chattering teeth is just a spasm in the buccinator muscles of your jaw. These are pretty big muscles (bigger on some people I know) and can produce enough heat to keep your head warm.

Some endotherms are exquisitely adept at regulating the temperature in different parts of their body, and can save lots of energy through this differential regulation. Ground squirrels in hibernation reduce their abdominal temperature to match ambient temperature down to 0˚C, and some birds can hold a body temperature just one degree above freezing all night. These types of animals are referred to as endothermic poikilotherms (poikilo = varied).

Lizards are ectotherms, so they have temperatures near

ambient. In this image, the ambient temperature was

76.1 ˚F, so the lizard remains near that temperature.

However, the human hand is much warmer, as it has an

internal source of heat.

We are biased toward believing that all animals control their body temperature just because we do, but the vast majority of animals are ectotherms (ecto = outside). They get most of their heat from the environment, and this works for them.

Ectotherms like reptiles and insects will have low activity when it is cool, but absorbing heat by sunning themselves will speed them on their daily errands. This is because the rates of most cellular activities increase with temperature right up to the point of boiling, but low temperatures slow them down greatly. So most animals need an external source of heat to allow them to hunt, protect themselves, or seek shelter.

Some ectotherms, like moths and bees, can have their wing muscles go into spasms in order to generate enough heat for them to take off. If they can raise their temperature in any way (sunning or spasming), they are called ectothermic poikilotherms, although some might call moths and bees partial endotherms, since the source of heat is internal. On the other hand, a few ectotherms like some fish, always have the same temperature as their environment no matter their activity or needs. These animals are referred to as ectothermic homeotherms.

There are many more ectotherms than endotherms in the world because it is a successful strategy for saving energy. It is extremely costly to maintain a high metabolic rate and a constant internal temperature, like running your furnace all winter to stay comfortable – we all know how expensive that can be. An adult human (endotherm) needs 1300-1800 kCal/day to maintain its temperature and activity, while a crocodile (ectotherm) of the same size requires only 60 kCal! Fewer calories needed means less energy expended hunting or foraging which makes surviving times easier when less food is available.

Even though they may be called cold-blooded, don’t assume that ectotherms are always cold. Rimicaris exoculata, an ectothermic shrimp that lives next to hydrothermal vents (undersea volcanoes that spew superheated water), is happy with an internal temperature of 350˚C (662˚F). The water doesn’t boil because it is under so much pressure (for every 33 feet of water, the pressure doubles); otherwise they would be shrimp toast.

On the left is a deep-sea hydrothermal chimney called a black smoker. The temperature in the hot water column is near 700˚F. The right side image is the vent shrimp, Rimicaris exoculata. The bright spots are the dorsal eyespots and are rich in rhodopsin. They glow like cat’s eyes when light is shone on them. In the deep ocean, there is no light, so the

shrimp don’t glow normally.

Also don’t assume that ectotherms are looking for a way to warm up. Some fish are perfectly comfortable in antarctic waters at (-2˚C to -4˚C; the ocean water doesn’t freeze because the salt disrupts crystal formation). For example, Dissostichus mawsoni fish have proteins that help important molecules resist cold damage (heat shock proteins) and to stay functional at low temperatures (chaperonins).

The left image shows Dissostichus mawsoni, the Antarctic toothfish,

swimming under an ice sheet. Up close, we can see the teeth, and

that he isn’t going to win any beauty contests.

D. mawsoni also has an antifreeze protein in its blood that binds to ice crystals and keeps the fish from freezing solid. Now that’s cold-blooded. These notothenioid (notothen = “from the south” in Greek) fish are successful enough in this environment to make up 90% of the fish biomass in the Antarctic.

Unfortunately, the terms warm-blooded and cold-blooded have become popular for all organisms. This is wrong on so many levels. We think of snakes as cold-blooded, but on a hot sunny day, the internal temperature of a snake will be much higher than that of a mammal. And we already talked about birds, endotherms of the highest order, that can allow the temperature of their feet to come within a degree of freezing. Now, which is warm-blooded and which is cold blooded?

And where do these terms leave plants? They don’t have blood – so they can’t be cold-blooded or warm-blooded – but they are ectotherms. Some plants are even poikilotherms- they can generate some heat at certain points in their life cycle. They can’t maintain or regulate it, so they are still ectotherms, but let’s not be prejudiced against them by calling them cold-blooded.

Low and behold, there are exceptions to the rules of body temperature – wouldn’t you know it. There is a plant that can maintain a constant temperature by producing heat – even if it is only for two days a year. And there is a mammal that seems to think ectothermy is the way to go. We’ll talk about these rule-breakers starting next time.

Shillito B, Le Bris N, Hourdez S, Ravaux J, Cottin D, Caprais JC, Jollivet D, & Gaill F (2006). Temperature resistance studies on the deep-sea vent shrimp Mirocaris fortunata. The Journal of experimental biology, 209 (Pt 5), 945-55 PMID: 16481583

Kiss AJ, Mirarefi AY, Ramakrishnan S, Zukoski CF, Devries AL, & Cheng CH (2004). Cold-stable eye lens crystallins of the Antarctic nototheniid toothfish Dissostichus mawsoni Norman. The Journal of experimental biology, 207 (Pt 26), 4633-49 PMID: 15579559

For more information, classroom activities, or laboratories on endothermy, ectothermy, or thermoregulation:

thermoregulation –

http://www.seas.
harvard.edu/courses/es96/spring1997/web_page/health/thermreg.htm

http://www.unm.edu/~lkravitz/Article%20folder/thermoregulation.html

http://www.biog1105-1106.org/demos/105/unit3/bodytempregulation.html

http://dwb.unl.edu/teacher/nsf/c01/c01links/www.science.mcmaster.ca/biology/4s03/thermoregulation.html

http://www.scienceforums.net/topic/37403-thermoregulation-activityexperiment/

http://www.teachersdomain.org/resource/lsps07.sci.life.reg.heatexchange/

endothermy –

http://planet.uwc.ac.za/nisl/biodiversity/loe/page_115.htm

http://www.ucmp.berkeley.edu/diapsids/endothermy.html

http://www.issuesinbiology.com/why-did-endothermy-evolve/

http://mysite.verizon.net/redslime/Lessons/endothermy.html

http://www.elp.manchester.ac.uk/pub_projects/2003/MNZO0MLK/lecture14.htm

http://amoebamike.wordpress.com/tag/endothermy/

http://edhelper.com/ReadingComprehension_54_27.html

ectothermy –

http://reptilis.net/cold-blood.html

http://hsc.csu.edu.au/biology/core/balance/9_2_1/921net.html

http://www.learner.org/jnorth/monarch/spring2011/journal022411.html

http://roberta-goli.suite101.com/what-is-an-ectotherm-a124054

www.ableweb.org/volumes/vol-24/13-hodson.pdf

Wednesday, March 26, 2014

Naked Mole Rats Don’t Feel The Burn

Biology concepts – thermoregulation, heat sensing, TRPV1, evolution, neurotransmitters, birds, ectothermy, diet-induced thermogenesis

BBC television has a very nice Sherlock Holmes show

running nowadays, but it has ticked off some mental

health professionals. Sherlock describes himself as a

high functioning sociopath. I have read several angry

letters from those in the profession saying that he

should stop doing so, he is using a mental disorder as

an excuse for just plain rude behavior.

Diseases of the mind are often more bizarre and more tragic than diseases of the body. Medicine and psychiatry use different terminology; terms of the mind are often less specific than terms of anatomy and physiology. For instance, what’s the difference between a psychopath and a sociopath?

There is an argument currently raging as to whether there is any difference between these two labels for anti-social personality disorder. The major similarity is in self-centered actions without remorse for doing wrong to others. The differences may lay in organization. Psychopaths are impulsive while sociopaths may plan things out and use charm to conceal themselves. Others say psychopathy is genetic and sociopathy is learned. But both groups are fine with breaking rules.

This blog has used rule breakers as models for explaining biology concepts, just as medicine uses them as ways to find corrections for when things go wrong. If one animal could be the poster child for rule breaking in biology, it would have to be the naked mole rat (Heterocephalus glaber). There are so many rules that this animal breaks or ignores, it makes one wonder if it's a sociopath or a psychopath.

In truth, the naked mole rat has no motivation for breaking rules. It’s merely a reflection of the evolutionary forces that its ancestors felt, adaptations to pressures over a long period of time. If rules had to be broken, so be it. It’s evolution that’s the psychopath.

The broken rules we are concerned with in this post relate to TRPV1. The capsaicin of chili peppers does not inflict pain on naked mole rats! H. glaber TRPV1 binds capsaicin just fine, it just doesn’t result in pain. The difference comes in the spinal cord. A 2008 paper shows that the connections from the TRPV1 expressing nociceptive neurons to those neurons that would convey the signal to the brain are different in naked mole rats, and the additional pathways result in a loss of the pain signal.

This isn’t just a pile of naked mole rats that someone

dumped out of a bucket. This is how they sleep. They

use each other to keep warm because they are cold

blooded. A cold-blooded mammal? Well, I never

imagined. But they break more rules – they live much

longer than other mammals, they don’t get cancer,

and they have a queen like in bee hives. Drum roll

please – the naked mole rat was named the vertebrate

of the year for 2013 by Science magazine.

There’s no inherent advantage in altered TRPV1 signaling via capsaicin for H. glaber; they don’t eat chili peppers. But remember that TRPV1 is activated by more than just capsaicin, so perhaps the advantage lies in stopping some other activation of TRPV1 and it just so happens that it also stops capsaicin–induced pain.

One of those other TRPV1 activators is acid. We said that a pH below 5.5 activates TRPV1 and a pH between 5.5 and 6.5 makes TRPV1 more sensitive. This is pertinent for naked mole rats because they live entirely below ground. Their tunnels are high in CO₂ from all their exhalations. Excess CO₂ in the tissues causes an acidosis, a low pH situation. If the H. glaber TRPV1 acted as it does in every other mammal, then naked mole rats would be in constant pain.

New research (2011) shows that the acid does indeed make naked mole rat TRPV1 channels open just as capsaicin does, but the neurons don’t fire. Acid suppresses a certain sodium channel downstream of TRPV1. Normally, the calcium influx mediated by TRPV1 activates the Na_v1.7 sodium channel, and the neuron is depolarized and fires. But acid destabilizes the Na_v1.7 channel and there is reduced firing.

This suppression occurs in all mammals, but it is a much stronger suppression in naked mole rats, because two amino acids are changed in their version of Na_v1.7. Now we have two TRPV1 activators (capsaicin and acid) that no longer result in pain, but for two different reasons. Is there more?

Substance P is a neurotransmitter and modulator

that is important for pain signaling, but it also works

in vomit regulation. In the medulla of the brain lies

the vomit center, and it uses substance P to induce a

reversal of the motion of the GI muscles that usually

moves food through the GI tract. A spike of substance

P in the wrong place, and here comes supper for a

return engagement.

A neurotransmitter called Substance P is also important in TRPV1 pain signaling. Are you getting the idea that this is a complicated system - I sure am. It turns out that naked mole rats don’t make substance P (2010). Now we have three different reasons that naked mole rats don’t transmit pain signals via TRPV1. This is an evolutionary overkill, but it makes the mole rats able to live where they live without fear of unnecessary pain.

But the question remains, did the mutations in TRPV1 signaling permit H. glaber to move permanently underground, or did living underground put pressure on the naked mole rat to adapt through TRPV1 mutations? It’s hard to tell which was the cart and which was the horse.

The naked mole rat’s lack of pain signaling via TRPV1 may help us humans. The more we know about the mechanisms of H. glaber TRPV1 action, the better we can design pain killers for ourselves. Exceptions can often be our savior.

Another rule that naked mole rats break is that they are cold-blooded (ectothermic) mammals. We know that TRPV1 is important in heat sensing, so does having altered TRPV1 pathways mean that the naked mole rat can’t thermoregulate and that’s why it’s ectothermic?

The rats can still probably sense heat. Remember that there are other TRPV proteins that are important in heat sensing; nothing says these aren’t functioning just fine. For that matter, there is no evidence that the TRPV1 of H. glaber is defective in heat sensing. It just doesn’t result in a pain sensation.

And it would be wrong to believe that ectotherms don’t need to sense heat. It may be even more crucial for ectotherms. Cold-blooded animals must find the heat in their environment and soak it in – but not too much. This means they must be experts at knowing how much heat they have and where they can find more.

Ectotherms also need to know where the shade is, so they can cool off if they get too hot. To prove this, we know that reptiles with mutated thermosensors don’t shuttle between warmer and cooler areas and can’t maintain a satisfactory physiologic temperature.

As weird as the naked mole rat is, birds also seem to break the rules when it comes to TRPV1; they can order their food spicy as well. Why is it significant that birds don’t sense capsaicin as burning pain? Remember that chilies are a group of plants with fruits. Those plants have evolved capsaicin to inhibit herbivorous predators and fungal infections, as we talked about last week. Now we have the fact that birds don’t react to capsaicin. How are these linked?

Squirrels are the bane of every backyard birder’s

existence. They eat ten times as much feed as the

birds, and they can find some creative ways to

reach the bird food. Try adding pepper flakes to

the bird feed, squirrels feel the burn, but your

song bird visitors won’t.

The answer is seed dispersal. It is important that chili peppers are consumed and the seeds are spread. This is crucial for the survival of the plant species. But if the fruits are spicy and there is avoidance of same by most animals, how will the seeds be spread? Well, there better be some animals that don’t react to capsaicin – birds.

The TRPV1 of most birds doesn’t have a vanilloid binding site. The channels work for heat sensing and do react to acids, but there is no activation by capsaicin. Since there is no capsaicin binding site, birds only taste the peppers, they don’t get the burn. It's possible that they taste the many different vanilloid compounds, so peppers may taste a little like vanilla to birds.

I still have one question – there's certainly a reproductive advantage for peppers when birds don’t sense capsaicin (for seed dispersal), but where is the advantage for birds? And how could pepper plants force the evolution of a different TRPV1 in birds? One possibility - maybe birds evolved a different TRPV1 to take advantage of a food source that other animals avoid. No competition for food would definitely be a reproductive advantage for birds.

But this explanation has exceptions as well. The TRPV1 of chickens is activated by capsaicin. It is weak, taking 3-4 times more capsaicin to get a reaction, but it does work. So if you own chickens, don’t give them very spicy feed. Ducks on the other hand, have a TRPV1 that doesn’t sense capsaicin or heat.

Some foods are considered negative calorie items. They

supposedly cost more to digest than the energy they

provide in calories. I’m not sure if I believe that all these

foods are negative calorie foods. If they were, there would

be a lot of starving vegetarians. If not dead, they would be

awfully weak and tired.

You can inject huge amounts of capsaicin extract into the veins of ducks without them having any kind of a thermal response. Since TRPV1 senses heat and then initiates a cooling process, capsaicin in the blood will result in too much cooling – a hypothermia. In chickens this hypothermia occurs, but not in ducks.

So thermosensing must be important. Even in most animals that don’t respond to capsaicin, their TRPV1 still works in thermoregulation. I can give you an idea of how intricate and detailed this thermoregulatory system is by talking about digesting spicy food. Your body uses energy and metabolism to digest the food you eat. This energy use produces heat as a byproduct, and warms you up a bit. This is called diet-induced thermogenesis. Celery is an excellent diet food because the energy you use to digest it is the same or more than the calories in the celery itself.

For some reason, spicy foods increase diet-induced thermogenesis; you expend more energy and heat up more when eating spicy foods than when eating the same foods without the capsaicin. Recent evidence indicates that including capsaicin and medium chain triglycerides in a meal will increase diet-induced thermogenesis by over 50%. This combination also makes you feel full sooner and therefore decreases overall caloric intake.

The spice also makes you use more energy for digestion, but it also makes your body think it is warmer than it is, so it tries to cool down. Cooling down also takes energy, so eating spicy food really does burn more calories - maybe because fat takes more energy to digest and capsaicin is a lipid-like molecule.

Some weird products include capsaicin for the supposed health

benefits. Here are capsaicin drinks. Including capsaicin in a diet

will help you eat less, but I am thinking it may be because you

just get tired of sweating and feeling like your mouth is on fire.

Another recent study shows that the decrease in energy your body expends when you diet (an evolutionary adaptation to try and maximize fat reserves) is prevented by consuming capsaicin. So you burn more calories with spicy food and your body doesn’t even realize your dieting. Somebody should try breeding a capsaicin-packed celery stalk.

Next week we'll see that TRPV1 is even more amazing. Not every spicy food contains capsaicin, there’s mustard, black pepper, horseradish, ginger, cinnamon, etc. Some of these even make your capsaicin seem spicier.

Smeets AJ, Janssens PL, & Westerterp-Plantenga MS (2013). Addition of capsaicin and exchange of carbohydrate with protein counteract energy intake restriction effects on fullness and energy expenditure. The Journal of nutrition, 143 (4), 442-7 PMID: 23406619

Clegg ME, Golsorkhi M, & Henry CJ (2013). Combined medium-chain triglyceride and chilli feeding increases diet-induced thermogenesis in normal-weight humans. European journal of nutrition, 52 (6), 1579-85 PMID: 23179202

Smith ES, Omerbašić D, Lechner SG, Anirudhan G, Lapatsina L, & Lewin GR (2011). The molecular basis of acid insensitivity in the African naked mole-rat. Science (New York, N.Y.), 334 (6062), 1557-60 PMID: 22174253

Park TJ, Lu Y, Jüttner 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

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

For more information or classroom activities, see:

Naked mole rat –

http://animals.nationalgeographic.com/animals/mammals/naked-mole-rat/

http://www.youtube.com/watch?v=eHi9FvUPSdQ

http://www.bbc.co.uk/nature/life/Naked_mole_rat

http://www.latimes.com/science/sciencenow/la-sci-sn-mole-rat-longevity-20131001,0,6184646.story

http://www.sciencefriday.com/video/10/24/2013/the-inner-beauty-of-naked-mole-rats.html

http://www.huffingtonpost.com/2012/07/03/naked-mole-rat_n_1646441.html

http://www.sciencedaily.com/releases/2013/07/130731093255.htm

http://www.scientificamerican.com/article/naked-mole-rat-named-vertebrate/

Substance P –

http://arthritis.about.com/od/arthqa/f/substanceP.htm