Sunday, October 31, 2010

Feeling A Little Rattled?

(Halloween issue...)

Halloween ghosts and goblins are mighty frightening to children, but not so much to adults. On the other hand, research has shown that small children have no innate fear of snakes -- the one animal that scares the willies out of most adults. By the time we're too old to dress up at Halloween, there are few things more frightening to us than the sight of a rattlesnake.

How did sluggish snakes get so good at rattling our nerves? Apparently, like a Halloween trick, we learned to scare ourselves.

Montana's only venomous snake species "watches" with super-sensitive heat pits between eyes and nostrils.

Studies show that children and adults both have an innate tendency to give spiders and snakes "preferential attention" over other shapes that they see. We evolved the ability to detect them quickly in a distracting background. But studies also show that adults who don't fear snakes are just as good at spotting them -- they have the same genetic "preferential attention" behavior.

While growing up, we are told over and over that snakes are bad (e.g. "The Jungle Book"), even evil (e.g. the "Garden of Eden" tale). And our genetic "preferential attention" gets churned into a fear response that is a learned behavior, not a genetic hardwire. Other than scaring us silly from time to time, is our fear of snakes justified?

Rattlesnake skeleton mounted in strike position.
Explainable, probably. Justified?  Probably not.

Big snakes that ate medium and large animals evolved about 100 million years ago, and they were one of the first serious predators faced by primates. (Big, carnivorous cats arrived later on.)

One intriguing theory (supported by recent research on human brain structure) is that evolutionary pressure on monkeys, apes and humans living near snakes helped to evolve close-up vision for predator avoidance, not for reaching or grasping as commonly believed.

A snake is only dangerous at close range. As primate vision and snake avoidance improved, some snakes evolved venom (about 40 million years later) that improved their odds of prey capture and survival, and the arms race was on.

So far, snakes are very successful as a group, but not by preying on large animals. Half of all reptiles are snakes, and they are generally thriving by targeting the smaller rodents while trying to avoid the most dangerous animals -- us.

Prairie Rattlesnake
According to the National Safety Council, in the US the lifetime odds of dying from a venomous snake bite are 1:479,992. Put into some perspective, you are three times more likely to die from falling out of bed, 116 times more likely to die from riding a bicycle, or 5,647 times more likely to die from riding in a car. And yet we don't teach children to fear beds, bicycles or cars.

Snakes are common all across the US. On average, about 45,000 people are bitten by snakes each year, and about 7,000 of these involve venomous snakes. Twelve or fewer deaths from snake bites were reported each year between 1960 and 1990. Rattlesnakes accounted for about half the poisonous snake bites and 70% of deaths.

Here in Montana, our only venomous snake is also our only snake with a rattle, the prairie rattlesnake (Crotalis viridis). They prefer our open, rocky country but can be found pretty much everywhere except in the northwestern mountains. The Deaconess Hospital in Bozeman, which is surrounded by rattlesnake habitat, averages less than one snake bite victim per year. 

Rattlesnakes are survivors. Being cold-blooded, their metabolic needs are low to begin with. They make efficient use of the occasional meal, and they can survive starvation by lowering their metabolic rates by up to 72% -- and continue to grow longer at the same time. Our adults here in Montana grow to 15-60" long, with males slightly larger than females. Reproductive success in females is correlated to the amount of fat stores in her body.

Rattlesnakes mate in summer, and give birth late in the following summer. Females will only reproduce every 2-3 years. They give birth to 4-25 live young that are between 7-13" long. This was displayed rather gruesomely in 1967 on Helena's one-hour live TV show, "Today in Montana." The interview featured a local rancher who brought in a live rattlesnake, stood on her head while slitting her belly, and spilled 13 live baby rattlers out onto the studio floor. Both hate mail and fan mail ensued.

When not being persecuted, pregnant female rattlesnakes often bask together on sheltered rock sites called "rookeries." They aestivate during hot spells and hibernate during cold months. Adults can range up to seven miles from their winter "hibernaculums," which can shelter as many as 150 adults plus juveniles and young of the year.

Hollow fangs fold backwards when not in use, and
they are shed about every 28 days (3 present here).

The adults are thought to reach breeding age/size when males have four and females have five tail rattles. They add one additional rattle each time they shed their skin, and they may shed 1-4 times per year, depending on food availability. Mice and rats make up the bulk of rattlesnake diets (about 90%), but they also eat ground squirrels, rabbits, birds and other small animals.

Rattlesnakes belong to a group called "pit vipers" that all have a pair of specialized organs located between their eyes and nostrils (visible in top photo). The heat pit is a deep pocket with a super-sensitive membrane stretched across it. It helps in two ways. The pit organ helps snakes find cool refuges during hot spells, and it helps them locate and identify warm-blooded prey items. It is sensitive enough to detect temperature changes of less than one-half of one degree Fahrenheit (0.6C).

In the snake's brain, a thermal image from the pit organ is overlaid on the eyes' visual image. Rattlesnakes use this thermal imagry to seek out cool, moist ambush sites to maximize the contrast between a warm meal and a background with low thermal radiation.

Unfortunately, seeking out these locations can sometimes bring them into contact with people, along with our heightened sense of snake awareness and disproportionate fear. The best thing for snakes is also the best thing for us -- mutual avoidance. We should teach children to avoid snakes, not fear them. Don't harass snakes, but let them continue to benefit us by helping control those really scary rats and mice (does Bubonic Plague still ring any alarms?).

Also, learn how to respond to a snake bite before you actually need that information. The National Institutes of Health goes over the do's and don'ts of emergency treatment. Read it before you need it at

DO keep the victim calm, restrict their movements, and keep the bite area below heart level. Remove any rings or restrictive clothing (area may swell), and head for the nearest emergency room ASAP.

DO NOT allow the victim to exert themselves, apply tourniquet or cold compress. Do not give pain meds or anything by mouth. Don't "cut and suck" (only removes 6% of venom at best) or raise bite wound above heart level.

Venomous snake bites are rarely fatal to adults, but they can cause painful tissue damage to children. Fortunately, full recovery is likely with prompt medical attention. One final thought -- the race to the emergency room is probably not a good time to tell the victim that riding in a car is 5,647 times more dangerous than his snake bite.

From the snake pit: I must confess that, although I enjoy watching snakes, the young prairie rattlesnake photographed here took me by surprise. I was sitting on a slab of exposed rocks and photographing animals a long ways away with a big lens. When I spotted this fellow watching me -- on the same rock, two feet from my right thigh -- I immediately levitated, camera gear and all. Of course, then I had to return with a close-up lens. I may not be my mother's smartest son, but I do have more hair left than my elder brothers, and some of it was standing on end.

Also, I can't let this one go without mentioning one of the odddest movies of all time. "Stanley"  (1972)  was about a Vietnam veteran's, uh, " relationship" with his namesake pet rattlesnake. I don't remember why or how I came to see this movie, back in the 70's, but when I described it to a friend years later, she accused me of lying. It's real, and you can watch the trailer here.