Prey abundance and predator reproduction: rats and pythons on a tropical Australian floodplain

A-5-yr field study examined the ways in which prey abundance can influence the reproductive rates of predators. Water pythons (Liasis fuscus) on the Adelaide River floodplain in tropical Australia prey almost exclusively on dusky rats (Rattus colletti). Rat numbers varied dramatically among years of the study. Feeding rates of pythons were highest when rats were abundant, and the snakes were in good condition (mass relative to body length) at these times. The proportion of adult female pythons that reproduced in a given year was also tightly linked to rat abundances. However, reproductive output per litter (offspring size, and fecundity relative to maternal body size) was unaffected by prey availability, perhaps because of high fecundity-independent costs of reproduction. Instead, female phytons reproducing in 'good' years where in better condition after oviposition. These data support an energy-limitation model for snake reproduction, whereby prey availability determines predator reproductive output, but the relationship between the two variables is complex. The degree to which prey availability influences feeding rates of phytons and the degree to which feeding rates influence body condition depend crucially on snake body size (due to size-related shifts in foraging abilities and metabolic costs). Thus, a given prey abundance translates into different energy reserves for different-sized predators. Also, females delay reproduction until they can gain enough energy for a large clutch of eggs, and reproductive output per litter does not then increase with additional energy availability. Such nonlinearities in predator responses to prey availability may be widespread.