Diet and energetic constraints of an earthworm specialist, the Mesa Central Blotched Garter Snake (Thamnophis scaliger)

Abstract

Optimal diet theory predicts that predators optimize energy intake by balancing costs and benefits of foraging. One extreme strategy of snake foraging ecology is shown by specialist species that forage on low-energy prey, such as Thamnophis scaliger (Jan, 1863) which feeds almost exclusively on earthworms. Compared with other prey types such as small mammals, lizards, or arthropods, earthworms are low-energy prey because of their small size and high water content. Given the importance of energy acquisition for fueling snake reproduction, we expect that a low-energy dietary specialist such as T. scaliger needs to forage frequently to store enough fat to reproduce. The high frequency of snakes containing prey, the presence of multiple earthworms in snakes, and the fact that females continue to feed when gravid suggest that T. scaliger is a voracious consumer of earthworms. Despite these foraging behaviours, females did not reproduce in sequential years, suggesting constraints in energy input to reproduce more frequently. A meta-analysis of the diet, body size, and reproductive frequency of some species of the genus Thamnophis Fitzinger, 1843 confirms that consumption of invertebrate prey is associated with small snake size, but not with biennial reproductive frequency within the genus.