Friday, August 20, 2010

Save that skin! Snake sheds can be useful for ID

The southern copperhead snake, Agkistrodon controtrix, with its
banded pattern blends in well with the forest floor.  Note the
triangular-shaped head.
A client came into our office last week with what was for me an unusual sample.  Snake skins (or sheds, as they are called) are not commonly brought to entomologists, though I suspect that most pest control businesses see snake samples regularly enough.  The shed in this case was discovered under a sofa in a home, and the alarmed homeowner wanted to know (reasonably enough) if it was from a poisonous snake.

Now it might not immediately occur to most people to worry about worry about what species of snake they have in their home.  A snake skin mysteriously appearing in the living room would be cause enough for total panic for most people, regardless of species.  But wanting to know whether a shed is from a venomous snake or not is a sensible request.  Shed skins can provide excellent clues to the identity of a snake and, if nothing else, can comfort a customer by relieving them of the worry that a fanged, potentially-lethal reptile might be lurking in the laundry basket.

To recognize venomous snakes, it's first important to know which snakes are venomous.  There are dozens of species of venomous snakes in the U.S., but it's relatively easy to recognize the three major genera and four major kinds of venomous snakes.  If you can recognize these snakes, you can recognize any of the bad players you're likely to encounter in this country.
  • Coral snakes.  The only North American members of the infamous snake family Elapidae, which includes the cobras, mambas, kraits and tiger snakes (found only in the Old World).  Coral snakes are relatively secretive and less likely to be encountered around residential areas.  They are strongly ringed with red, yellow and black; although there are several more common snake species that share this color characteristic.  What distinguishes coral snakes is that the two warning colors, red and yellow, touch each other.  Other, non-venomous mimics (like milk snakes and king snakes), always have a black band separating the yellow and red bands.  Hence the old Boy Scout rhyme really works: "Red on yellow, kill a fellow; red on black, friend of Jack". 
  • Copperheads.  Copperheads, water mocassins and rattlesnakes all belong to the family Viperidae, subfamily Crotalinae.  All pit vipers have triangular-shaped heads that are wider than the neck, and specialized facial pits between the nostril and eye that are used for sensing the body heat of prey.  Copperheads are brownish in color with bell-shaped markings on the sides (see image) and are difficult to see amidst dead leaves on a forest floor.  Copperheads are one of the more commonly encountered snakes around homes near wooded areas in the eastern half of the U.S. 
  • Cottonmouths (water moccasins).  Cottonmouth snakes belong to the same genus as copperhead snakes (Agkistrodon), but are associated with aquatic habitats.  One of the most feared venomous snakes, they are often confused with harmless water snakes.  Their pit viper characteristics, often aggressive behavior and white mouths help distinguish them from water and other dark-colored snakes. These snakes are less likely to be encountered by PMPs, unless the account is close to water and in the southeastern quarter of the U.S.
  • Rattlesnakes.  Two genera of snakes are classified as rattlesnakes, so-named because of a series of loose, horny segments born at the tip of the tail that make a rattling sound when vibrated.  Various species of these snakes are found throughout the contiguous U.S. These may be common around homes, schools and businesses in certain areas, especially in the more arid parts of the western U.S.
One way to distinguish most venomous from non-venomous snake sheds is to examine the scale pattern on the underside of the tail section.  Most snakes with double rows of scales from the anus to the tip of the tail can be assumed to be non-venomous (except for coral snakes).  Such scales should be visible on a complete shed snake skin.  Illustration taken from Roger Conant, A Field Guide to the Reptiles and Amphibians of Eastern/Central North America.
All snakes, both venomous and non-venomous, shed their skins several times a year.  Snake sheds may remain complete and intact, or may frequently come off in sections.  Sheds may show up in a yard, under a home, or even occasionally indoors.  If a customer calls to report finding a snake skin, ask them to keep it until you can visit.  Nathan Haislip of the Fort Worth Zoo's Department of Ectotherms (a fancy term for insects, reptiles, and amphibians) was kind enough to share his advice on how to recognize whether a shed is from a venomous or non-venomous snake. He suggests:
  • If the head section is present and intact, look to see if it's arrow-shaped (triangular) or if you can detect a pit located just below an imaginary line between the eye and nostril.  These two characteristics identify a snake as a pit viper, the most common family of venomous snakes in the U.S. The coral snake is the only other venomous snake in North America without these characteristics.  
  • If the head is missing or damaged, another character to look for is the pattern of scales on the bottom of the tail, behind the anus or ventor.  If the scales form more than one row, the skin is not from a rattlesnake, copperhead, or cottonmouth. 
  • If there is a tip to the tail of your snake skin, then you definitely know you aren't dealing with a rattlesnake because the tip is the rattle and the shed doesn't continue past the rattle. 
  • Looking for color patterns left behind on the skin can also be helpful.  Copperhead sheds, for example, may show the characteristic banding patterns of that species.  Diamondback rattlesnakes should have  the distinctive diamond pattern visible, depending on the quality of the shed. 
  • If you have access to a local herpetologist or zoo with a reptile collection, you may be able to get more precise on the identification from a skin alone.  Put the shed into a protective box and save it for an expert to evaluate. 
 All this illustrates why pest control is such an interesting profession.  You never know what kind of questions, or samples, you will encounter on a given day.

1 comment:

Mike Merchant, PhD said...

My apologies about a mistake that occurred in this story in its first posting. Turns out that coral snakes (true to their nature of being different from other U.S. venomous snakes) have a double row of scales on the underside of the tail. So if coral snake is a possibility for you in your area, you cannot rule that snake out when examining a snake shed with a double scale row as shown in the picture above.