Blood pythons in northeastern Sumatra display a series of discrete colour morphs, even among hatchlings within a single clutch. The first step towards understanding the maintenance of this polymorphism is to test the null hypothesis that colour variation in this species has no major biological correlates. Data on > 2,000 blood pythons killed for the commercial leather industry enabled us to test, and reject, this hypothesis. The four colour morphs differed significantly in most of the traits that we measured, including temporal and spatial abundances, sex ratios, age structures, mean adult body sizes, body shapes (tail length and body mass relative to snout-vent length), energy stores, numbers of gut parasites, prey types, feeding frequencies and clutch sizes. The causal basis for these associations remains unclear, but is likely to involve three processes: direct effects of colour, linkages between genes for colour and other traits, and correlated spatial heterogeneity in colour, morphology and ecology. The colour polymorphism may be maintained by frequency-dependent selection and genotype-specific habitat selection, because these sedentary ambush predators are under strong selection for effective camouflage to hide them from both predators and potential prey. In support of this hypothesis, similar colour polymorphisms have evolved independently in several other snake taxa that rely upon ambush predation.
- Chromatic polymorphism