Insular Radiation in Life History of the Lizard Eumeces okadae in the Izu Islands, Japan

Abstract

The life history of Eumeces okadae, an endemic lizard of the Izu Islands, Japan, was studied to determine whether life-history traits varied in relation to island biota. Analysis of predation revealed that populations of E. okadae were classifiable into three types that suffered predation mainly from insectivorous birds, snakes, or carnivorous mammals. Extremely high population densities (>100 lizards seen per hour of census) were found on islands where only avian predators were present. A suite of traits, including annual reproduction, large clutches (8.8) of small eggs (412 mg), and early maturity (2 yr) with a small body size at maturity (64 mm SVL), was found on islands with a high predation intensity mainly from carnivorous mammals. Biennial reproduction, small clutches (6.57.5) of large eggs (569-591 mg), and delayed maturity (3-4 yr) with a large mature size (72-75 mm) were found on islands with low overall predation but potentially severe predation on juveniles from insectivorous birds. Intermediate life-history traits were found on islands with moderate predation pressure from snakes. Analyses of prey abundance and stomach contents suggested that E. okadae on high-density islands experienced low prey availability, probably because of exploitation and intraspecific competition. Strong predation impact, along with variation in prey availability and intensity of intraspecific competition, have molded the ecological course of divergence in the life history of different E. okadae populations.