Brown Treesnake Frequently Asked Questions
Please see the following page on Snake-Proofing Your Home.
As a rear-fanged snake from the family of Colubrid snakes, brown Treesnakes are often classified as non-venomous. However, as snakes on Guam (with the abundantly available food) have grown to longer lengths than what is found in their native range, some reactions to the brown Treesnake bite have resulted on Guam, particularly with small children where there was evidence that the snake had an opportunity to chew on its victim. These cases are now treated very seriously at hospitals on Guam, and while no known deaths have resulted from these bites, the snake has been reclassified as mildly venomous. This snake is still not considered dangerous to an adult human.
To many people's surprise, the answer to this question lies not in what could possibly eat the brown Treesnake but instead what primarily immature snakes themselves eat. This food source, mainly small lizards, is far more limited in its native range (coastal Australia, Papua New Guinea, and a large number of islands in northwestern Melanesia) than the bountiful feast that awaited it on Guam and may be available for the snake on other oceanic islands. The combination of extraordinarily high lizard densities, the main food source for young snakes, and birds that did not evolve in the presence of snakes with no natural defenses against them allowed the snakes to successfully colonize Guam. The continued high lizard densities, mostly credited to introduced species that mature rapidly and reproduce frequently, has allowed the brown Treesnake to maintain densities of nearly 13,000 per square mile in some areas, even in the absence of native birds.
While the birds and mammals were a major part of the snake's diet when it first entered the scene on Guam, these were not the major food sources of immature snakes. These snakes, as mentioned above, feed primarily on small lizards, which were and still are abundant—so abundant that they constitute enough mass to more than support the massive snake population. Also, because the brown Treesnake is a generalist predator, it was able to adapt to the changing prey base as birds and mammals declined, so that now adult snakes primarily consume lizards as well. The snake population (particularly the adults) does seem to be stressed, as suggested by low fat reserves in captured specimens, but this does not sufficiently limit the population. There is still enough food to support breeding, and every year new young snakes abound.
5. Could more brown Treesnakes from the native range be introduced in the hope of introducing disease or parasites into the Guam population?
Very little is known about diseases or parasites and how they might exert control on a snake population. Again, the control in the snake's native range is food availability as opposed to any other one factor.
6. Why don't you introduce a predator to simply eat all of the snakes (particularly the mongoose or kingsnake)?
This is probably the most frequently asked question regarding the brown Treesnake. It seems like a simple and obvious solution; however, the many ecological concerns and implications accompanying such a move illustrate the dangers of this tactic. While introducing predators has been attempted in many situations across the world, it has more often than not met with disastrous results. Introducing the mongoose has been attempted for controlling snakes and rats on islands (e.g., in Japan and the Caribbean), but mongoose were found to commonly feed on nontarget species as well as snakes and, while the snake and rat populations continued to proliferate, the more vulnerable nontarget species suffered greatly.
As is the case with the mongoose, the kingsnake does eat snakes, but does not include them exclusively in its diet. Kingsnakes will eat other snakes, including rattlesnakes (they are immune to the venom), but they also eat lizards, frogs, birds and their eggs, and small mammals. Many of these species (native birds and lizards, especially) are already severely depleted on Guam and are the focus of major conservation attempts at present. Introducing another predator to further stress these struggling populations would cause an even greater crisis and would most likely be disastrous for these species.
A secondary issue regarding the kingsnake involves its habits and potential for effectively pursuing a brown Treesnake even if given the opportunity. The kingsnake is not a climber (arboreal) like the brown Treesnake. Since brown Treesnakes spend the majority of their time in trees and can climb all but extremely smooth surfaces, pursuit would be difficult for terrestrial kingsnakes. Also, brown Treesnakes are generally between 3 and 6 feet, but have been found primarily in urban areas with lengths of 8-11 feet! This would be a challenging meal for a kingsnake, ranging in length from 30 to 50 inches (2.5-4 feet) and a maximum of about 5 feet.
Some have also suggested that a nicely matched predator might be a hawk, owl, or roadrunner. Again, these birds have very specific habitat needs and would find Guam lacking in these areas. While some birds do indeed prey on snakes, including the short-toed bald eagle (Circaetus gallicus) which is probably the only bird that strictly feeds on snakes, they also require large habitat of forested areas--habitat that is limited on Guam. Also, many birds would be vulnerable to predation by the snakes on their eggs, as are the native and introduced species of birds on Guam.
Other specific suggestions have been made, but the bottom line is that introducing a predator to control another species rarely works the way it is envisioned. Often, these introduced species cause damage to unrelated species that become prey. They may also have negative impacts on indigenous plants, have the ability to bring with them mites or other parasitic organisms that can affect newly exposed wildlife, and may in the end fail to control the one intended species while introducing other long-term effects of their own. While some animals on Guam, including monitor lizards and feral pigs, are known to eat brown Treesnakes opportunistically, they don't seem to prefer the tough snake and have not caused any noticeable decline in snake numbers. Even humans have reported that these snakes taste bad! Yet an enterprising few have attempted to create appetizing snake dishes. While these recipes have found a limited audience, the occasional backyard barbecue or gourmet use of snake meat has had no effect on snake population levels.
There are a number of difficulties associated with attempting to poison snakes partially because of the way they forage in the wild. Snakes are very selective about what they eat. They will often refuse to eat real bird eggs if they have been refrigerated, have been washed, or are below a typical body temperature of a bird. All of this suggests that the snakes use more than the appearance of potential prey when considering their next meal. Also, evolution has helped snakes develop natural defenses against consuming potentially harmful substances: they will often regurgitate or pass indigestible materials if they are accidentally consumed.
Another difficulty in poisoning snakes involves the type of substance that needs to be used to attract snakes. Snakes are quite particular about their food and are usually not fooled by fake baits, so poisoned prey items such as eggs, meat, or other attractants would be needed. The problem with this idea is that many other organisms include the same food items in their diet, and in targeting snakes in this manner, birds, mammals, other non-target reptiles, and even household pets could be put at risk. However, research is ongoing in this area as behaviorists, physiologists, and chemists work to develop substances that will attract snakes without putting other animals at risk or perhaps to deliver a kind of snake birth control to keep those already in place on Guam from further breeding.
Trapping is ongoing on Guam in areas that require low snake densities such as cargo loading docks for air and ship traffic and endangered wildlife enclosures. This tactic works well for limited areas, but requires constant monitoring and on a greater scale is next to impossible. To effectively trap the snakes, traps must be set approximately 15 m apart and require daily monitoring. Since an artificial snake attractant has not yet been identified, live lures must be used, requiring stores of food and water and protection from the elements such as direct sunlight and rain, which cause further limitations. Certain areas, such as limestone cliffs, are largely inaccessible for humans maintaining traps but are prime snake habitat. Restricted military areas and private property cause additional difficulties. Finally, besides the cost involved in creating, setting, and maintaining traps, vandalism frequently disrupts efforts in populated areas.
8. Could a repellent be used to keep snakes away from people's home, electrical substations, or endangered species enclosures?
Research is ongoing in this area, but as yet no substance has been identified that can be used safely and without causing negative repercussions in the environment.
9. How about paying a bounty for snakes collected to the youth or adults of Guam to instigate removal of the snakes or commercializing the snakes for use in traditional medicines or foods to bring in revenue and reduce the snake numbers that way?
Both of these ideas would bring about the same basic outcome, although the specifics of each differ. With a bounty, people would be inspired to gather snakes, but they would do so in areas where the snakes were easiest to catch. When numbers were depleted, concentration would switch to areas of greater yield. This results in cropping instead of total collection, allowing the population to recover when efforts change. A short-term collection was tested on Guam, but the numbers collected were not enough to warrant additional attention. Also, in creating a cash crop in the snake by assigning bounties, people could decide to bring the snake to other islands in the hope of making money with future bounties—a situation to adamantly avoid.
The issue of commercializing snakes would again result in cropping instead of removal. However, while the meat and skins of some snakes are exploited unmercifully for leather or food, the brown Treesnake has little economic value. Its slender body has little usable meat, and its skin is narrow and much too fine for commercial exploitation as leather. Hand collection is also difficult even with experienced snake collectors, averaging about two snakes per hour in forest situations. To motivate people to hunt snakes, the price per snake might be inordinately high relative to the numbers of snakes that need to be caught.
10. Since Guam has a number of military bases, could the soldiers be lined up and marched across the island to catch the snakes?
Again, the snake habitat would make this extremely difficult, especially for people untrained in locating and dealing with snakes. Snakes are rarely encountered except at night, and even then many elude detection because they are high in trees, in dense vegetation, and or in places that are difficult to access. Additionally, private property, military security areas, and all sorts of situations would be inaccessible to human hunters. Also, because snakes have slow metabolisms and eat fairly large meals infrequently, even if every single foraging snake could be collected in one sweep of the island, many times the collected number could be expected to remain including the generally inactive gravid females.
11. Are the snakes attracted to the heat or high frequency oscillations of the power lines? If so, could something like this be developed to attract snakes?
Unfortunately, it isn't the heat or frequency of the current in the lines that attracts the snakes. For an arboreal snake, the poles and power lines are easy to climb and attractive to snakes because they are elevated. When searching for food and daytime refugia, these surfaces are as appealing as nearby trees and foliage.
12. If there is no easy solution, why don't you just give up and let nature take its course on Guam?
While it would be possible for the research on this problem to simply end and those involved to move on to other issues, the problem would not be resolved. The snakes would continue to cause problems for those living in Guam, and the threat to other island ecosystems would persist. This project has initiated detailed research on a previously unknown phenomenon: extirpation of native wildlife by an introduced snake. As human commerce and travel continues, the chances of brown Treesnakes and other species becoming established in new lands remains and will undoubtedly increase. The information gained in researching the brown Treesnake on Guam, including successes and failures, will be important to maintaining the biodiversity of other places.
13. I am doing a research project/report on the brown Treesnake. Are there some resources you could recommend?
Besides the information included here, there are many scientific papers available that detail the great deal of research that has been done regarding the brown Treesnake, particularly in the last 15-20 years. See the Bibliography section under Educational Resources for more details. There are also a couple of books that have been written specifically about this situation on Guam: for the more scientifically oriented, Problem Snake Management: The Habu and the Brown Treesnake published by the Cornell University Press, 1999; and for the non-scientist who is interested in the story behind the problem but doesn't want as much technical language, And No Birds Sing, by Mark Jaffe. These are both excellent resources and can be found at your local library or bookstore (although you may need to request that they be ordered). Finally, a nice overview article can be found in Bioscience, October 1997, entitled The Disappearance of Guam's Wildlife.
14. Why can't you control the snakes by using indigestible glass eggs or real eggs that have been laced with a poison?
First, snakes are extremely alert to the temperature, odor, and other chemical cues that help them discern real eggs from other objects that may be egg-shaped. Even if a snake were to ingest a glass egg it would probably regurgitate it when it proved indigestible. The idea of placing a poison (strychnine or other substances are often proposed) is also impractical for various reasons: