The Natural History of Skunk Stink

I never knew that fresh skunk spray smells exactly like burning rubber.

Instead, as my dog leaped into the back porch after his brief pre-dawn potty break in the yard, I thought it was just some new form of particularly odious flatulence.  This dog often has gas, and I had not yet had coffee.  However, as I bent down to wipe his feet and the odor increased sharply in its intensity, I realized that my dog wasn't directly responsible for the stench.  He seemed unconcerned, but he loves stinky odors, and the spray had missed his face.

There had been strong sulfurous fumes in the back yard for the previous week.  It was almost March, and the skunks were stirring, looking for food and for love after winter.  They are not shy about wandering around outside our house, although because they are normally strictly nocturnal we almost never see them. Skunks will spray predators that threaten them, but a female skunk will also spray persistent males bent on mating if she's really just not in the mood.  Skunks aim quite well, so I suspected that my dog had interrupted an unhappy love affair and been splattered by the consequences.

Striped skunk, Mephitis mephitis. Photo: NPS

Anal glands are used by many species of mammal as the creators of scent calling cards; when your dog sniffs around another dog's feces, she's most likely focused on the droplets of scent secretion that are released during defecation.  Possums also use anal gland secretion as a method of self-defense, but only skunks have the ability to take aim and fire.  Skunks' glands are quite large, over an inch or three centimeters long, and each of the pair is equipped with a nipple-like structure called a papilla that acts as a movable nozzle.  The glands contract with enough force to send the oily ooze flying ten feet or 3 meters distance. 

Skunks are born blind and helpless, but their glands contain musk by the end of their first week, and they are capable of spraying at the age of 17 days, more than two weeks before their eyes open for the first time.  Researchers found that young skunks were more likely than adults to spray than to run, hide, or charge.  I found no reliable reports of how long it takes for a skunk to refill its glands after a full discharge.  Presumably it takes some energy and time, because skunks give plenty of warning before firing, and don't seem willing to use all of their supply in a single encounter unless they must.

 

 Striped skunks.  Image from http://wdfw.wa.gov/living/skunks.html

Elements of the chemical composition of the spray were described as early as 1862 by German chemists, and a fellow German named Dr. O. Low did his best to continue the inquiry during an expedition to Texas in 1872.  Low commented on the abundant opportunity to collect skunks and their musk during his travels, but his companions objected so vigorously to the ensuing odor that he was forced to abandon his efforts.  He was further stymied from studying his few skimpy samples by colleagues and students in the college at which he worked "when the whole college rose in revolt, shouting 'A skunk, a skunk is here!' I had to abandon the investigation." 

 A more thorough treatment of the chemical elements of skunk spray was published in 1896 by Thomas Aldrich of Johns Hopkins University, who managed to obtain the glands of a number of skunks collected in Maine. “I have been more fortunate than my predecessors in being surrounded by those who, for the cause of science, would endure even the odor of a skunk in close proximity,” Aldrich wrote. 

 Aldrich found that the fluid’s vapors were highly flammable and gave off sulfur dioxide when burned.  He also discovered that even humans can sense the presence of these compounds at concentrations of ten parts per billion. This is equivalent to tasting a pinch of salt distributed across one ton of potato chips.  

 

Spotted skunk, Spirogale gracilis 

Image from http://wdfw.wa.gov/living/skunks.html

The composition of the musk is astonishing.  Over 150 compounds containing sulfur have been isolated.  Even more interestingly, each species of skunk has its own chemical signature. The striped skunk’s musk contains seven major volatile compounds, including three thiols, three thioacetate derivatives, and an alkaloid compound, 2-methylquinoline, which is also used in pharmaceuticals including anti-malarial drugs.

 The recurring skunky smell of a previously sprayed dog who seemed odor-free until the dry fur is dampened results from the reaction between water and one of the thioacetate derivatives remaining in the fur, creating one of the more volatile and smelly thiol compounds.  I found nothing in the literature regarding that first stench of burning rubber, but the odor had morphed into the sickening, familiar sulfur smell by breakfast time.  The highly volatile chemicals responsible for the first overwhelming impression must have already begun breaking down in the presence of water and oxygen in the air.


To get rid of the odor, the thiols need to be converted to other, less objectionable chemicals.  Exposure to oxygen will lead to their conversion to sulfonic acids.  Tomato juice or other acids do not accomplish this critical piece of chemistry, although a properly prepared solution of hydrogen peroxide will do the trick. Regarding the well-established folklore regarding tomato juice, the authors of the book Land Mammals of Oregon commented, "Don’t waste good tomato juice- add a little vodka and drink it; it won’t reduce the odor, but the odor won’t bother you so much!”


I didn't have any tomato juice or vodka on hand, only a bottle of doggy shampoo.  I went to work at once, and by dawn, the dog was still stinking with sulfur, but now had undertones of green tea and lavender.  At least he was clean.  Fortunately, olfactory fatigue had set in, and we stopped noticing the stench unless we left the house for a while.

 I'm not sure my dog has learned his lesson. I'll pay more attention to the atmosphere of early morning next year, making sure that no bad love is in the air before flinging open the door.  Or at least, I'll make coffee first.

References

Aldrich, Thomas B. 1896. A chemical study of the secretion of the anal glands of Mephitis mephitica (common skunk), with remarks on the physiological properties of this secretion.  Journal of Experimental Medicine 1:323-340.

Anderson, K. K., D. T. Bernstein, R. L. Caret, and L. J. Romanczyk, Jr. 1982. Chemical constituents of the defensive secretion of the striped skunk (Mephitis mephitis). Tetrahedron 38:1965-1970.

Medill, S.A., A. Renard, and S. Larivie. 2001. Onogeny of antipredator behavior in striped skunks, Mephitis mephitis. Ethology, Ecology & Evolution 23(1):41-48.

Verts, B. J., and L. N. Carraway. 1998. Land Mammals of Oregon. University of California Press, Berkeley, California.

Wood, W. F. 1989.  New components in defensive secretion of the striped skunk, Mephitis mephitis. Journal of Chemical Ecology 16:2057-2065.

Wood, W. F. 1999. The history of skunk defensive secretion research. Chemical educator 4:44-50.

Wood, W. F., B. G. Sollers, G. A. Dragoo, and J. W. Dragoo. 2002.   Volatile components in defensive spray of the hooded skunk, Mephitis macroura.  Journal of Chemical Ecology 28:1865-1870.

Possum Soup

The racket had gone on for quite some time, and it was getting hard to ignore.  I was up in my attic room, studying during the days of coursework for my master's degree in wildlife science.  The rattle of pots had gone from a brief shimmy of sound that you might think you really hadn't heard to a steady clatter.  I had a hypothesis.  A few days before, a rat had come in from the dairy barns, and chewed up a wooden spoon I'd used to make jam.  A repeat visit appeared to be underway.  I wasn't interested in facing down a rat in the cupboard, but the call to some kind of confrontation became unavoidable.  Reluctantly, I headed down the stairs.

I'm often told by people who want to become wildlife biologists that they are motivated by their love of animals and of being outside.  I try not to tell them that most biologist spend an obscene amount of time tied to a computer keyboard, because a few of us manage to escape that fate.  Who am I to accidentally discourage the next extraordinary naturalist-ecologist? For myself, however, some of my best wildlife encounters have occurred when I've been off-duty, although admittedly nearly always when I'm away from my desk.

That evening, I left my makeshift desk and climbed down the attic stairs.  Grabbing the broom as some means of self-defense, I flipped on the light switch and cautiously opened the cupboard door.  A sharp face decorated with glistening button eyes and spiky hair gazed back at me.  Not a rat.  A young possum sat in a saucepan, its tail arching up into a question mark, punctuating perfectly the look of bemused confusion on its face.

Much better than a rat, but if I left the possum there, I would get none of my work done, and I doubted that staying in the cupboard was what the possum had in mind for the evening's agenda.  The saucepan handle was angled toward me.  I cautiously extended a hand toward it, thinking the easiest solution was simply to carry the pan to the door and unceremoniously dump the possum into the bushes off the porch.

The possum froze for a moment, and hissed.  The hiss was almost exploratory, as if the possum had not yet had to do this before and wasn't quite sure of the etiquette of self-defense.  Despite its uncertainty, the impressive teeth were on full display.  I decided that this was not the time for experiential education, and withdrew my hand.

The possum then seemed to realize that it was in a compromising position.  It scrambled out of the saucepan and retreated to the back corner of the shelf, still facing me.  I thought about trying to prod it out with the broom, but playing broom hockey in the tight quarters of the kitchen with an upset possum wasn’t the resolution I was looking for.  Then as I squatted, staring into the dark recess of the cabinet and wondering what to do, I could have sworn the possum was shrinking.

After a moment or two, there was no doubt about it- there was much less possum left on the top shelf.  The shelf had been poorly cut, leaving a large gap in the corner chosen.  The possum's backside had started to descend through the gap, the back legs and tail hanging down into the lower shelf space, the front legs and paws now pinned to the side of its head as it sank still further.

I grabbed the largest kettle, and positioned it under the possum's dangling backside.  I coaxed the rest of the animal through by tapping gently on its head with the broom, and finally gravity brought the possum down into the kettle with an audible plop.  Moving quickly, before the possum could regain its composure, I slipped the lid onto the kettle and carried it outside, the possum scrabbling impotently against its enameled prison.  The possum's vague uncertainty about the whole misadventure prompted me to simply tip the kettle on its side rather than flinging the contents out into the blackberry thicket.  I stood and watched as the possum ambled out, blinked a few times, and then unhurriedly slipped off the edge of the deck and disappeared into the darkness.  Equilibrium was restored.

I wished the young possum well, and returned to my attic and the theoretical study of wildlife.  My housemate later admitted he'd left the door open all morning, and had even heard a faint scrabbling that must have been the possum settling into the cabinet to get out of the unwelcome early daylight. The noise then stopped, so he did not bother to investigate.  He laughed when I told him of our visitor, and I've never had such a good look at a live possum since.

I'd much rather encounter the natural world on its terms rather than my own, and watch what unfolds without the need to capture, tag, measure, or otherwise harass whatever it is I'm studying.  We can learn a lot from ecological research when we design it properly and all goes well.  It is an important kind of information.  But I'm convinced that we can learn just as much by being more attentive to the everyday interactions we have with the lives of all of those around us, no educational degrees required.