Ah, spring! Finally, we’ve arrived at my favorite month of the year. Hooray for the spring wildflowers! We’ve got warm weather and the early migrant songbirds. Nature is wakening up to begin the annual cycle of seasonal growth.
April also signals the emergence of our hibernating reptiles. This includes the timber rattlesnake and copperhead, our central Appalachian members of the pit viper subfamily (Crotalinae). Rattlesnakes are a truly American family of pit vipers, not found in the Old World, and all but two species found in the U.S. or Mexico. Pit vipers (145 species worldwide, 17 in US) include rattlesnakes, copperheads, cottonmouths, tropical America’s Fer de lance and Latin America’s bushmaster, the world’s largest pit viper, reaching lengths greater than 10 feet.
Our two Appalachian pit vipers are the only snake species that routinely den communally over the winter. Not only do these two species den together, they are occasionally joined by black rat snakes. They tend to leave the dens over a 2-3 week period in late April/early May in the Shenandoah National Park, depending on weather conditions. With warm spells lasting only 3-4 days, a second wave may be weeks later – up to six weeks after the first wave. The emergence is directly related to degree-days of the season, proceeding upslope at an average rate of 120’/day and 15 miles north/day.
Pit vipers, the most advanced of snakes, have a host of elegant adaptive features that separate them from more primitive snake species. They possess heat-sensing pits between the nose and eyes, perform elaborate mating challenges, practice delayed fertilization, and, of course, produces a highly toxic venom including various specialized toxic proteins.
The pit organs (Jacobson’s organ) serve as very sensitive heat detectors that detect both prey and predators. Believed to be used originally for defense, this organ has become an offensive tool for detecting prey (similar to the function of feathers, which initially served the role of insulation before becoming the tool for flight). Many vertebrates have a Jacobson’s organ. In the case of the lip-curling (flehmen) displayed by deer and elk during mating season, it is an effort to expose the Jacobson’s organ for optimal pheromone reception. Even humans have a Jacobson’s organ. It’s purpose in humans is debated; some consider it vestigial, serving no purpose; others believe it is a functional pheromone receptor, playing a role in sexual attraction and mate selection.
I have never seen two male timber rattlesnakes vie for a mate. They will rear up, intertwine, and seek to topple the opponent. The winner takes the female while the loser apparently becomes mentally demoralized for days. After losing such a competitive challenge, even far smaller males, which normally would be loath to pick a fight, will take on the loser and defeat it. It’s also been found that females may mimic a male, rearing up to an approaching male, and if the male backs off, the female will reject its mating overtures.
I have seen rattlesnakes mating. The male will rub his head along the female’s back and intertwine his body with hers, causing the release of chemical pheromones that stimulate both sexes to mate. For reasons not clearly defined, males have two copulatory organs, called a hemipenes (all snakes and lizards have two joined penises), ostensibly to ensure the job gets done.
Females have their own idiosyncrasies. Mating in the summer, they will hold sperm in their reproductive tract throughout the winter hibernation (like bats) and will not fertilize their eggs until the following spring, enabling a late summer birthing.
Pit viper venom is a mixture of neurotoxins, hemotoxins and other proteins; each causing its own toxic effect. The venom not only kills the prey, it starts the decomposition of the body, in essence, pre-digesting the prey. Since it may take days for the prey to be totally digested by the snake, the venom prevents the prey from putrefying within the snakes’ belly.
Untreated rattlesnake bites, especially from larger species, can be fatal. However, antivenom, when applied in time, reduces the death rate to less than 4%. It is estimated that between 7,000 and 8,000 people are bitten by venomous snakes in the United States each year, and about 5 – 10 of those die. By contrast, almost 30,000 people die every year in India from cobra and krait bites. In WV, between 1968 and 1992, 36 died of injuries caused by horses and cows, 26 died of bee stings, 5 died from insect and spider bites, while only 4 died of snakebites.
Pit vipers can control the amount of venom released in a bite. As often as 25% of the time, no venom is discharged; known as a “dry bite”. This is done when the snake is warning the recipient that it’s there and should be left alone – such as a deer in the field. On the other hand, when a snake makes a defensive strike against a predator, it will significantly increase the amount of venom used compared to predatory strikes of it’s own used to capture prey. Drop for drop, venom from newborns is actually more potent than adults; however their venom glands are smaller, thus less transmitted per bite.
There are two noteable exceptions to the toxicity of pit viper venom. The common Kingsnake is immune to the venom of rattlesnakes and other vipers. Not surprisingly therefore, rattlesnakes make up part of the Kingsnake’s diet (also the reason for the common name!).
The marsupial opossum is also immune to the venom. Opossum have been injected with sixty times the lethal dose of venom necessary to quell most mammals with the only affect being a slight change in blood pressure for a few hours. Yet, the venom of cobras can easily kill an opossum, who evolved in the absence of cobras.
(The following statistics pertain to the rattlesnake population in our central Appalachian region and come from papers written by William H. Martin based on 37 years of data collection.)
2010 had a record early emergence, early shedding, early mating and record early birthing. Emergence was about 2 weeks ahead of the average in the central Appalachians peaking in mid-April.
Den (hibernacula) populations average 30-60 snakes; large hibernacula contain 100-150 (10% of dens). Populations can have as many as 200 timber rattlesnakes (TRs).
TRs can live to 25 years of age (or longer).
TRs can be 5’ in length and weigh 5 lbs. Average length is 34-40” for females; 39-45” for males. (Eastern diamondbacks, which occur in Florida and lower Coastal Plain of the Deep South have been recorded over 7’ and 15 lbs.)
Typically, TRs come in two colors: some are almost solid black; whereas others are bright yellow, marked with splotches of brown. However, they can range from pale gray, buff, brownish gray, olive, dusky or chestnut.
Rattles represent shedding of skin, which happen once or twice a year (twice more often when young), but frequently break off over time.
TRs reach the peak of emergence 30 April-1 May. After a winter hibernating with body temperatures approaching 40 degrees, the snake’s body slowly begins to warm as it gradually makes its way to the surface in late-April.
Mating occurs from late July through early September.
Birthing peaks Aug 28- Sept 3. Rookery sites are used by most female members of a den, located within 500 yards from the denning site. Newborn (neonates) follow scents to denning site. After live birth, at these communal birthing rookeries, mothers remain to protect neonates for a week or more until they shed their natal skin and disperse. Occasionally, neonates follow scent of black rat snakes that may also den with rattlesnakes, thus the common name of “pilot snake” given to black rat snakes.
A litter contains seven to ten neonates, 9-12 inches in length on average.
TR northern (or high elevation) range is limited by short growing seasons, which preclude the gravid (pregnant) females from giving birth until after re-entering the hibernaculum in the fall. Successful neonates need to shed once (8-10 days after birth) and feed once prior to entering.
Fall entering the overwintering den occurs from late September to mid October.
Females are 7 to 8 years old before they are sexually mature, but 9 to 10 in New York, although they are physiologically the same age; in fact, they’ve molted the same number of times (shorter growing season). Females typically give birth every third year, but ranging from 2-5 years while for some northern and high elevation populations, taking longer to store adequate fat in their bodies, the typical birthing interval may be 4 or 5 years years.
Fangs are replaced three or four times a year. Next fangs can be seen in mouth.
TR population summer range typically stays within 2.5 miles of the denning site but individuals are known to have ranged out 4-5 miles.
The furthest eastern TR population in our region is on Sugarloaf Mountain near Frederick, MD, followed by Bull Run Mountain in northern Virginia.
The most common snakes in SNP are ring-necked snakes, followed by garter, black rat, copperheads, and rattlesnakes. Surprisingly, based on snakes seen on roads after dark, there about are ten times more copperheads than rattlers in SNP, however, they’re less often seen since they tend to hide under leaves.
Cottonmouths (water moccasins) are a southern species, found as far north as the Dismal Swamp, VA, and Hampton Roads with a few small populations found along the James River below the Fall Line in Richmond.
A southern geographic variant of the Timber, known as the Canebrake Rattlesnake, occurs from southeastern Virginia southward. (Recent DNA work does not support considering these snakes subspecies.)