Correlation of a Population of Roof Rats in Venezuela with Seasonal Changes in Habitat

Publication Type:Journal Article
Year of Publication:1960
Authors:N, JCGomez
Journal:American Midland Naturalist
Volume:63
Pagination:177-193
Date Published:1960
ISBN Number:00030031
Keywords:Rattus rattus
Abstract:

The general purpose of a study of Roof Rats (Rattus rattus) was to consider the correlation of seasonal fluctuations of the habitat on the rat population in respect to the population size, distribution, growth, movement, and probabilities of survival in an insular population in Venezuela. Zorro Island has an area of 6.5 hectares and is separated from the nearest shore by a distance of 750 meters. Its abrupt terrain is covered by a dense tropical deciduous seasonal forest. Besides the rats, other mammals found are bats and occasionally opossums. A great variety of birds and reptiles completes the fauna. The conclusions are based on the observations obtained from the systematic capture, marking, and release of individuals during a period of two years. A total of 153 different rats, 89 males and 64 females, were observed 327 times. No difference was noted between the trapability of males and females. Although the individual growth rate of the rats varies according to season, the average for both sexes ranges from 38 grams during the first month to 15 grams per month during the period between 3 and 6 months. Great seasonal changes affect the environment; maximum rainfall occurs in August reaching a monthly precipitation of about 200 mm, while during the first three months of the year there is slight or no rainfall. The temperature and humidity do not suffer such extreme fluctuations. These changes presumably produce unevenness in the supply and distribution of food, thus creating marked seasonal variations in the characteristics of the ecosystem's biotic components. During the rainy season the number of rats reached a maximum of about 40 on the island, later declining to about 10 at the end of the dry season. These individuals are not evenly distributed over the island. Furthermore the distribution also varies according to the supply of food and shelter. Immigration occurs on rare occasions, but emigration is unlikely due to local conditions. Individual movement appears greater for the males than for the females. During the dry season only sexually mature rats were observed but during the rainy season many immatures are present. Breeding occurs throughout the year. Sexual maturity is reached at a weight of about 70 grams and at an age varying from 1.5 to 2.5 months. The life expectancy appears to be influenced by the relation of the month of birth to the seasons. Rats born in the dry season survived through the wet period but few born in the wet season survived through the dry. Although some rats were observed during periods as long as 15 months the average life expectancy is about 5 months.

URL:http://www.jstor.org/stable/2422939
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