Ecological Correlates of Group-Size Variation in a Resource-Defense Ungulate, the Sedentary Guanaco

Abstract

For large herbivores, predation-risk, habitat structure and population density are often reported as major determinants of group size variation within and between species. However, whether the underlying causes of these relationships imply an ecological adaptation or are the result of a purely mechanistic process in which fusion and fragmentation events only depend on the rate of group meeting, is still under debate. The aim of this study was to model guanaco family and bachelor group sizes in contrasting ecological settings in order to test hypotheses regarding the adaptive significance of group-size variation. We surveyed guanaco group sizes within three wildlife reserves located in eastern Patagonia where guanacos occupy a mosaic of grasslands and shrublands. Two of these reserves have been free from predators for decades while in the third, pumas often prey on guanacos. All locations have experienced important changes in guanaco abundance throughout the study offering the opportunity to test for density effects. We found that bachelor group size increased with increasing density, as expected by the mechanistic approach, but was independent of habitat structure or predation risk. In contrast, the smaller and territorial family groups were larger in the predator-exposed than in the predator-free locations, and were larger in open grasslands than in shrublands. However, the influence of population density on these social units was very weak. Therefore, family group data supported the adaptive significance of group-size variation but did not support the mechanistic idea. Yet, the magnitude of the effects was small and between-population variation in family group size after controlling for habitat and predation was negligible, suggesting that plasticity of these social units is considerably low. Our results showed that different social units might respond differentially to local ecological conditions, supporting two contrasting hypotheses in a single species, and highlight the importance of taking into account the proximate interests and constraints to which group members may be exposed to when deriving predictions about group-size variation.

Spatial patterns of African ungulate aggregation reveal complex but limited risk effects from reintroduced carnivores

“…In the first step of model selection, we built a global model a priori that included carnivore UDs, carnivore kill occurrence, amount of hard edge (linear length in meters), and degree of openness (proportion of open habitat within a 250 m 2 cell). We included hard edge and openness covariates due to associations between open habitats, increased visibility, and aggregations of African ungulates (Evans 1979, Gerard and Loisel 1995, Pays et al 2007, Creel et al 2014, Marino and Baldi 2014. We then evaluated models developed from all possible linear combinations of these six covariates.…”

“…Here, we aimed to clarify the relative contribution of multiple forms of landscape-level predation risk on patterns of aggregation for six species of African ungulates exposed to multi-carnivore predation. A key objective of our study was to contextualize the topdown influence of carnivores on ungulate aggregation by comparing its effect size with that of an established relationship between ungulate aggregation size and habitat structure (i.e., aggregations tend to be larger in more open habitat; Evans 1979, Gerard and Loisel 1995, Pays et al 2007, Thaker et al 2010, Marino and Baldi 2014. In addition, our study evaluated the hypothesis that carnivore risk effects on ungulates will vary by hunting mode and by prey species, as has been indicated in predator-prey systems across a diversity of taxa (Preisser et al 2007).…”

Predator occurrence and perceived predation risk determine grouping behavior in guanaco (Lama guanicoe)

“…Aggregation and changes in prey group size as a response to risk have previously been reported in many herbivores (e.g., Banks, 2001;Creel et al, 2014;Moll et al, 2016Moll et al, , 2017. In the case of guanaco, Marino and Baldi (2014) reported larger families in populations coexisting with pumas than in predator-free reserves.…”

Vigilance behaviour of the year-round territorial vicuña (Vicugna vicugna) outside the breeding season: Influence of group size, social factors and distance to a water source

“…Territorial males were distinguished from bachelor males because they tend to be some metres away from females and calves of the group, often showing territorial displays towards other vicuñas, as chases and defecating (Franklin, 1983;Marino and Baldi, 2014). In contrast, bachelor groups are composed mostly by juvenile and adult males who perform different activities synchronously and are often aggregated (Vilá and Cassini, 1993;Marino and Baldi, 2014). Group size was recorded as the number of individuals in the group at the beginning of a sampling.…”