Guanaco social organization: variation in harem size supports Jarman’s hypotheses.

MARINO A.; BALDI R.; NOVARO A.

Mendoza, Argentina

Congreso; 10th International Mammalogical Congress; 2009

Since Jarman’s (1974) seminal study on antelope social organization, predation risk and feeding habits have been accepted as major factors shaping ungulate grouping patterns. However, few studies have assessed the effects of these factors simultaneously on a single species. According to current theory, high predation risk and predominance of grasslands favor the formation of larger social groups in open-dwelling ungulates. Guanacos (Lama guanicoe) have a flexible behavior both in their feeding habits and social organization, allowing for comparative studies in contrasting ecological scenarios. We modeled guanaco harem size (i.e. n° of adults in family groups) across five populations in order to test Jarman´s 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. Afterward, we tested the effects of predation risk, vegetation structure at patch scale, and local density by fitting a fixed effects model. We found that guanaco groups were larger at sites where predation risk was high than at predator-free sites, and harem size was larger in grasslands than in shrublands. These results support the idea that antipredator benefits induce group formation whereas dispersion and availability of key resources place the upper limit on guanaco group size.