Socioecology of the Vicuna
Abstract
The social organization of the vicuna (Vicugna vicugna) and its relationship to the environment were studied at the Pampa Galeras National Vicuna Reserve in southern Peru from May 1968 to April 1971. 2 Behavioral data were collected on groups within a grid of 12.4 km during 3,800 hours of observation. The vicuna's alpine grassland environment was characterized by greatly fluctuating and freezing temperatures, light precipitation, high evaporation, wind, short growing seasons, shallow and infertile soils and low plant production. Vicuna habitat quality was dependent upon high producing, preferred vegetation types and close access to permanent water and a sleeping area. Only one-third of the Reserve was good habitat and less than 20 percent of the Cupitay Valley study area was preferred by feeding vicuna. Territorial behavior has evolved in this environment where food resources are generally in short supply, annually renewed on locally abundant patchy sites, but predictable in time and space and defendable. Nearly 90 percent of females 2 years and older were pregnant at the beginning of the birth season and 90 percent of all births occurred between 22 February and 7 April. Ten to 30 percent of the crias (juveniles) died during their first 4 months. Five social units were distinguished, with the population primarily composed of Male Groups (MGs) and family groups: Permanent Territorial Family Groups (PTFGs) occupied good habitat and Marginal Territorial Family Groups (MTFGs) secondary habitat. Most PTFGs and MTFGs each occupied a year-round feeding territory in the day and a separate sleeping territory at night. Average group size for PTFGs was six (one male, three females and two crias) and they composed about 55 percent of the population. MTFGs were smaller (five), composed 40 percent of the population, had lower reproductive success and spent less time in their feeding territories than PTFGs. Non-territorial MGs averaged 20 animals and were chased out of zones occupied by PTFGs and MTFGs. As the number of groups increased, the frequency and intensity of territorial defense by males also increased. Territories provided a place free from intraspecific interference where females could feed, sleep, mate, give birth and raise their young. Feeding territories averaged 18 ha and sleeping territories 3 ha in size. A small cluster of sleeping territories on the flattened ridge formed a cummunal sleeping area for groups using the adjacent valleys. MGs and MTFGs made greater use of ridges and the non-preferred bl.lllch grass communities, because they were excluded from preferred areas occupied by PTFGs. Group size was significantly (P<.05) correlated with territory size and total forage production (P<.01) within feeding territories. The territorial male had a major role in the social organization of a population, since he established the location, borders and size of feeding territories. He also regulated the size of his family group by expelling the lowest ranking and youngest members before they became 1 year old (male and female crias), and by accepting or rejecting outsiders attempting to join. Marginal habitat within Cupitay Valley and vacant habitat in surrounding areas became filled by dispersing animals as the total population in the Reserve increased 36 percent during the study.