Population Dynamics of the Spiny Rat Proechimys semispinosus on Orchid Island (Panama)

Abstract

Seasonal variation in population density, reproduction, and mortality was observed in an island population of the tropical forest rodent Proechimys semispinosus. Average density was 8.5 individuals per hectare, with the maximum in August being 1.3 times higher than the minimum in April. Reproduction continued year round, although breeding was most intensive in the dry season. The highest proportion of trappable young animals in the population occurred in July-August. Mortality was relatively constant throughout the year, but increased slightly from the dry season to late in the rainy season. Observed seasonal variation in density of spiny rats is caused by seasonal changes in reproduction intensity. The timing of reproduction is such that most young are born in the period of the richest food supply. This guarantees optimal food conditions for lactating females and weaned offspring. MAMMALIAN HERBIVORES IN TROPICAL RAIN FORESTS are provided with a warm and relatively stable climate, an adequate food supply throughout the year, and stability of many other biotic factors. These allow the animals to avoid high and varying costs of thermoregulation, to maintain stable patterns of behavior, and to continue reproduction year-round. Yet, although seasonal variation in tropical forests may be less marked than in temperate habitats, it nevertheless may have important effects on populations of tropical mammals. Proechimys semispinosus (Tomes) is a large (body weight up to 500 g), nocturnal, frugivorous rodent, common in the tropical rain forests of Panama. Its ecology is known from earlier study by Fleming (1971) in Panama. This paper, besides supplying some new details to the existing body of information on the species ecology, presents the dynamics of a population confined to a small island, where no immigration or emigration was possible. This simplified situation allowed an assessment of the links between population dynamics and seasonal patterns of reproduction, corresponding with seasonal variation in food abundance.