The allometry of life-history traits: insights from a study of giant snakes (Python reticulatus)

Abstract

Many life-history traits are allometrically tied to body size, in snakes as well as in other animals. Two problems with previous analyses on this topic for snakes are that: (i) the range of body sizes within species is generally small, so that most analyses have relied on interspecific comparisons; and (ii) available data have been heavily biased towards small, temperate-zone colubroid species. We test the generality of results from previous work with data on a species of giant tropical snake that is phylogenetically and ecologically distinctive, and encompasses such a massive size range that we can examine intraspecific allometries. We use data from >1,000 field-collected reticulated pythons from southern Sumatra to ask two questions: (i) do life-history traits show intraspecific allometries similar to those revealed by interspecific comparisons?; and (ii) are mean values for life-history traits in a giant snake consistent with allometric trends in smaller species? As predicted, strong intraspecific allometry was evident for most of the life-history traits we measured, including reproductive output (e.g. clutch size, frequency of reproduction in females, testis volume relative to body mass) and energy stores (relative size of the abdominal fat bodies). For many traits (e.g. the means and variances of clutch sizes and maternal body sizes, relative offspring size, body size at maturation relative to size at hatching and maximum adult size), these giant pythons were near or beyond the extremes reported for smaller species of snakes, supporting the importance of allometry. None the less, reticulated pythons deviate from many of these previously-documented allometries in significant ways, suggesting that current generalizations about life-history allometry in snakes may be premature.