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.