Brown Tree Snake

## Description

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. This snake is infamous for being an invasive species responsible for extirpating the majority of the native bird population in Guam. It is also one of the very few colubrids found in Australia, where elapids are more common.

## Appearance

The Brown tree snake is a venomous arboreal snake native to Australia, eastern Indonesia (Sulawesi to Papua), Papua New Guinea, and many islands in northwestern Melanesia. This snake is infamous for being an invasive species responsible for extirpating the majority of the native bird population in Guam. The Brown tree snake is a member of the colubrid snakes, which is a group of roughly twenty-five species that are referred to as "cat-eyed" snakes for their vertical pupils. The snake is long and slender, which allows it to pass through tiny spaces in buildings, logs, and other shaded locations, where it seeks refuge during daylight. The coloration ranges 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.

## Distribution

Brown tree snakes occur in the eastern and northern coastal Australia, eastern Indonesia (Sulawesi to Papua), Papua New Guinea, and many islands in northwestern Melanesia. They are found 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. Brown tree snakes inhabit tropical rainforests, dry forests, mangrove swamps, and sparsely forested areas. They are most commonly found in trees, caves, and near limestone cliffs. They are also common in plantations, rural gardens, and human-disturbed habitats.

## Behavior

Brown tree snakes are solitary nocturnal reptiles. They are arboreal and use visual and chemical cues in hunting in the tropical rainforest canopy and/or on the ground. They live in trees but frequently come down to the ground to forage at night. During the day these snakes hide in the crowns of palm trees, hollow logs, rock crevices, caves, and even the dark corners of thatched houses near the roof. Brown tree snakes are generalist feeders known to eat a wide variety of foods; when threatened they are highly aggressive and tend to lunge and strike the aggressor repeatedly. Their venom is used to subdue and kill prey on which these snakes feeds; however, their venom is not considered dangerous to adult humans.

## 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 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. 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.

## Diet

Brown tree snakes are carnivores and active predators. They prey on birds, lizards, bats, rats and other small rodents in their native range. In Guam, these snakes prey on birds and shrews.

## Reproduction

The breeding season for Brown tree snakes takes place year-round. Females are known to produce 4 to 12 oblong eggs, 42-47 mm (1.7-1.9 in) long with leathery shells. They may produce up to two clutches per year depending upon seasonal variations in climate and prey abundance. The eggs are usually laid in hollow logs, rock crevices, and other sites where they are likely protected from drying and high temperatures. The incubation period lasts around 90 days. The young hatch fully-developed and are around 50 cm long. They are completely independent at birth and become reproductively mature at 3-4 years of age.

## Population

### Population threats

There are no major threats to Brown tree snakes at present. However, currently, their population on Guam is declining due to depleted food resources, adult mortality, and/or suppressed reproduction.

### Population number

According to IUCN, the Brown tree snake is locally common and widespread throughout its range but no overall population estimate is available. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.

### Ecological niche

As an active predator, Brown tree snakes control populations of various species they prey on in their natural habitat. These snakes also don't have natural predators in Guam where they have been introduced and thus have become there an apex predator. Brown tree snakes have become highly invasive there and have severely extirpated the majority of the native bird and reptile species.