Population Ecology of the Polynesian Rat, Rattus exulans, on Kure Atoll, Hawaii

ABSTRACT: A population of the Polynesian rat, Rattus exulans, was studied at Green Island, Kure Atoll, in the northwestern Hawaiian Islands from September 1963 through August 1965. The island covers 214 acres, of which 144 are vegetated, and all vegetated areas are utilized by the rat. Other vertebrates on the island and its shores include 14 species of breeding seabirds and the Hawaiian monk seal. Usual predators of rodents are absent and seasonal climatic fluctuations are slight. Juveniles are weaned and leave the nest at 2 to 3 weeks. A prolonged maturational molt occurs between the ages of 4 and 9 weeks. Most young, or nonbreeding, adults overwinter before attaining sexual maturity. Once sexual maturity is attained males remain fertile throughout the year. Reproduction is seasonal, most litters being produced from May through August and none from September through December. Mean litter size is four, and usually only one litter is produced per female per year for a mean of 4.77 young per mature female annually. About 360 young were produced on a 6.94-acre study area in one breeding season. The only ectoparasites recorded are the louse Hoplopleura pacifica and the mites Laelaps nuttalli and L. echidninus. Approximately 62 percent of the diet on Kure is composed of plant material, 30 percent of insects, and 8 percent of vertebrate flesh. Breeding males move significantly greater distances between captures and have significantly larger home ranges than breeding females. The size of the home range of breeding adults is not affected by density fluctuations or reproductive activity. Mean population density for the period studied is 45 rats per acre, or 6,480 individuals for the entire island, and density fluctuated from lows of 30 per acre in March of both years to a high of 75 per acre, about 10,800 on the island, in September 1964. Mean adrenal weight in subadult and adult animals is correlated with density, and also with reproductive activity in breeding adults. Mean body weight and extent of subcutaneous and mesenteric fat deposits are significantly reduced during the winter. Seasonal reproduction results in high population densities by early winter. Sharp declines in density are observed during the winter and early summer, in association with increased mean adrenal gland weight in early winter and decreased mean body weight and fat deposition in late winter and early summer. Because of the absence of predators and the apparent absence of epidemic disease it is suggested that population regulation is achieved chiefly through the interaction of density with available food resources; and with that portion of the population which cannot obtain sufficient food and/or shelter being eliminated during the winter to reduce numbers to a favorable balance with resources by early summer, at which time reproduction is again initiated among the surviving individuals