Conservation ecology of a nationally threatened reptile, the Green Python, Morelia viridis

A project undertaken at the Centre for Resource and Environmental Studies, Australian National University, and supervised by David Wilson and Rob Heinsohn

The green python (Morelia viridis) is an iconic rainforest species in Australia. It is used to promote values of pristine wilderness helps attract many people to zoos and reptile parks in Australia. The species is also famed for having some of the brightest colours and one of the most amazing colour changes in the animal kingdom. When young are born they are either a bright yellow or a brick red colour, but both morphs change to a vivid green later in life. This has made them one of the most popular snakes in the captive pet industry, increasing demand for wild individuals for private collections. What many people don’t realise is that we know almost nothing about this species in the wild – not even basic information about what habitat it requires, what it eats or how many there are. Due to this, its conservation status has never been determined.

This project sought to answer these basic questions and, hopefully, shed some light on both why there are two juvenile colour morphs and why they turn green.

All fieldwork was done during three wet season at the Iron Range National Park, on Cape York Peninsula, where most sightings of this species have occurred in Australia. There were three aspects of fieldwork – surveys to determine population characteristics, radio-tracking to understand individual behaviour, and photospectrometry to analyse colour influenced their ecology.

Surveys revealed that the population at Iron Range is large and long-lived. Over two hundred individuals were caught in the study area, which equates to a density of approximately 4 per hectare. They preferred primary lowland rainforest, but a few individuals were found in mature regrowth areas. Males mature in their third year, while females wait until their fourth year and both can live for at least 12 years. Juveniles hatch in late November, but reproduction is low in any given year. Only yellow juveniles were caught, with the colour change occurring at approximately 55 centimetres, which equates to animals that are about one year old.

Radio-tracking showed that males and females have different behaviours. Both sexes move slowly (an average of seven metres per day), however females have a home range while males appear to randomly roam throughout suitable habitat. Juveniles move less (an average of three metres per day), and are restricted to the edge of the rainforest or in treefall gaps. Juveniles also feed mainly during the day – for ground dwelling lizards and small invertebrates near the ground. Green individuals hunt primarily at night for terrestrial rodents, but sometimes they would hunt in the canopy for birds attracted to flowering trees. Juveniles are too small to physically swallow rodents – the minimum size this is possible corresponds to the size at which they change colour.

Measuring the colour of the pythons and that of their background allowed us to determine how camouflaged each colour is, both against its natural background, and against other backgrounds in the rainforest. This showed that yellow individuals were most camouflaged near the ground in open areas than anywhere else. Green individuals were most camouflaged in the canopy, but importantly relatively more camouflaged than yellow individuals in open areas. I was also able to measure captive red juveniles and these were most camouflaged near the ground in the closed canopy rainforest. This it appears that the two juvenile morphs have evolved to best survive in different environmental conditions, while the colour change has evolved to decrease the risk of predation once individuals are able to hunt at night.

After three years we now know much more about the ecology of the green python in the wild, and the reasons behind both its amazing colours, and the remarkable colour change. We also know that the species is common in suitable habitat and not threatened with extinction.

Publications resulting from this research:
Wilson, D. and R. Heinsohn. (in review). Geographic range, population structure and conservation status of the green python (Morelia viridis), a popular snake in the captive pet trade. Australian Journal of Zoology.

Wilson, D., R. Heinsohn and J. Endler. (in press). The adaptive significance of ontogenetic colour change in a tropical python. Biology Letters.

Wilson, D. (in press). Foraging ecology and diet of an ambush predator: the green python Morelia viridis in R. Henderson, and R. Powell, editors. Biology of the Boas and Pythons. Eagle Mountain Publishing, Eagle Mountain, Utah.

Wilson, D., R. Heinsohn, and S. Legge. (2006). Spatial ecology of dichromatic green pythons (Morelia viridis) in Australian tropical rainforests. Austral Ecology. 31:577-587

Wilson, D., R. Heinsohn, and J. Wood. (2006). Life history traits and colour change in the arboreal tropical python Morelia viridis. Journal of Zoology (London). 270:399-407

Figure 1. Adult green python in a typical humting posture.

Figure 2. Juvernile green python in a typical resting posture.

Figure 3. David and Ellie measuring an adult green python.