Cross checking censuses and a model of the annual cycle of mortality and reproduction in capybaras(Hydrochaeris hydrochaeris)

Abstract

Improved management of capybaras (Hydrochaeris hydrochaeris) for meat and hide production requires better knowledge of the status of the population throughout the year, its density, as well as monthly rates of mortality and reproduction. This study was made on 50,500 ha of a ranch in the Llanos of Venezuela. A model of the annual cycle of mortality and reproduction of a hypothetical population of 50,000 capybaras was constructed, based on age composition information from 1,269 capybaras harvested in March over a period of 7 years. Age was determined by the lens technique and the growth curve of the lens and a table of lens weights and corresponding age are presented. Because censuses of wildlife are dependent on assumptions which frequently are fulfilled to an unknown degree and fiducial limits may be large, here three cross checking censuses were made. The censuses varied from 3 to 8% of their mean. The analytical model predicted a 26.02% reduction of the population during the 2 months when the annual harvest occurred and the strip censuses showed a reduction of 27.63%, a degree of agreement lending credibility to both the model and the censuses. A table of the annual cycle of births based on age composition of a cohort of 358 capybaras is presented showing births throughout the year but with a peak in September and 56% being born from August to November. The mean monthly probability of dying for adult capybaras was calculated to be 0.075 based on a sample of 1.199 animals greater than 12 months old. The oldest capybaras were estimated to be 5 years of age.