Wednesday, February 19, 2014

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


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