Green Tree Python

The green tree python (Morelia viridis), also called the green python or simply tree python, is a non venomous constrictor that lives in the rainforests of New Guinea, nearby islands of eastern Indonesia and the tip of Cape York in northern Australia. It belongs to the family Pythonidae, and its closest relative is the rough scaled python of northwestern Australia. It was first described in 1872 by the Dutch zoologist Hermann Schlegel, from two specimens collected in the Aru Islands, and for much of the 20th century it was placed in its own genus, Chondropython. Its classification changed again recently. A genetic study published in 2020 by Daniel Natusch and colleagues found that green pythons form two species, separated by the high central mountains of New Guinea. Pythons from northern New Guinea are now the northern green python (Morelia azurea), while those from southern New Guinea, the Aru Islands and Australia keep the name Morelia viridis. Both look almost identical and live the same way, so this page covers them together. With its emerald body draped over a branch, it is one of the most recognizable snakes on Earth.

Few snakes are as striking as an adult green tree python. Its scales are a vivid green that blends into the canopy, and many individuals carry a line of white or yellow scales along the spine, while some show patches of blue. The head is large and clearly wider than the neck, the eyes have vertical pupils suited to low light, and the scales along the lips carry heat sensing pits that detect the warmth of nearby animals. Green tree pythons are slim for their length. Adults typically measure 150 to 180 centimeters, and large females reach about 2 meters. Males usually weigh 1.1 to 1.4 kilograms and females up to about 1.6 kilograms, so even a big one is far lighter than ground dwelling pythons of the same length. The tail, about 14 percent of the total length, is strongly prehensile and acts like a fifth limb, anchoring the snake to a branch while it reaches out or hangs down. The species' trademark is its resting posture: the body drapes over a branch in neat, saddle like loops, with the head tucked in the middle of the coils.

A newborn green tree python looks like a completely different snake. Hatchlings are either lemon yellow or golden to orange red, sometimes brick red or brown, and both colors can appear in the same clutch. They keep that color for about their first year. When a yellow juvenile reaches roughly 58 to 60 centimeters long, it turns green over just 5 to 10 days, a change that can be followed almost day by day. Studies of wild pythons at Iron Range in Cape York help explain why. Young pythons do not live where adults do: they hunt at the edges of the forest and in gaps where sunlight reaches the low vegetation, and there they eat mainly small lizards and insects. Against that brighter, more varied background, yellow and red are better camouflage than green. Some juveniles have also been seen wiggling the tip of the tail to lure prey closer. When they grow large enough to take mammals, they move into the closed canopy, switch to rodents and become green, the color that hides them best among the leaves.

Green tree pythons live in and around the rainforests of New Guinea and some nearby islands. On New Guinea they are found in both Papua New Guinea and the Indonesian provinces of Papua, from sea level up to about 1,800 meters, with the northern species on one side of the central mountain chain and the southern species on the other. In Indonesia they also live on islands of Maluku, including the Aru Islands, where the species was first described. In Australia the green tree python is restricted to a small part of Cape York Peninsula in far northern Queensland: the rainforests of the Iron, McIlwraith and Kawadji–Ngaachi ranges, covering an estimated 2,289 square kilometers. A survey there located 298 pythons but found none in the Lockerbie Scrub or the Jardine River catchment, despite old anecdotal records. Within the forest, adults live mostly in the trees, using the mid and upper canopy where leafy branches hide a coiled snake, while juveniles favor forest edges and clearings. Lowland and hill rainforest is their main home, though they also use monsoon forest, forest margins and secondary growth.

Despite its reputation as a bird hunter, the adult green tree python feeds mostly on small mammals, especially rats and mice. In a classic study of more than 1,000 stomach contents, the herpetologist Karl Switak found no birds at all, and later field work has confirmed that rodents are the main prey of adults, with geckos, skinks and other lizards taken too. Young pythons eat differently: they rely on small lizards and insects, which they find at the forest edge, and only switch to mammals as they grow. The green tree python is an ambush predator. It usually waits at night, anchored to a branch by its prehensile tail, with the front of its body hanging in an S shaped curve, ready to strike at prey moving below. In the dark it locates warm blooded prey with the heat sensing pits on the scales of its lips, which detect infrared radiation. Once it strikes, it holds the animal with its long front teeth, wraps it in coils and constricts it until it stops breathing, then swallows it head first.

By day, an adult green tree python usually rests motionless in its coiled posture on a favorite branch, well hidden among the leaves. At night it becomes active, moving through the canopy or coming lower to hunt, and radio tracking in Cape York has shown that pythons regularly change perches from one night to the next. It is a solitary snake. When disturbed it may draw its head back into an S shaped coil and strike, and it is known for being more irritable than many other pythons. Little is known about breeding in the wild, where mating has rarely if ever been observed, but in captivity females lay clutches of 1 to 25 eggs, in a sheltered spot, and coil around them to protect them and keep them warm and moist until they hatch. Life in the wild is hard: at Iron Range, pythons lived an average of only 3.4 years, though some reach at least 15 and the oldest recorded wild python lived 19 years. Their predators include rufous owls, goshawks and other hawks, the New Guinea harpy eagle, mangrove monitors, dingoes and New Guinea quolls.

Many people confuse the green tree python with the emerald tree boa of South America, and searches for "emerald python" usually mean one or the other. The two are a textbook case of convergent evolution: separated by the Pacific Ocean and only distantly related, they evolved the same bright green color, the same yellow or red babies that turn green, the same prehensile tail and the same habit of resting draped over a branch with the head in the middle of the coils. There are clear differences, though. The emerald tree boa (Corallus caninus) lives in the Amazon and Guiana rainforests, while the green tree python lives in New Guinea, eastern Indonesia and northern Australia. The boa usually has bold white bands across its back, like lightning bolts, while the python's white or yellow marks tend to run along the spine as a broken line or scattered dots. The heat sensing pits of the boa sit between the scales of its lips, while the python's are inside the lip scales. And the biggest difference is how they reproduce: the emerald tree boa gives birth to live young, while the green tree python lays eggs.

The IUCN lists the green tree python as Least Concern, because it has a fairly large range across New Guinea and nearby islands and much of its rainforest habitat remains. The IUCN does, however, point to illegal collection as a threat that needs monitoring. Green tree pythons are among the most sought after snakes in the reptile trade, and in Indonesia many supposedly captive bred animals are taken from the wild. An investigation covering 2009 to 2011 found that about 80 percent of the green tree pythons exported from Indonesia as captive bred had in fact been caught in the wild, around 5,337 snakes a year, and up to half of smuggled animals are thought to die along the way. This pressure falls hardest on small, isolated island populations and on color forms that collectors prize. Logging, oil palm plantations and mining also clear rainforest in parts of New Guinea. In Australia the species is protected, and its small Cape York range lies largely within protected land such as Kutini-Payamu (Iron Range) National Park, but its restricted area makes it vulnerable to fire, cyclones and illegal collection.