The work presented in this thesis represents the first detailed ecological information on the green python in the wild, and provides an important demonstration of both the adaptive significance of different colour morphs and the evolutionary advantage of ontogenetic colour change.
Key Findings
Several major conclusions can be drawn from this study:
i) Growth is described by the von Bertalanffy growth curve (von Bertalanffy 1957), with males maturing at 2.4 years and females at 3.6 years of age. Growth is rapid over the first few years and is indeterminate after approximately 10 years. There is no sexual dimorphism and the adult sex ratio is at parity.
ii) Breeding is strongly seasonal with hatching predicted to occur in late November of each year. Females reproduce on a less than annual basis, and reproductive rates appear to be low in the population studied.
iii) Males and females have dichotomous movement strategies. Females maintain a home range, the size of which is correlated with her snout-vent length, while males adopt a ‘roaming’ strategy through all suitable habitats. There was large overlap between individuals of the same sex, between the sexes, and between adults and juveniles.
iv) The colour change from yellow to green occurs between 53 and 58 centimetres, which equates to individuals of approximately one year old. This colour change is associated with changes in habitat preference, diet and behaviour. Each colour appears adapted for a different habitat – yellow for more open areas on the ground, red in closed canopy environments with few leaves and green individuals in the canopy. v) Green pythons have a high density at Iron Range (~4 per hectare), however they are restricted to relatively small areas of suitable habitat. In Papua New Guinea their predicted distribution is much greater, but densities in these habitats are unknown.
106 There are three important areas of research that remain unanswered.
i) We still need to understand the reproductive ecology of the green python in the wild. Despite spending considerable time in the field I never observed any reproductive activity. I saw few very young individuals, and none of the females I tracked became pregnant. From the growth rate we know that the hatching period is restricted in time, strongly seasonal and occurred while I was in the field. Further, very few juveniles were located during this study. This all suggests that breeding is rare, occurs on a greater than annual basis and clutch sizes are small or few neonates survive to a size where they can be captured. This information on the reproductive biology of green pythons is vital to understand fully how the population may change in the future. Specifically, the most important aspects that need to still be determined are how often females lay clutches of eggs, and what is the survival rate of individuals from hatching to adulthood.
ii) We need to have a greater understanding of green python ecology in New Guinea. This study focuses on a small, isolated population at the edge of the species’ range, and there is no indication as to whether or not behaviour recorded at Iron Range is typical of the species in New Guinea. There are a number of additional pressures in New Guinea that are not present in Australia which may impact adversely on populations. These include harvesting for food, habitat loss due to logging and poaching for the international pet trade. Although green pythons are eaten by local people (Igag pers. comm., 2002; Pasveer (2004)) they are not commonly encountered and considered to be rare in Papua New Guinea (pers. obs). Hence, although subsistence harvesting for food does not appear to have a high impact overall on populations of green pythons, it may maintain local population levels below maximum levels. In contrast, both logging and poaching may have significant widespread effects on populations. Although local people use small-scale slash and burn agriculture for farming, large-scale clearance by international logging companies may threaten large areas of potential green python habitat. Small numbers of adult green pythons were recorded from regrowth areas in Australia following logging; recolonisation of logged areas may be possible given a suitable lag period. Poaching for the captive pet-trade may also severely deplete local populations. The true effect of this trade is hard to quantify, although it may be
On green pythons 107
107 WCMC CITES trade database). While we know these threats exist, their effect cannot be properly quantified until we have a detailed understanding of the species’ ecology in New Guinea.
iii) There are still aspects of the green python colour morphs that are not fully understood. Many tropical and arboreal snakes are green, and this has always been attributed to them being more camouflaged in the canopy (Cott 1957), although this study provides the first supporting evidence. The two juvenile colour morphs are only matched by the emerald tree boa of South America, and this species has not been studied to determine the adaptive significance of its colour (Stafford & Henderson 1996). Although we know that the change from yellow to green in M. viridis occurs so rapidly and over such a short range of snout-vent lengths, the proximate reason and cue for the change at this restricted size is still unknown. Additionally, although my
analysis has shown that the red morph is most camouflaged in sub-canopy
environments with few leaves, this hypothesis needs to be tested in natural conditions. I hope that one day someone will find and study a group of red juveniles in the wild.
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