Quelques aspects de la socioécologie de la Vigogne Lama vicugna

Résumé En

A population sample of vicunas has been studied during 1979-1980 in the Pampa Galeras National Reserve in Southern Peru. Since 1965 the populations of this species which were previously on the verge to extinction, have increased sharply following effective measures of protection and good pasture conditions favoured by a succession of wet years. The social structure of the population studied includes groups of bachelor males roaming over extensive ranges, and harems settled on smaller territories and defended by an adult male. The observations were mainly focused on two family groups : — the Canllapampa Valley harem, composed of one male, one adult female and her young, one 2-year old female and one yearling female ; — the Llamaiso Valley group, composed of one male and one adult female with her young. Since 1974, there has been a severe drought in the Reserve, causing changes in the social organisation and the social structure of the whole population, as well as in the pattern of range use. Therefore, our 1979-1980 observations are compared to those of Franklin, carried out in 1971, during a typical rainy year. Since that time annual censuses have shown that the population has increased regularly until 1978 and that its demographic structure changed, through an increase in the nomber of bachelors. Whereas the size of reproductive territorial groups, as well as the surface area of their territories, decreased during that period, the mean reproduction rate changed from 74 % in 1971, to only 33 % in 1979 due to numerous miscarriages. Our 1979-1980 observations indicate that the males on Canllapampa became aggressive towards juvenile females during the dry season (from May through November 1979) before expelling them. Moreover the one adult female studied suckled her young until February, whereas weaning occured between September and December in 1971. This indicates a tendency to breed only once every second year. All the year round, vicunas presently spend an average of 89 % of their time grazing, without any significant seasonal variation. During the dry season vicunas may temporarily leave the male territories and cover great distances to graze on other pastures. All these changes which have occured after several dry years seem to indicate that the vicuna population is presently facing very harsh living conditions on very poor pastures. Indeed, the population began to decrease in numbers during 1979.