Brown tree snake
Conservation status
Least Concern (IUCN 3.1)[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Squamata
Suborder:	Serpentes
Family:	Colubridae
Genus:	Boiga
Species:	B. irregularis
Binomial name
Boiga irregularis (Merrem, 1802)
Synonyms
Coluber irregularis Merrem, 1802 Boiga irregularis Fitzinger, 1826 Dipsas irregularis Boie, 1827 Triglyphodon irregularis Duméril & Bibron, 1854 Dipsadomorphus irregularis Boulenger, 1896

The brown tree snake (Boiga irregularis), also known as the brown catsnake, is an arboreal rear-fanged colubrid snake native to eastern and northern coastal Australia, eastern Indonesia (Sulawesi to Papua), Papua New Guinea, and many islands in northwestern Melanesia. The snake is slender, in order to facilitate climbing, and can reach up to 2 meters in length. Its coloration may also vary, some being brown, green, or even red. Brown tree snakes prey on many things, ranging from invertebrates to birds, and even some smaller mammals. It is one of the very few colubrids found in Australia, where elapids are more common. Due to an accidental introduction after the events of World War II, this snake is now infamous for being an invasive species responsible for extirpating the majority of the native bird population in Guam.^[2] Currently, efforts are being made to reduce and control the population on Guam and prevent the snake from spreading to other locations.

Description

The brown tree snake is a nocturnal and arboreal species that uses both visual and chemical cues when hunting, either in the rainforest canopy or on the ground.^[3] It is a member of the subfamily Colubrinae, genus Boiga, which is a group of roughly twenty-five species that are referred to as "cat-eyed" snakes for their vertical pupils.^[4] The brown tree snake is generally 1–2 m (3–6 ft) in length in its native range. The snake is long and slender, which facilitates its climbing ability and allows it to pass through tiny spaces in buildings, logs, and other shaded locations, where it seeks refuge during daylight. Variations in coloration occur in the snake's native range, ranging from a lightly patterned brown to yellowish/green or even beige with red, saddle-shaped blotches. They are rear-fanged, have a large head in relation to their body, and can survive for extended periods of time without food.^[4] The median age of maturity for these snakes is estimated to be 3.1 years in males and 3.7 years in females.^[5]

Owing to the availability of prey and lack of predators in introduced habitats such as Guam, they have been known to grow to larger sizes than their normal 1 to 2 m (3.3 to 6.6 ft) in length.^[2] The longest recorded length of this species is one found on Guam measuring 3 m (9.8 ft).^[2]

Distribution and habitat

The brown tree snake is native to coastal Australia, Papua New Guinea, and many islands in northwestern Melanesia. The species occurs on variably sized islands, extending from Sulawesi in eastern Indonesia through Papua New Guinea and the Solomon Islands and into the wettest coastal areas of Northern Australia.^[6] The snakes on Guam represent the only documented reproductive population outside the native range. Since January 2016, however, four snakes have been sighted on the island of Saipan in the Northern Mariana Islands.^[7]

The brown tree snake is not restricted to forested habitats, as it can also occur in grasslands and sparsely forested areas, as well. In Papua New Guinea, it occupies a wide variety of habitats at elevations up to 1,200 m.^[8] It is most commonly found in trees, caves, and near limestone cliffs, but frequently comes down to the ground to forage at night. It hides during the day in the crowns of palm trees, hollow logs, rock crevices, caves, and even the dark corners of thatched houses near the roof.^[9] Based on the frequency of sightings of this snake, in relation to buildings, poultry, and caged birds, the snake is considered to be common in human-disturbed habitats.^[10] Although the snake has been found in nearly all habitats on the island, it is most common in primary and secondary forest habitats on Guam.^[11]

Diet

In its native range, the brown tree snake preys upon birds, lizards, bats, and mice and other small rodents.^[2] Those of a smaller size focus on smaller prey such as frogs and lizards, while the larger individuals prey on larger vertebrates such as mammals and birds.^[12] The larger snakes are often seen shifting their diet towards endotherms rather than consuming ectotherms due to relative prey size.^[12]

On Guam, this shift is prevalent due to the prey being available on the island being mostly larger ectotherms such as lizards, which are the primary prey for the smaller snakes on Guam, and birds.^[12] The snake preys on a large variety of invertebrates, including their eggs, their young and the adults of many of the preyed upon species.^[12] A study was done on the Guam population where 683 snakes were necropsied and their stomach contents were examined.^[11] The majority of stomach content consisted of reptiles and reptile eggs, followed by birds and bird eggs, and then lastly mammals.^[11] The snake also preys on shrews on Guam, and threatens native bird and lizard species.^[13][2]

Behavior

Reproduction

The reproductive characteristics of the brown tree snake have not been widely studied.^[2] The female is known to produce 4 to 12 oblong eggs, 42–47 mm (1.7–1.9 in) long and 18–22 mm (0.71–0.87 in) wide with leathery shells.^[2] Females may produce up to two clutches per year depending upon seasonal variations in climate and prey abundance.^[2] The female deposits the eggs in hollow logs, rock crevices, and other sites where they are likely protected from drying and high temperatures.^[2] Eggs can then hatch around 90 days after being laid.^[14] Unlike the native population in Australia, populations in Guam may reproduce year-round as there is no evidence of seasonality in the population.^[15]

There have been observations of courtship behavior in brown tree snakes, some of which are different than other snakes within their family.^[16] Male courtship behavior involves tongue-flicking, head-jerking, and chin-rubbing on the female in preparation of mating. If females are receptive, mating will take place almost immediately.^[16] Interestingly, the female brown tree snakes have been observed to display courtship behaviors that previously have only been used by male individuals.^[16] These behaviors include the tongue-flicking and head-jerking behaviors, which can lead to the female mounting the male to elicit courting behavior from the other snake. Oftentimes, if females displayed these behaviors towards a male, the male would respond and continue until copulation occurred.^[16] Female snakes also displayed a tail-lifting behavior in which they release cloacal secretions in response to male courtship behavior.^[16]

Predatory behavior

The brown tree snake is a generalist feeder known to eat a wide variety of foods. When threatened it is highly aggressive and tends to lunge and strike the aggressor repeatedly. The snake has numerous teeth, but only the last two on each side of the upper jaw have grooves, which inject venom as it bites. Therefore, the snake's mouth must be opened as wide as possible to insert and expose its fangs. A chewing movement is used by the snake to inject the venom by means of capillary action along the grooved fangs. The venom is used to subdue and kill prey on which the snake feeds; however, the venom is not considered dangerous to adult humans. In addition to subduing its victim with its venom, the brown tree snake often wraps its body around the prey, like a constrictor, to immobilize the prey while chewing and consuming the animal.^[2]

Lasso locomotion

A 2021 study found that brown tree snakes in Guam can use "lasso locomotion" to climb large smooth cylinders. This type of locomotion has not previously been observed in snakes.^[17][18] This novel form of movement has been observed to be physically demanding, as snakes seen utilizing it make frequent stops and move more slowly than other forms of locomotion.^[17]

Seasonal activity

In the northern Australian city of Darwin, the brown tree snake is significantly more likely to be encountered in suburban areas during the wet season months of January–March.^[19] This is indicative of shifts in snake behaviour or movement across the year, and may partially be driven by differences in seasonal detection probability. Potentially, in months of lower activity, brown tree snakes may remain in tree canopies, where chances of detection by members of the public are unlikely.

Venom

The brown tree snake is a nocturnal, rear-fanged colubrid, possessing two small, grooved fangs at the rear of the mouth.^[20] Due to the placement of the fangs and their grooved rather than hollow architecture, the venom is difficult to convey into a bite on a human, and thus is only delivered in small doses. The venom appears to be weakly neurotoxic and possibly cytotoxic with localized effects that are trivial for adult humans; serious medical consequences have been limited to children, who are more susceptible because of their low body mass.^[2] The snake has been reported as aggressive,^[2] but is not considered dangerous to an adult human.^[20] The venom seems to be primarily used to subdue lizards, which can be more easily positioned in the rear of the mouth for venom delivery.^[2]

Invasive species

Shortly after World War II, and before 1952, the brown tree snake was accidentally transported from its native range in the South Pacific to Guam, probably as a stowaway in ship cargo or by crawling into the landing gear of Guam-bound aircraft.^[2][20][21] As a result of abundant prey resources on Guam and the absence of natural predators apart from the native Mariana monitor and feral pigs, brown tree snake populations reached unprecedented numbers. Snakes caused the extirpation of most of the native forest vertebrate species; thousands of power outages affecting private, commercial, and military activities; widespread loss of people's pets; and considerable emotional trauma to residents and visitors alike when snakes invaded human habitats with the potential for envenomation of small children. Since Guam is a major transportation hub in the Pacific, numerous opportunities exist for the brown tree snakes on Guam to be introduced accidentally to other Pacific islands as passive stowaways in ship and air traffic from Guam.^[2] To minimize this threat, trained dogs are used to search, locate, and remove brown tree snakes before outbound military and commercial cargo and transportation vessels leave the island.^[22]

Numerous sightings of this species have been reported on other islands including Wake Island, Tinian, Rota, Okinawa, Diego Garcia, Hawaii, and even Texas in the continental United States.^[23] Hawaii is especially at heavy risk from the snake, as direct military flights between Guam and Hawaii are allowed and brown tree snakes are regularly intercepted at landing areas. A successful introduction could pose an immense threat to the already highly threatened endemic birds of the islands.^[24] An incipient population was thought to be established on Saipan after sightings around the port;^[2][25] however, after 20 years without a sighting it appears that Saipan's biosecurity inspections have worked and the island is free of them.^[25] Acetaminophen has been used to help eradicate the snake on Guam.^[26]

Physiological evidence for reproductive suppression

Environmental stressors such as lack of shelter, climate change, overcrowding, and loss of prey have been researched as primary causes of diminished snake density, as they have been found to have direct correlation with the reproductive success of the snake. Current research on the breeding patterns of the brown tree snake is being conducted in hopes of further understanding how these environmental stressors are affecting the population density of the snake on Guam.^[27]

A study conducted by I.T. Moore predicted that low body condition would correlate to high levels of stress hormones and low levels of sex steroids in free-living brown tree snakes on Guam when compared with the native snake population in Australia and snakes held in captivity on Guam. After extensive research, the body condition in the free-living snakes was found to be significantly different from the body condition of native and captive snakes.^[27] The results determined, "depressed body condition and elevated plasmacorticosteron levels in the free-living animals suggest that a lack of food resources was placing individuals under chronic stress, resulting in suppression of the reproductive system." The study suggested that snakes living under stressful conditions such as high population densities or low prey resources had suppressed reproduction at multiple stages, including steroidogenesis and gametogenesis.^[28]

Current status

Currently, the brown tree snake population on Guam is declining with an equilibrium population size predicted to be roughly 30 to 50 snakes per hectare (12-20 per acre). The decline in snake population may be identified as a result of depleted food resources, adult mortality, and/or suppressed reproduction.^[29] The brown tree snake population on Guam has exceeded the carrying capacity of the island.

Species status and effect

Effect of early introduction

The introduction of the brown tree snake on Guam after WWII has had a significant impact on the community dynamics of the island. Upon its introduction the brown tree snake population exploded and spread across the entirety of Guam. The brown tree snake population on the island has reached peak densities of greater than 100 snakes per hectare.^[29] This population spike was caused by the copious amount of resources newly available to the brown tree snake upon its introduction. The limitations on the snake's population in its native range is predominantly food based. The snake's food source is far more limited in its native range than on the island of Guam as the prey in its natural range boasts significantly more natural defences to the snake than the prey on Guam.^[30]

The predominant population affected by the snake's introduction was that of native bird species such as the Mariana fruit dove, the Guam flycatcher, the rufous fantail and the Micronesian myzomela. The introduction of the brown tree snake into Guam has resulted in extinction of twelve native bird species in total. The Guam National Wildlife Refuge is attempting to prevent the extinction of additional bird species endangered by the snake.^[31] Other species significantly affected by the invasion of these snakes were small lizards and small mammals.^[27] Research has indicated a direct correlation of the spread of these snakes across the island to the decrease in the populations of these native species. Furthermore, the introduction of the brown tree snake has had an indirect, negative impact on vegetative diversity as its intense predatory nature has decreased populations of vital pollinators including native birds and fruit bats. Data collected from nearby islands lacking brown tree snake populations depict a significant difference in vegetative species richness, that is, islands close to and similar to Guam in which the brown tree snake has not been introduced have greater vegetative species diversity. Overall, the vertebrate fauna and native flora of Guam have suffered tremendously because of the introduction of the brown tree snake.^[29]

Economic damage on Guam

The negative effects of the brown tree snake are not limited to extirpation of native species and fauna, as they have also had sizable effects on the economy of Guam as well. One of these economic facets includes Guam's energy sector. The snake, due to its large invasive population, caused almost 2,000 power outages in a 20-year span.^[32] The outages, depending on the scale, can cause millions of dollars of revenue loss to Guam and can affect either the whole island or smaller parts of it. According to a study done by Thomas H. Frittz, there was an average of 133 power outages per year from 1991 to 1997 that were caused by the brown tree snake. A majority of these outages were considered major disruptions to the power grid of the island.^[32] The current estimate of total economic loss due to the outages caused by these snakes is 4.5 million dollars.^[32] The snakes have also become attuned to the predation of poultry, which is another important sector of Guam's economy.^[33]

Population control methods

Due to the success of the brown tree snakes on Guam, there have been many efforts to both eradicate the invasive population and to prevent its spread to other places such as the mainland United States and other islands such as Hawaii. Other goals for the control of this species include intercepting snakes that are either coming to or leaving Guam, such as those that stowaway on planes, protecting sensitive areas on Guam from further destruction, and exterminating any other populations that exist outside of their native range.^[34] Current control techniques and search methods for the brown tree snake include trapping, spotlight searches, baiting and poisoning, the use of search dogs, barriers, fumigation, and area reduction of prey.^[34] There is also a public outreach campaign that raises the issue of the snake to the regular public and the military population. Educational tools such as workshops, demonstrations, and brochures are used to raise awareness and keep the public informed.^[34]

Capturing and poisoning methods

Given the environmental impact of the brown tree snake, studies have attempted to provide a capturing methodology to alleviate the detrimental effects of the tree snake. The use of mice as bait has shown considerable reduction effects when combined with acetaminophen, to which the snake is particularly sensitive, in a mark-recapture experiment leading to potential widespread application in Guam.^[35] When utilizing a precisely defined treated plot with results corrected for immigration and emigration, the additive effect of both acetaminophen and mice usage shows a 0% survival rate of the brown tree snake. In the study, 80 mg of acetaminophen was inserted into mouse carcasses.^[28] In addition, one study showed that increasing inter-trap spacing would not only increase efficiency, but also not compromise efficacy as 20-, 30-, and 40-metre long perimeter trap lines were compared and no difference was found.^[36] Another study echoed the aforementioned notion of increasing inter-trap spacing.^[37]

Predation on brown tree snakes

An investigative study was performed to find predators of the brown tree snake that could possibly serve as a population control method. In this study two actual predators were identified and 55 potential predators were identified: the two actual predators identified were the red-bellied black snake and the cane toad.^[38] Actual predators were identified by evidence showing that they would actually prey upon and consume the brown tree snake in a natural habitat whereas potential predators were identified as species that were only physically capable of consuming the brown tree snake.^[39] The research collected in this study suggested that even with the introduction of brown tree snake predation, it was unlikely this would serve as an effective brown tree snake population control method.^[38] One reason for this conclusion was that the identified actual predators of the brown tree snake are generalist feeders and would cause further detriment to other native island species.^[39]

Another possible negative outcome of introducing species as a control method for the brown tree snake population is predation on juvenile cane toads and red-bellied snakes by brown tree snakes themselves, because they are opportunistic and generalist feeders.^[39] This investigation determined that the environmental and ecological risk associated with the introduction of these predators was too high to implement.^[38] Lastly, red-bellied snakes could pose a threat to the health of humans. The cost of introduction of such predatory species outweighs the benefits and is not practical.

Use of airsoft guns

The use of firearms is a control method that is often paired with invasive vertebrate species, including snakes. A study published in 2018 that was conducted at the Brown Treesnake Project laboratory on Guam looked at the use of airsoft guns, which are a lower-powered and less expensive alternative to firearms, as a form of lethal control for the brown tree snake.^[40] Using both dead and live snakes, the study looked for evidence of wounds and how they affected the snake's bodies as well as the live snake's reactions after being shot. The study found that higher power guns with larger ammunition was preferrable in the control of brown tree snakes.^[40]