Who Tastes Best?

Biology concepts – taste/gustation, aposematism, carnivore, herbivore, omnivore, Jacobson’s organ, palatal organ, parasite

What animal do you think has the most taste buds? There are bunches of animals out there, so let’s make it a bit easier. What sorts of animals do you think would need more taste buds? Omnivores might be a good choice, since they eat more different kinds of foods.

Cows are big, so they have big tongues. It isn’t too surprising

that they have many taste buds. However, the density (number

per unit area) is higher than most animals too, since they eat

plants, and they have to pick the non-poisonous plants from the

poisonous. Some people eat beef tongue – Bill Cosby said he

didn’t want to taste anything that might taste him back. I would

add that I don’t want to eat anything that could have been

in a cow’s nose.

Carnivores could be a choice, since they have to worry (most of them, vultures excepted) about rotted or diseased meat. But then again, herbivores might be the answer; they eat plants, and plants are the source of most poisons. The same could be said about insectivores, the insects they eat are often toxic – formic acid in ants or stored plant toxins.

Let’s take a survey of animals and see if a pattern develops. Humans have about 10,000 taste buds on average. Young people have more than the elderly; you lose about half your taste buds by age 65. Women generally have more than men. And supertasters can have double the number of non-tasters. But 10,000 is a good number to go by.

Pigs have about 15,000 taste buds while cows have 25,000 and rabbits average 17,000. Lions and other cats come in at about 450 taste buds, but dogs fair better, averaging about 1200. Birds have very few and some fish have alot, but we will talk about them below.

Sort the numbers out and it appears that herbivores have the highest number of taste buds, carnivores and insectivores the fewest, and omnivores lie somewhere in the middle. It looks like keeping an eye (or tongue) out for poisonous plants is the most important mechanism for taste. Carnivores’ strict diet means they need fewer, as we saw when we discussed the loss of sweet taste receptors in cats.

But the winner in the taste bud count? Believe it or not – catfish! Even small catfish (6 in/15 cm) have about 250,000 taste buds. They are literally covered in taste buds. Mind you, we’re talking about taste buds, not just taste receptor cells like we discussed last week. These are full-fledged taste buds, with at least five different tastes represented (sweet, salt, bitter, sour, umami).

On this light colored fish skin, you can see the dots that

are the microscopic taste buds. I eat fish skin, but I don’t

equate it to eating cow tongue.

Most fish have the majority of their taste buds in their mouths - that makes sense. Some fish, especially bottom feeders like carp, are designed so that food coming into contact with the back of the roof the mouth will stimulate an area called the palatal organ which has thousands of taste buds. When the muscular palatal organ tastes food, it automatically clamps down on the food particle. Everything else - water, stones, inedible things - are then flushed from the mouth by blowing them out. Only the food is left and they swallow it.

That’s pretty much it for fish with scales, but catfish, sharks, and some other fish don’t have scales, they are covered with tissue that is more like skin. In these kinds of fishes, it’s like their tongues have migrated all over their bodies. They taste with everything – mouth, lips, gills, fins, body, tail, whiskers.

The catfish, as ironic as it may be, doesn’t have taste buds on its tongue! Why would it be important for catfish to have so many taste buds but none on their tongues? It’s because they live in muddy water. Their sight is impaired, so the taste buds on their body allow them to find food. In truth, most fish have very few taste buds on their tongue (called a basihyal amongst ichthyologists). 

This is fortuitous for a fish parasite called Cymothoa exigua. It may sound disgusting, but this isopod (iso = identical, and pod = foot) parasite makes its living by getting into a fish’s mouth through its gills, eating its tongue and then replacing the missing tongue with its own body. Now, everything that comes into the fish’s mouth can be nibbled on by the parasite.

C. exigua doesn’t so much eat its host’s tongue as it causes it to disappear. It grabs on to the tongue, and sucks the blood out of it. The longer this goes on, the more the tongue atrophies (a = no, and trophic = feeding) and shrinks away to nothing. Then the parasite grabs hold of the stump with its back legs and takes its place.

An amazing picture of the isopod parasite C. exigua acting

as the tongue of a captured fish. It lives in the gulf of

California and usually selects the rose snapper as a host.

Every once in a while, a fish lands in the grocery store

with a parasite still hanging on, looking for the next meal.

I’m guessing that fish gets returned and a lot of store

credit is issued. Photo credit to Dr. Nico Smit.

Talk about exceptional; this is the only known instance where a parasite functionally replaces a host’s own organ. The isopod is willing to act like a tongue, holding food up against the small teeth on the roof of the fish’s mouth, because this is how it ensures food for itself.

We know that taste is important for animals to separate toxins from those foods that are good for them, so it is probably fortunate that fish don’t use their tongue for tasting. If losing their tongue caused fish to die more often from ingested poisons or from starvation, then the parasite would be sealing its own fate – it wants its host to survive.

Moving on to something a little less disgusting. We talkedlast week about solitary chemosensory cells and their work in non-gustatory organs. An interesting 2013 paper studied the SCC and taste buds of fish and compared them to those of mammals. Their results indicate that taste buds did not evolve from SCCs or vice versa, the two developed independently. This means that taste, smell and nutrient sensing all developed on their own, yet they have similar and overlapping structures and functions. A good smell idea is a good taste idea.

Snakes show again the similarities between taste and smell. Most snakes (some sea snakes excepted) have taste buds, often lined up near their teeth. What is different is the vomeronasal organ (Jacobson’s organ) for sensing volatile chemicals. Present in amphibians, most reptiles (not crocodiles or chameleons), and some mammals, the VNO is for sensing pheromones and other scents.

On the left is a cartoon showing the snake veromonasal organ

(VNO). It sits in the roof of the mouth and neurons from it go

to the olfactory bulb. As the tongue pulls in molecules, it

presses the tongue up into the VNO. On the top right, you see

how the snake spreads its tongue out to pick up as many signals

as possible. Now you know why snakes have forked tongues.

On the bottom right, you see that the VNO doesn’t even

communicate directly with the nostrils, the moth is more

important for this smelling.

In snakes, however, the scents are not brought to the organ by breathing in. In fact, the VNO in snakes isn’t even open to the nasal compartment. Instead, snakes bring the scent molecules in with their tongue, and press them into the VNO for sensing. This makes it a quasi-direct chemosensory organ, like taste. And thus, we run up against the crossover of smell and taste again.

Humans can taste some volatile compounds as well. One nasty example is gasoline. If confronted with gasoline fumes in sufficient density, some will enter your mouth and mix with saliva. If enough molecules come into contact with your taste buds, you will taste the gas – not an especially pleasant taste.

Even more exceptional, you can also taste a gas you didn’t breathe in. There is a chemical called dimethyl sulfoxide (DMSO) that is used in many laboratories. It’s famous for being a great solvent. A solvent is a liquid in which other chemicals will dissolve. Water is a very good solvent, but there are many things that are not water-soluble.

Once inside the body, DMSO starts to be metabolized by your cells. One of the products of its breakdown is dimethyl sulfide (DMS). This travels around in your blood, but wants to a gas, so it passes from your blood to your lungs. From here you breathe it out. When you exhale DMS, you taste it. It tastes and smells like garlic, so if you taste this and haven’t had Italian for lunch, then you might have contaminated yourself with DMSO - oops.

DMSO is a by product of the wood industry and is a great

solvent. It is polar solvent, meaning that it has a partial

positive charge and a partial negative charge within its

structure. Scientists first thought that the sulfur (S) oxygen

(O) bond was a double bond, but it turns out to be a single

bond with the excess charge being shared as a huge dipole.

Being so much more polar makes it a great solvent, it can get

in between nonpolar, positive or negative structures to

help things dissolve.

The oops is because the structure of DMSO allows almost anything to go into solution – it would put your car into solution if you have enough of it. Its structure also makes it so DMSO can be absorbed directly through your skin to your bloodstream. Any molecule dissolved in DMSO will also be carried straight into your tissues.

This works out well for medicine and chemistry, as certain tests or drugs require that the chemical be dissolved. But, if you happen to be using a toxic chemical in DMSO and get some of it on your skin or mucous membranes - you’ve now been poisoned and you just hope it wasn’t enough to kill you.

Back to taste buds. At the low end of the taste bud spectrum are the birds. They have merely dozens (Japanese quail) to a couple hundred taste buds; chickens and songbirds are especially taste-poor. But that doesn’t mean they can’t be interesting. In the 1970’s, it was discovered that ducks had taste buds in a weird place – on their beaks.

Mallard ducks therefore have about 400 taste buds on their jaws, since beak parts are extensions of the maxilla (upper jaw) and mandible (lower jaw) bones. The taste buds are located in five concentrations, four on the maxilla and one on the mandible.

These positions just happen to correspond to where the duck grasps and holds food as it decides whether it is safe to swallow. Once again, life is protected by taste sense.

The leopard lacewing caterpillar on the left warns predators

with its bright colors that it is poisonous. This is

aposematism, although some animals aren’t above

faking that they are poisonous by just adopting the colors.

On the right is the crimson speckled moth, it is bright and

patterned, but it does it for a different reason. This is a

secondary sex characteristic for attracting mates.

So birds can taste their food, but they don’t rely just on that. It was back in Darwin’s day that the question was raised as to why birds would eat green and brown caterpillars but leave bright colored caterpillars alone. This led Alfred Russel Wallace (the other guy who came up with natural selection) to demonstrate the concept of aposematism (apo = away, and semantic = sign), coloration to warn of toxicity in order to ward off predation.

The bright color says, “Don’t eat me, I’m poisonous.” This is important because even a single peck with a beak could be lethal to a caterpillar. Birds learned quickly to avoid the bright caterpillars, suggesting that birds could taste. This tale is well recounted in the 2012 book of Tim Birkhead, called Bird Sense.

In a strange twist, there are a few birds that are toxic because of their diet, the pithoui and the ifrita. We have discussed these birds in terms of toxins. These birds are brightly colored, so they are using the concept of aposematism to keep themselves safe, just like many potential bird snacks do.

Speaking of caterpillars, we’ll get to arthropod gustatory sense next week. It turns out that some insects taste with their wings, while others taste different things based on their jobs.

Kirino M, Parnes J, Hansen A, Kiyohara S, & Finger TE (2013). Evolutionary origins of taste buds: phylogenetic analysis of purinergic neurotransmission in epithelial chemosensors. Open biology, 3 (3) PMID: 23466675

Tim Birkhead (2012). Bird Sense: What It Is Like To Be Bird Walker Publishing, New York

For more information, see:

Cymothoa exigua –

http://www.wired.com/wiredscience/2013/11/absurd-creature-of-the-week-the-parasite-that-eats-and-replaces-a-fishs-tongue/

http://phenomena.nationalgeographic.com/2013/02/28/tongue-eating-fish-parasites-never-cease-to-amaze/

http://www.talksciencetome.com/treasures-the-tongue-eating-sea-louse/

Sneaking Up On A Snake

Biology concepts – thermosensor, sight-hunters, snake hearing, mutation, TRPA1, pit vipers

We have been talking about taste sense for many weeks. I

remember a 1975 movie called, A Boy And His Dog, starring a

very young Don Johnson. It was a post-apocalyptic story of a

guy, his dog, and cannibalism. The best line of the movie? “Well,

she might not have had good taste, but she sure tasted good.”

Of course, this isn’t the kind of tastes we have been talking about.

We’ve come a long way since we started talking about taste sense. We have learned about how TRPV1 capsaicin receptors sense pain and heat. We have also learned that TRPV1 capsaicin receptors have cousins that sense cold - TRPM8 and TRPA1. They may generate pain, and they certainly help to warm us when we are cold.

We have even learned that in rare cases, the cold receptors can be heat sensors, like in chickens and insects where TRPA1 sense hot instead of cold. And this leads us to today’s exception. It’s time to talk about how these relatives of taste receptors help animals to become better hunters and to better sense their environment. Today let’s focus on snakes.

Snakes have a number of ways to catch prey (see this post). Some lie in wait, blending in with the jungle or background until a moving potential dinner catches their eye and moves across their path. Vision is their primary way of finding dinner. As a consequence, most sight-hunting snakes are diurnal (active in daylight).

Here is the southern black racer. You can see it has big eyes with

round pupils so lots of light can enter – it’s a sight hunter. Many

grow to be 5 ft. (1.5 m) long, so they can look intimidating. But

they are not venomous and will usually exit the seen if disturbed.

The non-venomous Southern black racer (Coluber constrictor priapus) is a sight-hunting snake of North and Central America. It’s called a racer because it is quick, reaching 4 mph (1.8 kph) in a very short time. Even though it is a constrictor, it typically doesn’t coil around the lizard, mole, or bird (I said they were quick) that it catches. It prefers to crush them into the ground to suffocate them. Sometimes nature can be a little rough around the edges.

Other snakes use the combination of scent and taste that we talked about a while back. The Jacobson organ (more scientifically called the vomeronasal organ, VNO) in their mouth can sense the molecules that the tongue pulls in from the air. Like it or not, every organism has molecules floating off of them continuously. Snakes' VNO can pick these up. See this post for more on the VNO.

Some snakes “hear” their prey coming. True, snakes don’t have an outer ear opening or the small bones that convert sound waves into mechanical waves in our middle ear (see this post for an explanation). But they do have a cochlea, the organ for sensing the vibrations and converting them to a nerve signal. Many snakes can sense the vibrations that their prey generate when they move through the environment using this cochlea and their lower jaw.

Similar to something called bone conduction hearing in animals with ears like ours, vibrations that travel through the bone can also cause movement in the hairs of the cochlea. As we discussed previously, the bending of the sensory hairs of the cochlea are transduced to chemico-electrical signals that travel to the hearing centers of the brain.

This is from a scientific paper showing the bone hearing of a python.

The red is the lower jawbone. The bark blue is the quadrate bone

and the green is the equivalent to our stapes bone of the middle

ear. The light blue is the inner ear space and the purple is where

the cochlea is housed. Vibrations go from red, to blue, to green to

light blue, to purple. You can see how sound waves would find it

tough to get to the cochlea.

A 2008 study showed that many snakes rest their jaw bones against the ground. The vibrations caused by moving animals are transferred from the ground to the bone, and from the bone to the buried cochlea. The sensation in the brain is a lot like muffled knocks, not unlike the bass that is turned up too loud in peoples’ cars.

This was followed by a 2012 study that showed pythons have very sensitive vibratory hearing, but poor sound pressure hearing. Almost all their hearing input comes from the vibrations they sense in the ground or tree, or whatever they happen to be lying on. So be on tip toes, that snake may hear you coming.

But how does any of this relate to a receptor for painful cold and controls mammalian breathing rate? Well, another way some snakes find their prey is by sensing the heat they give off – even from a few meters away.

Pit vipers are a subfamily of the Viperdae family, called Crotalinae. There are two types of vipers; all of them have hinged fangs, the ones that are folded up into the upper jaw when the mouth is closed, but protrude for striking as the mouth is opened. Pit vipers differ from true vipers in that they have pits (duh!); more about these below. True vipers live exclusively in Africa and tropical Europe and Asia.

In America, where I live, there are a lot of pit vipers. Cottonmouths, rattlesnakes (all 30 species), water moccasins, copperheads – these are all pit vipers. From southern Canada to Argentina, and from Eastern Europe to parts of Asia, pit vipers are not rare. Eyelash vipers (Bothriechis schlegelii) of South America are arboreal (live in the trees). They have bright coloring, but sit still and wait for their prey to happen by. They strike from above, so they scare the heck out of jungle hikers.

On the left is the eyelash viper. You can see it doesn’t mean business

because its hinged fangs aren’t extended. In the middle is the two-striped

forest pit viper. It is protecting it’s young, so the fangs are extended. On

the right is a sidewinder rattlesnake. Sidewinders are amazing and will

get their own post soon.

The amazing thing is that there aren’t any pit vipers or true vipers in Australia. The land of a million weird and painful deaths has nothing to offer in the way of hinged fang venomous snakes. I’m sure there’s a movement to import some.

But it’s specific part of the pit viper that we are interested in today – namely the pit. The pit organ is located between the eye and the nostril, on each side of the snake’s head. It is a hollow pit, so the actual business end of the pit organ is inside the snake’s skull.

The pit is lined with epithelium, but it also has a membrane that is stretched across the base. As a consequence of the location membrane, there are air pockets on each side of the membrane. The trigeminal nerve innervates the membrane and there are thermosensors in the cells of the membrane.

So, the pit organ is a thermosensor that helps them locate prey animals (or predators). But wait you say. Sure, pit vipers may use a thermosensitive ion channel to sense the heat given off by passing prey animals. But we just said they use a COLD sensing ion channel, TRPA1. What gives?

The pit on a pit viper is located between the nostril and the eye.

It would be easy to mistake the pit for the nostril. The cartoon

shows the pit anatomy. The air chamber helps cool the air

quickly and stops the TRPA1 receptors from firing again. This

is so the snake won’t get a residual image of something warm,

when the target may have moved in the interim period.

The explanation is two fold. 1) We said a couple of weeks ago that TRPA1 might sense painful cold on its own, or may work with other TRP’s to respond to very cold temperature. But whichever way it works, it is very similar to TRPV1 for heat sensing and TRPM8 for cold sensing. 2) Remember that in birds, lizards, and many insects, TRPA1 actually senses heat, not cold.

So maybe it’s not so terribly bizarre that pit vipers use TRPA1 to sense their prey. But before they touch it??? We eat chili peppers and we react to the capsaicin in our mouths and noses. We go out on a summer day, and the heat activates our TRPV receptors in skin and other tissues. We eat something cold (or menthol) and we feel the cold sensations it touches or tissues. But snakes feel the heat of their prey before they eat, from a distance away! There must be more at work.

And there is. The TRPA1 ion channels in the pit organs of pit vipers have a mutated version of TRPA1. Here’s how things work according to a 2010 study that identified TRPA1 as the heat sensor. The pit is a hole with a membrane stretched toward the back. Consequently, there is an air chamber on both sides of the membrane.  The membrane is highly vasculature and has the sensitive nerve endings with the TRPA1 channels.

The TRPA1 receptors are always firing, but at a low rate. Neutrally warm objects don’t change the firing rate, but warmer objects (as little as 0.001 ˚C warmer than background) will increase the firing rate. The receptor is mutated according to a 2011 study, with 11 amino acids of the pit TRPA1 divergent on only pit-containing snakes. These changes make the receptor so sensitive that it can react to infrared light signals (heat) from several feet away. That would be like our mouth burning over a chili pepper that we walked past in the supermarket.

Since the sensors are spread across the entire membrane, the effect on locating the source is sort of like vision or a pinhole camera. Light passes through the pupil and diverges before it hits the retina. This provides for a larger spread of the “image” across the membrane and allows for precise two-dimensional map of the target. The difference in heat between the target and the background gives a “picture” of the object that is warm.

The Taylor’s Cantil viper will play dead and then strike, but this

brings up an important point. DON’T get near a pit viper, even if

you are sure it’s dead. The pit is wired directly to the brain and

muscles. A dead snake, even one with a severed head, can still

strike as long as there is any residual neural electrical flow. People

die every year from snake bites from dead snakes.

The picture generated is also a little like hearing, since the heat will reach one pit earlier or more strongly. By comparing the timing and the strength of the signals from each pit, the distance and direction to the target can be detected by the brain (see this post for localization of sound waves).

Because the heat “picture” pit vipers pick up is based on the difference between the temperature of target and background, most pit vipers hunt when coolest, so temperature gradient between environment and prey is greatest. Prey will stick out the most.

Snakes can also use the pit more conventionally, as a thermosensor for its whole body. The basal rate of firing will tell the snake when to move to shade if it’s too warm or move to sun if it’s too cold. This is how it regulates its body temperature.

Pythons and boas can also have heat-sensing pits, but they are

5-10 times less sensitive because of their differing anatomy.

The amazing thing is that they evolved the same special power

independently from pit vipers, although they both use mutated

versions of TRPA1. The nostril has a black arrow and the pits

have red arrows.

The exception to today’s exception: some non-pit vipers have pits. In terms of evolution, pits evolved once in pit vipers, but they have sprung up several times in boas and pythons. These pits are less sophisticated (no membrane or air chambers), are less sensitive, and are located in different places.

Boas and pythons with pits have 3-4 simple pits in their upper lips. They don’t have the suspended membrane for sensing temperature, the TRPA1 sensors are housed within the epidermal cells at the back of the pit.

Next week – vampire bats and mosquitoes get into the mutated thermosensor act as well.

Christensen CB, Christensen-Dalsgaard J, Brandt C, & Madsen PT (2012). Hearing with an atympanic ear: good vibration and poor sound-pressure detection in the royal python, Python regius. The Journal of experimental biology, 215 (Pt 2), 331-42 PMID: 22189777

Gracheva EO, Ingolia NT, Kelly YM, Cordero-Morales JF, Hollopeter G, Chesler AT, Sánchez EE, Perez JC, Weissman JS, & Julius D (2010). Molecular basis of infrared detection by snakes. Nature, 464 (7291), 1006-11 PMID: 20228791

Geng J, Liang D, Jiang K, & Zhang P (2011). Molecular evolution of the infrared sensory gene TRPA1 in snakes and implications for functional studies. PloS one, 6 (12) PMID: 22163322

For more information or classroom activities, see:

Pit vipers –

http://www.kidzone.ws/lw/snakes/facts11.htm

http://animals.howstuffworks.com/snakes/pit-viper-info.htm

http://animals.pawnation.com/pit-viper-snake-4957.html

http://www.newton.dep.anl.gov/natbltn/600-699/nb681.htm

http://www.biologyofthepitvipers.com/

http://news.nationalgeographic.com/news/2011/07/110713-new-species-pit-viper-china-snakes-animals-science/

https://www.youtube.com/watch?v=lySW2-eYilg

https://www.youtube.com/watch?v=f_G4Q1NAqoA

http://www.nature.com/news/2010/100314/full/news.2010.122.html

http://www.mapoflife.org/topics/topic_312_infrared-detection-in-snakes/

http://www.nsf.gov/news/special_reports/science_nation/infraredsnakes.jsp

http://www.phschool.com/science/science_news/articles/snake_pits.html

Bone conduction hearing –

http://www.healthyhearing.com/content/articles/Research/Hearing/26145-Do-snakes-have-ears

http://what-did-you-say.org/2012/07/08/out-in-the-wild-hissy-bone-conduction-hearing-could-prevent-a-bite/

http://jeb.biologists.org/content/215/2/ii.full

http://en.wikipedia.org/wiki/Bone_conduction

http://www.hearinglosseducation.com/treatments/direct-bone-conduction

http://electronics.howstuffworks.com/gadgets/audio-music/bone-conducting-headphones.htm

http://www.oticonmedical.com/Medical/YourTreatment/About%20bone%20conduction/How%20does%20it%20work.aspx#.U5dGWIVf13I

http://www.britannica.com/EBchecked/topic/72920/bone-conduction

VNO (Jacobson organ) -

http://chemistry.about.com/cs/medical/a/aa051601a.htm

http://www.neuro.fsu.edu/~mmered/vomer/snake.htm

http://reptilis.net/smell.html

http://chemse.oxfordjournals.org/content/26/4/433.full

http://www.sciencedirect.com/science/article/pii/S187972961100010X

http://www.catbehaviorassociates.com/what-is-the-vomeronasal-organ/