Blog Carnival: Ecology of Snake Sheds - Reptilenesia

I've already written a series of posts about identifying snake sheds, which is definitely the most common question people ask about them (those three posts make up over a third of all the traffic on this site). People ask other questions about snake sheds much more rarely. In fact, I never stopped to ask some basic questions myself. What are snake sheds made of? What are they used for, and by whom? Where are they found? Do they make substantial contributions to ecology? You might think that because snake sheds are so insubstantial that they don't have much of an impact, but several facts about snakes lead us to believe otherwise.

Part I: Contributions to nutrient cycling

Snakes can occur at high densities, although their population density can be difficult to measure because snakes are so hard to find. Some estimates provided by snake population ecologist JD Willson in his dissertation included 4-14 vipers per hectare in Scandinavia, 275 vipers per hectare on Shedao Island in China, and over 1000 ring-necked snakes per hectare in Kansas. Aquatic snakes in Ellenton Bay, South Carolina, where I did my undergraduate field research, can reach densities of  170 snakes/ha. I did a couple of back-of-the-envelope calculations using these estimates, plus those for snake shed frequency and shed energetic content, and found that all snakes shedding across the entire continental United States probably generate close to 1.6 billion pounds of shed skin each year, which contain about 3.6 trillion calories of energy. That's enough for everyone in Alabama to survive eating nothing but snake sheds every day all year long (ma, not this for dinner again!), if they could somehow collect all the snake sheds from the entire country. So it isn't an unimaginably immense amount of energy, but it's not insubstantial either. Given the results of this rather bizarre thought exercise, I think it's safe to say that shed snake skin contributes substantially to nutrient cycling in areas where snakes frequently shed.

What exactly do I mean by nutrient cycling? Think of it as nature's ultimate recycling. It's one example of the services that ecosystems provide for free, and it's why you have regular access to clean water to drink, air to breathe, food to eat, and other essentials, without having to manufacture or engineer systems to produce these things. The cycles of carbon, nitrogen, sulfur, and other elements in and out of the water, air, soil, and the bodies of plants, animals, and microbes, are critical to maintaining a healthy ecosystem. Perturbations can lead to serious imbalances, like the changes to the global carbon cycle that result from the burning of fossils fuels. Few people have investigated the roles that amphibians and reptiles play specifically in nutrient cycling, but we are beginning to suspect that they are important components of many ecosystems. They may be small, but there are a lot of them. For instance, redback salamanders in forests in the northeastern US outnumber all other terrestrial vertebrates combined. On some tropical islands, lizards occur at densities of over 67,000 per hectare. Snakes can occur at really high densities as well, partly because they are so efficient at converting food into biomass as a result of being ectothermic (cold-blooded) and partly because feeding as infrequently as they do reduces the effects of competition with other snakes. By one estimate, snakes are 25 times more efficient at turning food into biomass than carnivorous mammals of equal size, and occur at population densities 20 – 1400 times greater, meaning that they probably contribute disproportionately to nutrient cycling. Explicit investigation of this phenomenon is underway in turtles, which have large bony shells that probably contribute to cycling of calcium and phosphorus, but to my knowledge no one has so far studied this in any snake, let alone for shed snakeskin.

A red-tailed green ratsnake (Gonyosoma oxycephalum) sheds its skin

In the wild, shed snake skins disintegrate in about a week, although if you collect one and put it in a plastic bag, they can last decades. The chemical composition of snake sheds is poorly known, but they contain some keratin and some lipids, among other things. Some fungi feed on keratin, including those that cause athlete's foot and ringworm as well as the chytrid fungus that has caused amphibian declines worldwide (with disastrous consequences for the snakes that specialize on them), but these species mostly grow on living organisms. Although we don't know for sure, it seems likely that numerous fungi and microbes have probably evolved to take advantage of the abundant energy found in snake sheds. Of course, the dead bodies of the snakes themselves also eventually contribute to nutrient cycling, but depending on the source of mortality, many of those are probably eaten by predators, and fewer probably decompose compared with snake sheds.

Part II: Use by other animals

An Eastern Indigo Snake (Drymarchon couperi) getting ready to shed

Snakes shed their skin in order to grow bigger. You do this too, just not all in one piece. Once a snake sheds its skin, it's typically done with it. However, both snakes and you might be surprised to learn that snake sheds are frequently used by other animals for a variety of purposes. As I mentioned previously in my article on conservation successes with Eastern Indigo Snakes, snake sheds are really smelly, and specially-trained dogs can sniff out even individual scales left over from a decomposing snake shed. This might be one reason that, although snakes usually spend several days inactive at their shedding site prior to shedding, they don't normally hang around for long afterwards - their predators might have an easier time finding their stinky sloughs than they would finding the snakes themselves. This could be especially true when those predators are other snakes. Some evidence suggests that dogs have an easier time sniffing out snake skins than actual snakes - the indigo-snake -sniffing dogs correctly identified a concealed snake 4 out of 5 times, but they got the sheds right every time. Dogs have also been used to help search cargo on Guam for hitchhiking Brown Tree Snakes, an invasive species which has spread around the Pacific. No word on whether the Brown Tree Snakes were shedding or how this affected the dogs' ability to smell them.

Most shedding sites are protected in some way, because snakes are vulnerable prior to shedding - they cannot see and other functions may be impeded as well. Shed sites used by Black Ratsnakes in Ontario include old barns, old mining machinery, cracks in building foundations, old hay piles, large hollow logs, rock crevices, and standing dead trees. Most of these things sound like something somebody might want to "clean up", but the fact is that they are important habitat features that many amphibians and reptiles use for shedding and also for hibernation. Many burrowing snakes come to the surface to shed, and shedding snakes may remain on the surface even during cold weather, when other snakes have retreated underground.

Sometimes other animals exploit the stink of snake sheds. Ground squirrels in California use them to scent themselves - first they chew up shed rattlesnake skins, then vigorously lick their own fur, which results in  a type of olfactory camouflage that reduces a rattlesnake's ability to correctly identify snake-scented ground squirrels as prey. Rattlesnakes and ground squirrels in California are partners in a coevolutionary relationship that goes back millions of years and has been well-studied by scientists from both the predator's and the prey's point-of-view.

Birds use snake sheds in their nests, something people have noticed since at least as far back as the 1800s. Although birds cannot smell, ornithologists (who should study snakes more often) wondered whether the shed skins helped protect eggs or nestling birds by deterring would-be predators. Recently, two experiments have helped determine which predators might be frightened off and whether the strategy really works. Ecologists at Arkansas State University conducted a study to test whether snake skin is an effective deterrent to predators. They found that flying squirrels, a major nest predator, ate the eggs out of 20% of nests without sheds, but didn't depredate any nests with sheds. Because flying squirrels are themselves vulnerable to predation by snakes, this makes intuitive sense. Interestingly, they also noticed that the deterioration rate of the snake skins in their experimental nest boxes (which were not occupied by birds) was much faster than that in real nests, where birds were actively raising chicks. Many of the sheds were eaten by ants, which would probably have been eaten by birds maintaining active nests. Ornithologists in Slovakia found opposing results - nests of great reed warblers festooned with snake sheds were no more or less likely to be depredated by birds and small mammals. However, over a third of reed warblers incorporated grass snake (Natrix natrix) sheds into their nests. When given a choice, two thirds of female reed warblers elected to use sheds left near their nests, whereas only 10% used ribbons of a similar length and color. If they weren't deterring predators, what were they for? The researchers suggested that because snake skins were mainly incorporated by female birds early in the nest-building process, they may have functioned as a signal to male reed warblers that the nest-builder was good at finding rare nest materials, which might lead the male to invest more heavily in helping share the duties of parental care later on in the nesting season.

Humans use snake sheds too. Because of their many similarities with the outermost layer of human skin, shed snake skins are used as model membranes in membrane permeability research, which primarily includes studies of ways to better transport pharmaceuticals into target cells, including some drugs that are inspired by or derived from snake venom (another ecosystem service). Snake sheds are a good alternative to using human, mouse, or synthetic skin, because they are cheap, large, and lack hair. This work is just one of many examples of snakes being used as model organisms to study general concepts in biology. Snake sheds can also be very aesthetically pleasing - many people have taken to creating beautiful snake shed jewelry.

Finally, snakes are themselves very olfactory creatures. Skin lipid pheromones have been shown to play important roles in male combat and in mating behavior, which could mean that sexual selection could act on these chemicals, creating species-specific diversity and dimorphism between males and females, which is mostly lacking in other snakes (except for a few species, including Langaha from Madagascar, where snake play many important cultural and ecological roles). Because most of these pheromones are in the skin, what's the potential for snakes to use their shed skins to mark territories, communicate information about their reproductive stage, select ambush sites, or perform other functions? Really, no one knows. Although territoriality is not the norm in snakes, some species have been suggested to be territorial and others may exhibit other types of social behavior. I hope that by understanding more about the important roles snakes play in ecosystems, people attending this carnival will be more likely to see them as valuable and less likely to fear them. As I hope I've been able to communicate, the old axiom that 'the only good snake is a dead snake' is just not true.

ACKNOWLEDGMENTS

Thanks to JD Willson, Angie Luebben, and Volker Wurst for their photographs and to everyone who helped publicize this blog carnival. A special thanks to the other #SnakesAtYourService blog carnival participants. Be sure to check out their contributions:

Social Snakes: Good Neighbors Make a Greater Impact: How Viper Behavior Increases Their Effect on Prey Populations by Melissa Amarello, @socialsnakes

Living Alongside Wildlife: Kingsnakes Keep Copperheads in Check by David Steen, @AlongsideWild

Nature Afield: Pythons as Model Organisms by Heidi Smith, @HeidiKayDeidi

Ophidiophilia: Converting Ophidiophobes to Ophidiophiles, One Kid at a Time by Emily Taylor, @snakeymama

The Traveling Taxonomist: Snakes of Madagascar: Cultural and Ecological Roles by Mark Scherz, @MarkScherz

Strike, Rattle, & Roll: Snakes and the Ecology of Fear by Bree Putman, @breeput

Australian Museum: When the Frogs Go, the Snakes Follow by Jodi Rowley, @jodirowley

SnakeBytes: The Brown Tree Snake of Guam by Brian Barczyk (@SnakeBytesTV) 

REFERENCES