Guanaco social organization: harem size variation supports Jarman´s hypotheses

MARINO ANDREA; BALDI RICARDO; NOVARO ANDRÉS J.

Mendoza

Congreso; 10th International Mammalogical Congress; 2009

SAREM

In his seminal study on antelope social organization, Jarman (1970) proposed that predation risk and feeding habits were major factors shaping ungulate grouping patterns. According to current theory, higher predation risk, as well as the predominance of grasslands, would favor the formation of larger social groups. However, few studies have assessed these effects simultaneously on a single species. Guanacos have a flexible behavior both in their feeding habits and social organization, allowing for comparative studies in contrasting ecological scenarios. We modeled guanaco group size (i.e. n° of adults in family groups) across five populations in order to test Jarman predictions. We fitted a nested random model to account for variation inherent to each site at different scales, where vegetation type at stratum (km2) and patch (m2) scales where the random factors selected. Subsequently, we tested the effects of predation, vegetation structure at patch scale, and local density, on group size. As predicted by Jarman, guanaco groups were larger at sites where predation rate was high than at predator free sites, and group size was larger in grasslands than in shrublands.