Wednesday, October 14, 2009

Are Bites from Baby Venomous Snakes More Dangerous Than Those From Adults?

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The short answer: No.

Full Column:

For all the fear and hatred they evoke, snakes inspire fascination like no other group of animals. Those that kill snakes on sight will eagerly take every opportunity to share stories of their encounters with serpents. Animal lovers will hold court with tales of large snakes they have seen and those they hope to find. And perhaps most interestingly, rational-minded people, even those that spend much of their time outdoors, will often believe the most far-fetched ideas about snake biology.

When it is revealed that I am a researcher that specializes in reptiles, I am often confronted by curious individuals wanting to know the veracity of a particular legend. I’ll never forget the woman who earnestly asked me whether I knew what kind of snake would grow into separate, fully-functioning individuals when it was cut into pieces with a garden hoe. I believe I noted that I wasn’t familiar with that species.

They say that every myth has some basis in reality, so it shouldn’t be surprising that there are some legends that seem more reasonable. Perhaps the question that I am posed most often relates to the relative danger of young versus adult rattlesnakes.

The legend goes that young snakes have not yet learned how to control the amount of venom they inject. They are therefore more dangerous than adult snakes, which will restrict the amount of venom that accompanies a bite. It’s repeated so often that it’s become a sort of mantra among laypeople and biologists alike.

It seems like a simple enough suggestion, but to examine this topic requires some examination of the assumptions implicit within the framing of the question as well as delving into some hot topics in biology. There are four main assumptions when the question is framed in this manner: 1) snakes are able to control the amount of venom they inject, 2) there is some disadvantage to a snake when it injects all of its venom in every bite (otherwise why not inject all of their venom all of the time?), 3) as a result, a snake will learn of these disadvantages and change its behavior as it matures, and finally, 4) a full envenomation from a young snake is more dangerous than a partial envenomation from an adult snake.

First things first, can a snake control the amount of venom they inject? This is actually a contentious issue among snake specialists. There are some who believe snakes do indeed control the amount of venom they inject, they are proponents of what is considered the Venom Metering Hypothesis (among scientists, a hypothesis is a preliminary explanation of observed phenomena; these explanations haven’t been rigorously tested. This is a step below a scientific theory, which is a conclusion based on observations and experimentation). Past studies have indicated snakes inject different amounts of venom in different situations, but the trends are sometimes inconsistent.

A recent review of studies on the subject suggested although some researchers have documented trends in venom injection, there isn’t compelling evidence to suggest that it was necessarily controlled via any decision by the snake. They came to this conclusion because the trends didn’t seem to indicate the amount of venom the snakes injected would have any consistent benefit in the wild. And if there was no apparent benefit in the wild, then why would snakes be choosing to exhibit this behavior?

An alternative hypothesis has been termed the Pressure Balance Hypothesis, which suggests the amount of venom a snake injects is due to a combination of snake anatomy and the properties of the object the snake is biting. This would explain why snakes tended to inject different amounts of venom into different targets with no clear benefit to the serpent.

For the purpose of this discussion, let’s say that snakes can control the amount of venom they inject. The second assumption states there must be some disadvantage to a snake injecting all of its venom when it bites; otherwise, a snake would just inject everything every time. Why not?

This is another interesting question. It may be beneficial for a snake to keep some venom on hand in case its intended prey requires a second dose, or if a first prey item escapes and another quickly appears. Another scenario is that a snake does not want to inject all of their venom into their food just in case they are suddenly confronted by a potential predator of their own. Finally, it takes some time for a snake to produce more venom, and energy that goes into venom production is energy these animals could use for other important tasks, such as growth or reproduction. Consequently, common sense would suggest that there are some disadvantages to a full release of venom at every opportunity. It may be possible to confirm this suggestion via experimentation by testing whether snakes that frequently inject all of their venom experience slower growth, lower reproductive rates, or high mortality. To determine this would require a complex study, one that has not yet been attempted.

The third assumption states that as a snake matures, they learn there are disadvantages to delivering full venom loads during every bite and as a result, they change their behavior. For learning to occur, there must be positive or negative reinforcement. If we state that a snake may keep venom on hand in case a prey item (or one that appears shortly after the first prey item) requires a second bite, this snake must have experienced a number of incidents where they injected a fraction of the venom they had into a prey item only to have this prey item escape. Over time, they may learn that it’s beneficial to keep some venom for a successful attack later. This may make sense superficially, but one might think that it would be more likely that the snake learns to inject more venom with their first bite and increase the chance of a fatality than saving venom just in case they experience another opportunity to bite their intended food again. A commenter has rightly pointed out that there are a number of other potential scenarios we need to consider as plausible.

If we state that a snake learns to withhold venom from their bites in case a potential predator quickly appears and attempts to eat them, a snake must have learned that it’s somehow beneficial to do so. This snake would have had to experience numerous predation attempts and survive to know the costs associated with their venom injection behavior. If an “empty” snake were to be eaten by an owl or bobcat, then it would know that it should’ve kept some venom (but it’s too late to do anything about it because it’s dead). For a snake to learn it’s beneficial to keep venom ready, it would have had to survive an attack, and if it survived an attack without any venom left over from a previous feeding attempt, then I guess it didn’t really need that venom anyway. So, by logically extending the third assumption, we find that it’s difficult to envision a scenario that would enable a snake to eventually learn that it’s beneficial to withhold the amount of venom it injects with their bite (check the Comments to read about why this sentence was misleading). Remember, it would likely require that this scenario happen numerous times for a snake to eventually learn the consequences of their behavior.

It’s possible that there are evolutionary advantages to a snake retaining some venom in case it’s needed in a defense against an attack by a predator. It’s easy to conceive how snakes that tended to have venom on hand would be more likely to survive longer and produce young. If this behavior had a genetic component, the surviving snakes would pass on the tendency to conserve venom to their offspring. This is not learning however, and the behavior would be innate (i.e. something they’re born with) or instinctual.

Finally, the fourth assumption states that a bite from a young snake that has no control over the amount of venom it injects is more dangerous than a learned adult. But, there are some big snakes out there, and just a fraction of their total venom capacity could be more than 100% of a young snake’s potential venom output. So, I don’t think this final assumption is always valid.

To summarize, although it’s possible that this legend is true and baby snakes are more dangerous than adults because they haven’t learned to control the amount of venom they inject when they bite, it’s safe to say this is unlikely to be the case. And, there's really no evidence to suggest it's true. Due to the complexities of the original question, I doubt this statement will ever be tackled in a manner that sufficiently addresses all of its assumptions. But until then, try not to get bitten by any venomous snakes, no matter how old they are.

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Sunday, October 4, 2009

Gopher Tortoises of Alabama

Having recently returned from this year's annual meeting of The Gopher Tortoise Council, I'm inspired to reprint a brief article about gopher tortoises I wrote for the Alabama Coastal Foundation's newsletter. Laura Wewerka of the GTC and Wally Smith of Alabama PARC and Alabama's second-most distinguished university provided input:

Gopher tortoises, Gopherus polyphemus, are perhaps one of the most recognizable animals found in Alabama. Plodding around the southeastern United States from South Carolina down through Louisiana, this shelled reptile can be found in Alabama south of the Fall Line.

They’re the only tortoise native to the southeastern United States, and adults can be distinguished from the box turtle, another terrestrial turtle, by their large size and gray un-patterned shell. Hatchlings and young tortoises typically have square brown markings encircling a yellow center on their carapace (top shell); these markings tend to fade with age.

The presence of tortoises is most often detected first by stumbling upon their characteristic burrows, shaped like half-moons. These animals spend most of their life underground in these holes, which they dig with their extremely strong front limbs. But they’re not the only ones who benefit from their frequent tunneling. Dozens, if not hundreds, of different species of insects, amphibians, reptiles, mammals, and even birds may find refuge within tortoise burrows. Some animals, like rabbits and ground-dwelling sparrows, may hide in tortoise holes when fire burns through an area. Others, like the federally threatened eastern indigo snake, spend their winters residing with the dark recesses of the tortoise lairs. When tortoises are removed from a forest, this may have serious implications for the remaining species that rely on their burrows for shelter.

Gopher tortoises are closely associated with longleaf pine forests, and the ranges of the two overlap across the country almost precisely. Tortoises like these savanna-like forests because a lot of light reaches the ground; this light encourages plant growth which provides food for the tortoise. This is why it’s so important to manage longleaf pine forests with periodic prescribed burning; the fire helps limit invasion of hardwood trees that would shade the area and eventually crowd out the pine trees. Tortoises living in a longleaf pine forest that isn’t managed with frequent fire will eventually try to disperse to more suitable areas or find refuge in sub-par habitats with more light streaming in, such as powerline corridors. Recent research, in fact, has suggested that the reintroduction of periodic burning has an incredibly beneficial effect on gopher tortoise densities when fire has been historically suppressed in an area.

Unfortunately, just as the longleaf pine forests have shrunk in size due to development and mismanagement, tortoise populations have declined as well. Perhaps the decline of the tortoise is linked most closely to the vanishing longleaf pine ecosystem, but they face other threats too, including collisions with cars, poaching for food, and disease. Without conservation efforts aimed at protecting tortoises and their habitat, they have a bleak future. Fortunately, there are some encouraging trends: Alabama recently outlawed the practice of gassing tortoise burrows. This irresponsible and environmentally destructive practice was undertaken to drive out rattlesnakes, which could then be either killed immediately or brought to rattlesnake roundups, where the snakes are killed later. Tortoise tended to stick around in their burrows after they were gassed, exposing the animals to lung damage and eventual death. Recent laws banning this technique, in concert with land conservation and proper forest management, may help ensure that future generations can look forward to encountering the gopher tortoise in Alabama.