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.

Backyard Boa

Billie appeared at my doorstep in the heat of the afternoon, her hands cupped firmly together and held against her body.  "I've got a boa," she said.  "It came out of a weedy area I was mowing, and I don't want it to get into the road.  Can we release it in your rock wall?"

By all means.  The elusive rubber boa, Charina bottae, is one of those common and utterly overlooked creatures that echos greater things.  It is the small and nearly invisible cousin of the impressive anacondas, pythons, and boa constrictors, massive snakes large enough to eat small deer and peccaries.  The boa family line has some unusual characteristics among snakes.  The members carry two lungs, rather than the single lung considered adequate by most snake species, and boas retain their eggs in their bodies, giving birth to fully-formed young rather than laying the eggs like many other snakes do.  For the uninitiated, boas hunt by lunging out at unwary animals, encircling their prey in coils of their thick, smooth bodies, and strangling their dinner.

 

Rubber boa. Photo: J. A. Gervais

But the boa family is probably most noteworthy for another reason: they are snakes with legs.  They still retain the tiny nubs of what were once fully functional limbs.  Both sexes carry small spurs placed along their sides above the vent, used primarily by the males in courtship. 

The northernmost branch of the family ranges into North America in the form of two species, the rosy boa and the rubber boa, which extends up into British Columbia.  These are the poor cousins of the spectacularly patterned and impressively sized South American relatives.  The rosy boa boasts some handsome stripes but its northern cousin, the rubber boa, is notable for its plainness and the fact they occur in a wide variety of habitats although they are rarely noticed.  The body is olive or brown or pinkish, the belly lighter and possibly more shaded with yellow, and there are no lovely patterns of brown or black or green, no rings or stripes or spots or really anything of interest at all.  Their scales are small and very smooth and hardly offer any texture to break the monotony of their skin.  All in all, they are spectacularly uninteresting, other than the chance to see those relict legs that are now reduced to small spurs.

Except for two things: rubber boas have tiny eyes.  And they have thick, knobby tails that are as large as their heads.  With the very fine scales and tiny eyes, you might mistake one for a large fat worm.  The legs are pretty surprising, but they're easy to overlook.

Rubber boa head. Photo: J. A. Gervais

Rubber boas spend much of their time underground, haunting rodent burrows and cracks in the earth, sheltering under appropriate human junk.  They seem to do much of their hunting at night, and they are the scourge of the cozy nests of mice, shrews, and other small mammals.  The snake will eat all of the babies if it can, holding the mother at bay with that thick, knobbed tail.  One report also documents a snake consuming both a mother ensatina salamander and her clutch of eggs, which the salamanders brood until hatching.  These snakes are the real cradle-robbing monsters of nightmares.

 However, the boas themselves are often heavily scarred, and some voles and mice will counter-attack repeatedly to defend their offspring.  They may even kill the attacking snake if the snake is a small one.  Bigger boas will also eat birds if they can catch them, and smaller snakes feed on the eggs of other reptiles.  There are reports of big boas trying to eat smaller ones, although these incidents were seen in captivity.  There's a lot of tabloid-style drama going on out in the fields that we never notice.

Billie put out her hands and slowly opened them so I could see the boa.  It was olive in color, with a yellowy underside, about two feet long, and much more interested in hiding its head and presenting its tail to us than escaping.  The rubber boa is also one of the most docile of snakes, moving slowly and nearly impossible to provoke into biting.  They are far more likely to release musk from their vent to deter rough handling, and need to be harassed to do even that.

Down a mouse hole. Photo: J. A. Gervais

Rubber boas in the northwest breed in early spring.  The spurs are part of the courtship ritual, used primarily by the male snakes, whose spurs are more mobile.  Females' spurs are also more conical in shape, where the male snakes have spurs shaped like hooks.  Adult females are larger than males, with shorter tails, and more tail scarring, possibly because the demands of producing young require more hunting.  Females may not eat the entire summer that they are carrying their young.  Instead they seem to spend as much time as they can keeping their body temperature as high as possible to speed the babies' development and birth.  They will be born in August, not long before temperatures drop and all of the snakes must find sites to spend the winter.

Rubber boas are homebodies, frequently caught over and over in the same small area year after year.  They are also impressively long-lived, perhaps reaching a half-century even in the wild.  Their habit of rarely showing themselves in the open keeps them safely off the roads that take the lives of so many garter and gopher snakes, and helps ensure their reputation as the boa you never knew about living right in your backyard.

We looked the snake over closely but missed seeing the spurs, which I didn't know about at the time.  The snake might well have been a female, whose spurs are hardly larger than her scales and may not even be visible.  We released it in the grass not far from the half-buried line of concrete rubble that runs along the edge of our yard, the remains of an old barn foundation we had recycled into snake habitat.  There are too many mouse droppings in the bike shed.  I hope this snake sticks around.


References:

•  Dorcas, M. E., and C. E. Peterson. 1998. Daily temperature variation in free-ranging rubber boas. Herpetologica 54(1):88-103.
• Hoyer, R. F. 1974. Description of a rubber boa (Charina bottae) population from western Oregon. Herpetologica 30(3):275-283.
• Hoyer, R. F., and G. F. Stewart. 2000. Biology of the rubber boa (Charina bottae) with emphasis on C. b. umbratica. Part I: capture, size, sexual dimorphism, and reproduction. Journal of Herpetology 34(3):348-354.
• Hoyer, R. F., and G. R. Stewart. 2000.  Biology of the rubber boa (Charina bottae) with emphasis on C. b. umbratica. Part II: diet, antagonists, and predators. Journal of Herpetologica 34(3):354-360.
• Macey, R. M. 1983. Charina bottae food. Herpetological Review 14(1):19.
• Peabody, R. B., J. A. Johnson, and E. D. Brodie, Jr. 1975. Intraspecific escape from ingestion of the rubber boa, Charina bottae. Journal of Herpetology 9(2):237.
• Rodriguez-Robles, J. A., C. J. Bell, and H. W. Greene. 1999. Gape size and evolution of diet in snakes: feeding ecology of erycine boas. Journal of Zoology 248(1):49-58.

Grossbeak Convention

The male evening grossbeak may be one of the most brightly colored birds we overlook right in plain sight.  At least, I always think this in the last week of April and the first two weeks in May, when flocks composed of hundreds of individuals suddenly descend on the center of Oregon State University's campus and spill over into the tall trees in the historic downtown area.  To be fair, the evening grossbeaks stay high in the trees, and their green, black, and yellow plumage, which seems so bright, blends in astonishingly well with the new leaves against a startling sunny sky.  It is as if suddenly someone turned on the color after a long monochrome winter, and all that green and yellow is too overwhelming to pick apart into individual components. 

Photo: J. A. Gervais

Evening grossbeaks aren't wallflowers.  They are big, they are loud, and they are mobile.  But they do like the upper parts of the canopies of the tallest trees, making them somewhat difficult to see.  Binoculars help.  Watching the flocks of grossbeaks inevitably invites watching the crowds of people thronging the campus between classes, to see who else is paying attention, and if anyone else is craning their neck to see the treetop show.

It's hard to tell who actually notices the loud clear calls and buzzy whistles and the rain of empty seed pods wafting down from the heights, but isn't actually trying to look.  There are schedules, after all.  The grossbeaks keep them too, arriving on campus typically in mid-morning, having flown in from the hills northwest of town.  In the late afternoon, they filter back up into those hills, apparently refusing to spend the night in town.  I hear their morning commute while walking the dogs before I commute to town myself.  They fly over in tight flocks, urging each other on with hurry-up calls.  I have no idea if anyone is taking attendance.

I've asked people on campus if they'd like to borrow my binoculars on occasion, if they don't have their eyes fixed to the pavement while moving at warp speed, and if their ears are clear of electronic devices.  Most people have been shyly curious, and delighted when they finally get a glimpse of the handsome males.  These birds look like they belong on the tropics, not on campus with the drab sparrows and juncos that students might ordinarily glimpse on the way to class.  Suddenly there are brighter possibilities afoot, the campus a lot more interesting-  People start to notice the trees and the birds again, if only for a little while.  I take this as a hopeful sign.

Evening Grossbeak, Coccothraustes vespertinus.  Photo: USFWS

Grossbeaks are nomadic, known for their wanderings far out of their typical haunts if good seed years are followed by very bad ones, so that there are too many hungry grossbeaks and a mass exodus ensues.  The flocks around campus come every year with adamant regularity, however, and spend these few weeks frantically eating and courting.  When the speed dating session is over, they disperse; presumably, they're heading into the Coast Range and the Cascade Range to get down to the business of breeding.

How they all know to come through Corvallis, Oregon at this time of year to feed on the seeds of the elms around campus and around town is an open question.  Presumably the juveniles learn by following the older birds, and now there's some kind of tradition- "Let's meet on campus!" followed by, "See you next year!" 
Just as suddenly as they arrived, the grossbeaks leave, the banquet over. I'll still hear solitary, plaintive raspy contact calls in the woods above our farm for a few more weeks, before those too go silent for another year.  Probably the local little brown birds breathe a big sigh of relief to be rid of the big noisy showboats from out of town.

The menu.  Photo: J. A. Gervais

I hope the little brown birds continue to be so overwhelmed every spring, as many parts of the country are seeing declines in these gorgeous finches.  It isn't entirely clear what's going on, because evening grossbeaks get around and don't mind showing up in new places where they may be entirely unexpected.  Their stubborn predictability here is a treat.

Imagine

We were trying to herd the sheep into their small three-sided shelter one evening recently, so we could worm the ewes and trim hooves.  It always amazes me how good animals are at reading human body language; they often seem to know what we're up to before we even know ourselves.  Somehow, when the order of business switches from feeding them to moving them around, the sheep recognize it instantly, and they are not always on board with the agenda. They weren't on board this time, behaving as if we were driving them into a dragon's den.

Part of the problem is that sheep really don't like going from bright light into a dark space where they can't see well.  Individually, they enter this shelter on their own all the time, but being pushed as a group made a significant difference in their willingness to enter when we were pressuring them.  Time and again we almost got the lead ewe through the wooden gate, and at the last second she dodged aside, the younger ewes and the lambs quickly following her lead to the far end of the field.  This went on until the light softened with the setting sun, reducing the contrast of the dark interior of the shed.  The old ewe finally decided that it wasn't so frightening in there after all, and led the flock inside.

Photo: J. A. Gervais

Taking the risk of going forward isn't always easy, even if you're a human.

We're facing an unprecedented global environmental crisis because we've modified the atmospheric chemistry enough to begin raising global temperature.  The science behind this fact is well established; the devil is, as usual, in the details.  We don't know precisely at what point increasing carbon dioxide concentrations may begin to force positive feedback loops, whereby the rate of warming is increased still faster.  We do know that this could happen when the frozen methane hydrates in the high-latitude ocean sediments and in the tundra's permafrost begin to melt, releasing large volumes of methane.  Methane is a potent greenhouse gas.

We know that even the deep ocean is warming rapidly.  We don't understand how exactly that will alter ocean currents, or how quickly warmer water will begin releasing the frozen methane in marine sediments, or breaking the ice dams that have stabilized some of the world's largest ice sheets by slowing their entry into the seas. We don't know at what point melting ice sheets will reach a point of no return, where nothing we do could stop the enormous volume of glacial water from pouring into the oceans.  We know from the paleoclimate record that sea level has risen in the past as much as a meter in a quarter century.  We just don't quite understand the exact conditions that would be needed to trigger an event like that again.

But we can guess that we may be getting close.

These are enormous facts, dark facts, rooted in a past most of us cannot imagine and extending into a future that we cannot see, even though that future may be only a few decades away.  Facts fail to resonate with most of us, leaving them in the realm of simply facts, hard-edged, immutable, and seemingly irrelevant to the impulses and instincts that drive most of our behavior.  What does it take to make these inert facts part of our living consciousness?

I wonder if part of the problem is that we don't run forward into the utter unknown very well.  For the vast majority of our evolutionary history, not launching off into the abyss was probably very wise; you take those sorts of risks only when going back or staying where you are is absolutely untenable.  Pushed hard enough, the unknown ahead is less horrific than what you know is behind you, and then you jump.  The trick is recognizing when you are truly at the point of jumping or being pushed, that instant between having some mastery over your fate and losing any hope of control.

 Photo: Jeff Gervais

We may actually be at one of those tipping points, where we could manage to make our planet's climate very difficult for human civilization to persist, at least as we've known it for the last few centuries.  Are we able to perceive that we may have no choice but to accept the fact that the way of life we know now cannot continue, and we can choose, or not choose, to consciously guide its transformation?

Beyond any other species that has ever existed on Earth, we have the capacity to imagine future states and plan out the actions needed to achieve them.  I wonder if our apparent inertia in the face of an increasingly dire body of scientific evidence isn't rooted in two paradoxes.  The first is that although we are intelligent enough to create civilizations that ultimately threaten our own life-support systems, and even to realize it, we seem unable to emotionally grasp and process the enormity of the danger we collectively face.  Rational choice requires emotional roots.  Can an incipient catastrophe carry sufficient emotional weight, or do we have to live through it first?

The second paradox lies in the fact that despite our technological prowess and extraordinary cultural and social diversity as a species, we  appear unable to imagine any other world, any other set of human societies, than the ones we currently know.  So we are caught, unable to emotionally respond to the danger that our current position threatens, and unable to imagine a future different from the present, one that is worth risking the unknown to achieve.  Both block proactive planning and execution of those plans.

Imagine first the world you would like to live in, or the one you would like to bequeath to those who follow you.  What things and beings, tangible or intangible, would this world have in it?  If you listed them all out, how many of them deal with the roots of survival and of happiness, versus the trappings of our civilized lives that bring as much stress or ambivalence as they do real value?  What aspects of our society are most necessary to achieve those core needs?  If no one steps forward to lead us all, how can each of us fill that leadership void, in our own communities, in our own individual way?  How will we break the impasse, and will we manage to get in front of the wave of changes we've unleashed, or allow it to utterly overwhelm us?

Photo: Jeff Gervais

Of Love and Dancing

The albatrosses began to return in late October.  One day I looked out from our cook tent and there were perhaps a half-dozen of the huge white birds standing serenely on the sand, looking both grave and comical with the stark, sharp beauty of sea-gray wings folded crisply across their backs.  They waddled uncertainly over the dunes to just the right spot, there, that's the nest, right there.  A few days later, there were hundreds, and a week later, thousands.  Their season on the tiny speck of sand in the middle of the Pacific had begun.

 Laysan albatrosse pair on Laysan Island.  Photo: J. A. Gervais, 1992

You can find Laysan Island on most world maps, although it is only about a mile and a half long, a mile wide, and cradles a large super salty lagoon.  Essentially it is nothing more than a large sand dune perched on a remnant volcano.  There isn't much else out in this part of the world, a thousand miles northwest of Honolulu, and Laysan is the second largest of the Northwestern Hawaiian Islands.  For a few years over a century ago, it was inhabited, briefly, by humans, who mined the guano, harvested the albatrosses' eggs, and killed the birds for their feathers.  It has belonged to the birds for many thousands of years.

Land is a rare commodity in the middle of the ocean, and all seabirds are tied to it for breeding.  The birds of Laysan do a time-share, where different species come in from the far reaches of their wanderings and breed at different seasons.  Winter belongs to the bonin petrels, the black-footed albatrosses, and Laysan albatrosses.

Blackfooted albatrosses dancing, Laysan Island.  Photo: J. A. Gervais, 1992.

The albatrosses were mostly quiet at first.  It must be very strange, to land on unyielding ground for the first time in nine months, and the newest arrivals seemed to suffer from the same "sea leg" syndrome that people do.  They wobbled around and studied the clumps of bunchgrass and their neighbors in silence.  But as the few birds swelled to thousands, the singing and dancing began in earnest.

Albatrosses dance.  These are spectacular dances, involving wild bows and snapping beaks, with some individuals becoming so excited they gape and scream, whipping their heads back and forth.  They whinny and moo and clap that huge beak.  Sometimes they throw themselves up on tiptoe and point skyward, with a soulful moan at the apex.  The black-footed albatrosses have a different dance than the Laysan albatrosses, but both dances are exotic, energetic, and incredibly noisy.  Living on a colony numbering tens of thousands of pairs is like being in the middle of a demented barnyard.

Laysan albatross skypointing.  Photo: J. A. Gervais, 1992.

We quickly learned that young albatrosses were so anxious to get going with their adult lives that they would throw themselves into a frenzied performance if we just waved two fingers back and forth in front of them, mimicking the first moves of the dance.  When we failed to deliver the correct response partway into the performance, they would retreat hastily, looking flustered.

I tried coaxing a few of the pairs who settled into spots right around my tent into a cross-species tango.  These birds' mates had already arrived, and after a few passionate rounds of dancing it seemed that old ties were renewed well enough to get down to the business of breeding.  Waving fingers in front of these birds elicited a sidelong look.  The albatross would draw in its chin, that huge, hooked beak held down along its neck, and waddle emphatically away.  Sometimes interspecies communication is shatteringly clear.

Every now and then, a Laysan albatross and a black-footed albatross mate, and raise a chick.  The chick, however, is doomed to be completely unlucky in love, because its dance is stuck halfway between the species.  Nobody seems to want a partner who can't do all the right moves.  The vast majority of birds belong to one species or the other, however.  Nearly all find a partner and stay together for many years, renewing the relationship each autumn with the ritual of the dance.

Blackfooted albatross dance on Laysan Island.  Photo: J. A. Gervais, 1992.

I worked on Laysan for a magical four months, picking away at invasive grass that provided shelter for none of the birds, but crowded out the native bunchgrass that nearly all of them need.  Although the work was far from special, sneaking by sea turtles and monk seals, admiring the antics of boobies and albatrosses, and watching tropicbirds and frigatebirds engage in aerial warfare made even plodding over sand dunes carrying backpack sprayers full of herbicide the best job I ever had.

I left Laysan Island on a November morning nearly twenty years ago, and I still dream sometimes of the intense color of the sea, the enormity of the sky, and the noise of all those birds.  It was an early love, one you don't forget, even if you go on to fall in love with many other places.

Road Kill

There are two more dead deer lying along the road that ends just a few miles from town.  One is a doe, probably pregnant, the other a young buck just out of rut.  Someone has stopped and sawn off the antlers of the buck but both carcasses remain in the ditch, apparently inaccessible to scavengers.  They are hardly visible from a car, and possible to overlook even from a bicycle, as the cold weather has prevented the usual tell-tale smell.

We think of roads as opportunities for our own rapid movement and convenience, if we think about them at all, not the ribbons of death that wind through the home ranges or migration routes of many animals.  Stand at a rest stop along a busy interstate and see how many seconds pass in which there are breaks in traffic from one side of the highway across all lanes to the other side.  Not surprisingly, research has shown that roads can be major obstacles to animal movement.

How much roads cut off movement depends on not only the traffic load but the animals themselves.  Some animals such as urban gray squirrels seem almost oblivious to the traffic despite the risk and frequent near-misses.  There are video clips of urban wildlife using crossroads and even apparently waiting for lights to change before they cross.  They include deer, gray squirrels, coyotes, and Japanese crows.  The crows, of course, can easily fly over the traffic, but it seems that some of them have figured out that if they drop nuts into the crosswalks, cars will run over them, and the cracked nuts can be retrieved when the light changes.  These animals have adapted to live in the new world we've created for them.

But the vast majority haven't adapted.  Desert bighorn sheep populations are already showing reduced genetic diversity in just four decades after interstate highways threaded among them, raising the risks of extinction for the now-isolated populations.  Cougars avoided two-lane paved roads although dirt roads did not deter them.  Both wolves and elk in the Canadian Rockies avoided roads and trails in national parks as their traffic increased, but elk were less repelled and actually used the areas near moderately busy trails as predator-free zones because the wolves appeared more sensitive to the disturbance.  There are gradients in responses, and consequences.

At the other end of the spectrum, freshwater turtles don't seem to recognize the danger.  Turtles moving between two wetlands in Florida were willing to attempt to clamber over a barrier made of plastic netting to cross the busy highway separating the wetlands; nearly all those that succeeded were killed.  Populations of turtles near roads have a sex ratio skewed towards males relative to populations away from roads. More dead  female turtles are found on roads than males because the females are driven to leave the safety of water to find nesting sites.

The biggest issue may not be just the body counts, as staggering as they may be (upwards of 350 million wildlife deaths per year in the United States).  The worst thing about road kill may be what it reveals about our own fundamental thoughtlessness.  We are the one species that seems able to contemplate killing in an abstract way, evaluating the moral and ethical consequences of taking another's life.  The vast majority of us would not describe ourselves as careless killers, and would claim to avoid causing senseless death when possible.  Then we get into our cars.

The speed limit on the road with the dead deer is fifty miles an hour.  Every branch of this winding road ends a few miles farther into the hills, serving an exurban bedroom-community development.  People claim to like living in the country because they enjoy nature, but nature had better not get in the way of easy access to town.  Of course, very few wildlife-vehicle collisions occur on purpose; after all, people are often also victims of severe injuries or death when large animals like moose are involved.  But nearly all animals that are hit die.

When animals die of natural causes besides predation, they tend to die in places where scavengers can get to them, so that the occasion of death is also an occasion for the continuation of other life.  One of the elements of roadkill is its wastefulness, because the traffic may prevent scavengers from at least cleaning up after our carelessness.  Worse, other animals may be attracted to the bounty then also die in traffic.  This is only one aspect of the particularly repugnant facets of road kill.  Another facet is its anonymity, with neither killer or victim aware of the other.  It is the ultimate in thoughtless take.

The vast majority of victims are not ever seen before being hit, especially if they are low to the ground and cryptic, like snakes, lizards, or salamanders.  It is easy to ignore the carnage if you never see it, easy to believe that you don't contribute to it, easy not to think about it at all.  What if we did think about it?  What if seeing a dead animal, even a snake or a slug, required a moment of reflection and grief?  We would have to slow down enough to see the corpses.  If we slowed down, there would be far fewer of them.  That would be one benefit.  The process of acknowledging the losses might have a far greater benefit, that of helping us recognize our own place in the scheme of life, the first critical step to salvaging our planetary home.

Tale of Two Skinks

There are two edges to the sword of change that is slicing through so much of the biological skin of our planet.  The first is the staggering loss of biological diversity, adding up one of the great waves of extinction in Earth's history.  Along the trailing edge, ecosystems and the species that evolved in them are under increasing pressure from species new to the system.  We pay a lot of attention to the problem of extinction, but much less to invasion, even though it can and does contribute to the former.  It is perhaps a bit easier to rally to save a charismatic species such as a panda rather than organize to fight the diffuse threat posed by signal crayfish or purple loosestrife or kudzu.  However, scientists have started asking why some species become so destructively overabundant even though close relatives may be on an endangered list. 

Behavior alone won't dictate success.  Species that establish in new places also tend to be introduced over and over,  increasing the odds that eventually, a few individuals will survive the initial colonization event.  This in turn may be dictated by who lives where the most common transport routes start.  However, individual behavior seems to play a larger role than we expected.

Kudzu consuming a barn in North Carolina. Photo: NASA

If you want to successfully colonize a distant planet, you might consider taking a close look at how this works right here at home.  First, you've got to get yourself on some kind of transport vehicle, whether in the gut of another animal or the hold of a trading vessel or cargo plane headed elsewhere.  Realize you're probably not a welcome passenger, so you've got to be discreet.  You have to survive the journey, which means somehow finding or maintaining conditions in which you can live, with enough water, warmth and energy to avoid death.  Once you arrive, you need to sneak out of the way lest you be caught in the act and exterminated.  Very few stowaways make it this far, but the journey isn't over yet.  You need to find appropriate food, water, and shelter in these new, unknown surroundings.  Eventually, you need to reproduce successfully, which means you need to find, recognize, and successfully interact with a mate.  Next, your children must also raise children.  The resulting little community must avoid being found and eradicated, and finally, new colonists must leave and establish more communities before the invasion can be considered a success.

The odds, in short, are heavily stacked against you.  However, even if animals don't get help (in the form of deliberate introductions by people, such as starlings in North America, cane toads in Hawaii and Australia, or possums in New Zealand), some species still manage to pull it off.  We've just started to think about how animal behavior influences the risk of successful invasion.

Delicate skink. Photo: Peter Robinson, Museum Victoria, Australia

The delicate skink, Lampropholis delicata, and its cousin the garden skink (Lampropholis guichenoti), illustrate the point.  The delicate skink is native to eastern Australia, but has managed to colonize New Zealand, Hawaii, and Lord Howe Island.  The closely related garden skink, however, has stayed home even though it is quite similiar to its more adventurous cousin in many ways.  The two skinks are similar in size, have similar diets, and similar life histories.  Both species live together in urban areas in Australia, close to transport hubs such as major shipping ports and airports.  Both are common, and occur at high densities, suggesting no lack of individuals available for export.  What, then, is different?

Although both skinks like to explore new environments, the delicate skink was far more willing to move through a tube when it couldn't see the exit, enter a small black box in the test cage, and walk up a graveled ramp to reach a heat lamp suspended above the cage floor.  A greater willingness to explore, then hide, may explain a good part of why delicate skinks are called plague skinks while garden skinks have been at worst temporary tourists who never established outside their native range.  The opportunity to become a problem appears to be the same, but the behavior of the animals influences who takes advantage of that opportunity.

Garden skink.  Photo: Peter Robinson, Museum Victoria, Australia.

Not all delicate skinks successfully found the elevated basking site; animals, after all, are individuals.  Other research found that individual mosquitofish vary in their tendency to strike out for new horizons.  Interestingly, fish that chose to disperse also seemed less tolerant of other mosquito fish, and these personality traits were consistent in individuals over the study.  There may need to be a range of personalities and behavioral tendencies to support a successful invasion from start to establishment. 

We don't often think of the individuality of wild animals, or how much that might matter to the survival of a species.  If we were better at recognizing the individuality of non-domesticated animals, how might that change our view of them?



Sources
Chapple, D.G., S. M Simmonds, and B.B.M. Wong. 2012. Can behavioral and personality traits influence the success of unintentional species introductions? Trends in Ecology and Evolution 27:57-64.


Chapple, D.G., S. M. Simmonds, and B.B.M. Wong. 2011. Know when to run, know when to hide: can behavioral differences explain the divergent invasion success of two sympatric lizards? Ecology and Evolution 1:278-289.


Colautti, R.I., I.A. Grigorovich, and H.J. MacIsaac. 2006. Propagule pressure: a null model for biological invasions. Biological Invasions 8:1023-1037.


Cote, J., S. Fogarty, K. Weinersmith, T. Brodin, and A. Sih. 2010. Personality traits and dispersal tendency in the invasive mosquitofish (Gambusia affinis). Proceedings of the Royal Society B 277: 1571-1579.