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The Brown Treesnake
Colubridae
Boiga irregularis
Distribution
The
brown Treesnake is a member of the Family Colubridae, a diverse assemblage
of primarily harmless snakes, and is native to coastal Australia, Papua
New Guinea, and a large number of islands in northwestern Melanesia.
The species occurs on both large and small islands, extending from Sulawesi
in eastern Indonesia through Papua New Guinea and the Solomon Islands
and into the wettest coastal areas of Northern Australia. Individuals
of this species have been discovered on several extralimital islands,
but the snakes on Guam represent the only documented reproductive population
outside the native range.
The brown Treesnake is reportedly fairly common in New South Wales and is likely to be locally common everywhere else where it occurs in Australia. It is one of the most common snakes in the Central Province of Papua New Guinea and perhaps through most of lowland New Guinea. Curiously, the species is reported to be uncommon in the Solomon Islands, which may be more attributed to its cryptic coloration in vegetation than actual absence. On the different islands where the brown Treesnake is common, its coloration and scale patterns vary. Based on this variation, the snakes on Guam are most similar to those of the Admiralty Island group of northern Papua New Guinea. This area had large military bases in World War II and is the most probable region from which the snakes on Guam were inadvertently carried as accidental stowaways in military cargo.
Habitat
Despite its common name, the brown Treesnake is not restricted to forested habitats but occurs 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 palms, hollow logs, rock crevices, caves, and even the dark corners of thatched houses near the roof. Based on frequent mention of this snake in relation to buildings, domestic poultry, and caged birds, the snake is common in human-disturbed habitats and second-growth forests.
The local abundance of the brown Treesnake in its native habitat frequently causes people to ask, "What limits this snake's populations in its native range?" The answer to this question is that the snake's food source is far more limited in its native range than the virtual feast that awaited it on Guam, and equally optimal conditions remain a possibility on other snake-free oceanic islands if the snake colonizes them. 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 and had no natural defenses, 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.
Identification
Generally 3-6 feet in length in its native
range, this snake has been able to attain lengths of over 10 feet on Guam
(where food supplies are more plentiful and conditions for long survival
ideal). The snake is long and slender, which facilitates its exceptional
climbing ability and allows it to pass through tiny spaces in buildings
where it seeks undisturbed refuge during daylight hours. 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. On Guam, the coloration is more consistently patterned
in a brown/olive green with shadowlike markings, which camouflage it well
in the vining foliage. 
Biology
Diet
The brown Treesnake is known to eat a wide variety of foods, a factor that helps make it such an effective colonizer. These snakes eat frogs, lizards, small mammals, birds, and birds' eggs. In Papua New Guinea, eggs and chicks are regularly consumed, but mammals are more frequently taken. Frogs and other snakes are eaten occasionally. Small snakes depend primarily on lizards, small birds, and eggs of lizards and birds, whereas larger individuals feed to a greater extent on adult birds, mammals, and larger prey items. Having nearly depleted the bird populations on Guam, larger snakes have been found scavenging garbage and even sneaking in to steal a hamburger off the barbeque!
Reproduction
The reproductive
characteristics of the brown Treesnake are poorly known. The female
produces 4-12 oblong eggs, 42-47 mm long and 18-22 mm wide. They
have a leathery shell and often adhere together after the shells dry.
The female deposits the eggs in hollow logs, rock crevices, and other
sites where they are likely protected from drying and high temperatures.
Females may produce two clutches per year, but the timing may depend on
seasonal variations in climate and prey abundance. Like other snake
species, the female may be able to store sperm and produce eggs over several
years after mating.
Defensive Behavior and Toxicity
When threatened,
the brown Treesnake is extremely aggressive and likely to lunge and bite
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.
Thus, the mouth must be opened as wide as possible to insert these fangs,
and a deliberate chewing movement is employed 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 normally feeds but is not considered
dangerous to adult humans. The snake often wraps its body around
the prey to immobilize it while chewing on the animal to inject the venom
with its grooved teeth.
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