It's the kind of research that's not for the queasy, with scientists figuring that a cocktail of biological odors is most likely to offend all people, regardless of cultural background. With this in mind, researchers are making an odor bomb that will smell like rotting garbage, human waste, vomit, and burning hair. Foul as they sound, the advantage of using such odor bombs is that they offer a less aggressive means of crowd control than tear gas or pepper gas. People caught in their midst may feel nauseous but won't come to any real harm.

Similar tactics are sometimes used to control animal populations, too. In a curious story from Australia last week, wildlife officer Bruce Thomson describes unusual ways of keeping flying foxes away from lychee crops. Thomson plans to try odor bombs that smell of rotten eggs and ammonia to encourage the flying foxes to look for food elsewhere. He's even considering adding the odor of pythons, the natural enemy of the flying fox — if he can figure out a way to replicate that particular smell. "Maybe we could talk a perfume company into producing odor of python," Thomson joked in an article from Australia's Courier Mail.

• Read the article, "Smell may End Flying Fox Stink," from the Courier Mail.

In fact, an entire industry has been built on deterring animals through their sense of smell. The methods are environmentally- and creature-friendly because they don't involve traps or poisons. If you love deer but don't like them nibbling on your prized plants, try coyote urine in a box. Or if you want to keep geese from grazing on your lawn, sprinkle it with a concoction made from the bitter-tasting, smelly part of concord grapes — guaranteed to offend the discerning goose. Other methods include using castor oil to discombobulate moles, red pepper spray to irritate squirrels, and mustard to bother rabbits. Such methods offer non-harmful ways of discouraging critters from calling your yard home.

Dogs can get this kind of information from smells because they live in a much richer smellscape than humans do, thanks to their turbo-powered noses (that's why they're so useful as rescue helpers and at finding illegal drug shipments). Bloodhounds, the champion sniffers, have about 220 million smell receptors in their noses, compared with humans' 5 million. Imagine the kind of "smell information" that a bloodhound receives as it sniffs the air!

• Learn more about the dog's sense of smell from Petsmart.com.

The bloodhound's sensitive nose is likely to be accosted by the many animals that leave an unmistakable impression with the foul odors they emit. Certain animals kick up a stink to defend themselves; others use pungent liquids to mark territory and attract a mate. As smart dogs and their humans know, the most notorious odor-maker is the skunk.

The smell of skunk spray comes from the chemical compounds it contains, thiols. Thiols are also responsible for the odor of rotten eggs. Interestingly, thiol compounds are also found in great-tasting foods such as garlic and coffee — but in minute quantities compared with the thiol-loaded skunk spray. So, a little thiol contributes to making a good smell, but a lot of thiol is bad. This curious nuance has yet to be explained by scientists who study olfaction ("olfaction" is the name for the sense of smell).

• You can find out more about the chemistry of skunk spray from William Wood's Web site. William Wood is a specialist in chemical ecology from Humboldt University.

For all its virtuosity, surprisingly, the skunk is not the champion of foul odors in the animal kingdom. Last year, the Discovery Channel's Nigel's Wild Wild World listed the top ten stinky animals and the skunk got second place. Here's the complete list but be warned — some of these animal behaviors are quite grisly!

1. Tasmanian devil
These aggressive creatures emit a foul odor when they get stressed out.

2. Skunk
As you've read, skunks spray a malodorous liquid as a means of self-defense.

3. Hyena
Hyenas mark their territory with a smelly substance secreted from anal glands and glands between their toes.

4. Turkey vulture
When threatened, these large birds may vomit as a defensive maneuver. Also, when they're hot, they may excrete on their own legs to cool themselves. It sounds awful, but it works!

5. Wolverine
These predators have scent glands that produce a strong, musk-like odor for marking territory.

6. Musk ox
These animals produce urine that smells pungent enough to make your eyes water. They use it for marking territory, an important part of the mating process.

7. Stinkbug
Some of these bugs are pests, damaging crops such as tomatoes and rice. One type of stinkbug, T. Papillosa, can "spit" its secretions a distance of 6-12 inches — quite an achievement for a small bug.

8. Beaver
These cute dam-builders have scent glands near the anus that produce a musky liquid called "castoreum." The liquid is thought to play a part in attracting a mate.

9. Fox
Red foxes have scent glands near the anus and in the tail. The secretions smell like skunk spray and are used for marking territory. Their urine also has a strong, "skunky" odor.

10. Porcupine
Porcupines exhibit a very smelly behavior. Before mating, the male porcupine showers the female with urine. If she does not object, it shows that she is ready to mate.

• Can you think of some ways in which you depend on your sense of smell? (Consider food and taste.)
• How do we use references to smell in everyday speech? For example, "Wake up and smell the coffee" or "Smells like trouble." Can you think of other examples?
• Helen Keller said, "Smell is a potent wizard that transports you across thousands of miles and all the years you have lived." Do you think humans use their sense of smell as much as they could? Do you think humans depend more on one sense as they interpret their environment? How do the senses work together?

We'll answer some of these questions in next week's article on odors and how we smell them — "What the Nose Knows."