Geographic variation in sexual size dimorphism within a single snake species (Morelia spilota, Pythonidae)

Unusually among reptiles, Australian carpet pythons (Morelia spilota) display substantial geographic variation in mating systems and sexual size dimorphism. We studied a population of the south-western subspecies (M. s. imbricata) of this widely distributed taxon, on Garden Island near Perth, Western Australia. Our data greatly expand the range of variation previously documented for populations of this species. Unlike eastern Australian populations where sex differences in mean adult body size are relatively minor [<10% in snout-vent length (SVL), <30% in mass], female M. s. imbricata grow to over twice the length and more than 10 times the mass of adult males. Mean adult size averages 104 cm SVL (305 g) for males versus 214 cm SVL (3.9 kg) for females. This sex difference is a consequence of cessation in growth by males, in turn due to a reduced rate of feeding. Males display low feeding rates even in captivity, suggesting that their "dwarf" sizes reflect genetic control rather than local prey availability. Observations of free-ranging snakes suggest that males do not engage in overt agonistic interactions during the mating season, and that larger body size does not enhance male mating success. These results fit well with previous interpretations of the relationship between mating systems and sexual size dimorphism in snakes, including other populations of carpet pythons. M.s. spilota displays the greatest geographic variation in sexual size dimorphism yet recorded for any vertebrate species.