Brown tree snake
The brown tree snake, also known as the brown catsnake, is an arboreal rear-fanged colubrid snake native to eastern and northern coastal Australia, eastern Indonesia, Papua New Guinea, and many islands in northwestern Melanesia. The snake is slender, in order to facilitate climbing, and can reach up to 2 metres 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. Currently, efforts are being made to reduce and control the population on Guam and prevent the snake from spreading to other locations. Researchers are employing various methods including aerial baiting to attempt to control the population while various agencies continue to fund and support the removal of the species.
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. 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. The brown tree snake is generally 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. The median age of maturity for these snakes is estimated to be 3.1 years in males and 3.7 years in females.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 in length. The longest recorded length of this species is one found on Guam measuring 3 m.
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. 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.A separate species, Boiga flavescens, was described from Sulawesi in the nineteenth century. Subsequent authors identified both B. flavescens and B. irregularis on the island, and flavescens was synonymised with irregularis in 1985 by In den Bosch. Weinell et al. found significant DNA differences between the Sulawesi and Sundaland populations, proposing in 2021 that all members west of Weber's Line be designated B. flavescens.
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. 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. 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. 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.
Diet
In its native range, the brown tree snake preys upon birds, lizards, bats, and mice and other small rodents. 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. The larger snakes are often seen shifting their diet towards endotherms rather than consuming ectotherms due to relative prey size.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. The snake preys on a large variety of invertebrates, including their eggs, their young and the adults of many of the preyed upon species. A study was done on the Guam population where 683 snakes were necropsied and their stomach contents were examined. The majority of stomach content consisted of reptiles and reptile eggs, followed by birds and bird eggs, and then lastly mammals. The snake also preys on shrews on Guam, and threatens native bird and lizard species.
Behavior
Reproduction
The reproductive characteristics of the brown tree snake have not been widely studied. The female is known to produce 4 to 12 oblong eggs, long and wide with leathery shells. Females may produce up to two clutches per year depending upon seasonal variations in climate and prey abundance. The female deposits the eggs in hollow logs, rock crevices, and other sites where they are likely protected from drying and high temperatures. Eggs can then hatch around 90 days after being laid. Unlike the native population in Australia, populations in Guam may reproduce year-round as there is no evidence of seasonality in the population.There have been observations of courtship behaviour in brown tree snakes, some of which are different than other snakes within their family. Male courtship behaviour 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. Interestingly, the female brown tree snakes have been observed to display courtship behaviours that previously have only been used by male individuals. These behaviours include the tongue-flicking and head-jerking behaviours, which can lead to the female mounting the male to elicit courting behaviour from the other snake. Oftentimes, if females displayed these behaviours towards a male, the male would respond and continue until copulation occurred. Female snakes also displayed a tail-lifting behaviour in which they release cloacal secretions in response to male courtship behaviour.
Predatory behaviour
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 immobilise the prey while chewing and consuming the animal.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. This novel form of movement has been observed to be physically demanding, as snakes seen utilising it make frequent stops and move more slowly than other forms of locomotion.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. 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. 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 localised 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. The snake has been reported as aggressive, but is not considered dangerous to an adult human. 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.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. 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. To minimise 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.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. 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. An incipient population was thought to be established on Saipan after sightings around the port; however, after 20 years without a sighting it appears that Saipan's biosecurity inspections have worked and the island is free of them. Acetaminophen has been used to help eradicate the snake on Guam.