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

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Title:Population Ecology of the Polynesian Rat, Rattus exulans, on Kure Atoll, Hawaii
Authors:Wirtz, William O. II
LC Subject Headings:Polynesian rat
Date Issued:Oct 1972
Publisher:University of Hawaii Press
Citation:Wirtz WO II. 1972. Population ecology of the Polynesian rat, Rattus exulans, on Kure Atoll, Hawaii. Pac Sci 26(4): 433-464.
Abstract: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.
Appears in Collections: Pacific Science Volume 26, Number 4, 1972

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