Ecological comparisons of island and mainland populations of Australian tigersnakes (Notechis: Elapidae)

A recent study by Schwaner of tigersnakes from offshore islands of South Australia documented inter-island variability in average body sizes, degree of sexual size dimorphism, adult sex ratio, and population structure. These variations were interpreted as adaptations to different prey availability (especially, prey sizes) on different islands. From this hypothesis, one would expect less variability in these characteristics in tigersnakes from mainland areas because of the greater geographical homogeneity in types and sizes of prey available on the mainland. 

This prediction was tested by measurement and dissection of 860 tigersnakes from throughout the taxon's range. Results were generally consistent with prediction. Mainland snakes fed mainly on frogs whereas island snakes fed mainly on mammals and birds. Island snakes averaged larger than mainland conspecifics. Very few juvenile snakes were collected on islands, perhaps reflecting low juvenile survivorship because of scarcity of small prey. Geographic variation in sexual size dimorphism and adult sex ratios occurred but may be due to sampling error rather than any real biological difference. Data are presented also on sex ratio at birth (1:1), fecundity (x̄ = 18.4, range 7-39), offspring size (12-22 cm SVL), incidence of infertility among oviductal ovocytes (11%), seasonal timing of the female reproductive cycle, and proportion of adult females that were reproductive (52%, suggesting "biennial" reproduction). No significant geographic variation was evident in reproductive biology. Overall, the ecological differences observed between island and mainland tigersnakes were consistent with the hypothesis that these variables are influenced by the types of prey available.