The majority – nearly three-quarters – of the guanacos in the sampled population were migratory. These migrations were typified by relatively short distances along a steep elevational gradient. Similar to other ungulate species in mountainous landscapes (49), and previous observations for guanacos elsewhere (35,50), guanacos in our study site spent the summer at elevations up 295–1105 m above their winter ranges. These elevational movements appeared to be influenced by both vegetation and snow phenology of the study area. Generally migratory guanacos avoided snow-covered areas by occupying snow-free areas at lower altitudes during the winter. This strategy also provided access to higher-quality forage as guanaco’s use of seasonal ranges coincided with the peak of vegetation green-up in both winter and summer ranges.
Guanacos exhibited a combination of fidelity and plasticity regarding migratory characteristics. Even though our data on multiple migration periods was limited, we found individuals that displayed fidelity to migration (for up to three migratory cycles) as well as individuals that switched strategies from one year to the next. Guanacos with multiple years of data that did not switch between strategies, tended to revisit the same summer and winter ranges, and exhibited similar residency times in their migratory ranges. With one exception, all guanacos that switched between strategies had summer ranges that, while still at higher altitudes than their winter ones, were not located in the high elevation plateau most guanacos migrated to, but were rather neighboring their winter ranges. This change between strategies could be due to a combination of the fact that guanacos often presented exploratory movements outside of their home ranges, coupled with lower NDVI years that could force resident guanacos out of their usual boundaries for foraging purposes. We also observed important behavioral plasticity around the timing and duration of migration, both within and between individuals, with interindividual variation being highest. Indeed, migration departure dates differed by > 100 days, and duration of migration ranged from 1 to 21 days. Changes in migration timing could be related to annual variations in climatic conditions, as we found a trend for a later summer migration departure date in 2020, which registered a longer period of snow cover compared to 2019 and 2021, leading to a later snowmelt date.
Variation in migratory characteristics has also been described for a number of other ungulate species (51). Typically, differences in migration timing, distance and duration have been associated with individual attributes, like age, sex, and reproductive status (16,31,52). Although we did not detect notable differences between male and female guanacos, we did not account for guanacos age, nutritional or reproductive status. Given the tight social nature of guanacos that tend to migrate in family groups, we would not expect major behavioral differences driven by individual attributes.
While most collared guanacos were migratory, we observed variation in migration propensity, with nine individual-years, corresponding to 7 guanacos, that remained resident in the winter ranges. This could be related to density-dependent resource release that follows departure of migratory individuals (49,52), as well as to the absence of domestic cattle in the winter range that has been shown to compete with guanacos for resources (53). Partial migration appears to be common for guanacos across their distributional range (50,53,54). Moreover, partial migration seems to be the predominant strategy among ungulates, with > 27 species exhibiting a mix of migratory and resident individuals within the same population (15,52). This strategy has been related to increased fitness and resilience, given the capacity of the population to respond to environmental changing conditions as well as to fluctuations in population density and predation risk (55). Because pumas (Puma concolor), guanacos main predator, have been found to not follow migratory individuals (50) and occur throughout the study area, and resident guanacos move as much as migratory ones within their home ranges, we do not believe risk of predation or energetic costs associated with long distance movement are influencing guanaco’s decisions to migrate or not. Instead, migratory guanacos likely benefit from accessing newly emerged vegetation, and resident guanacos may benefit from reduced competition during mating and breeding season, while avoiding risk related to encountering roads and fences during migration.
Multiple studies have shown that ungulate migrations follow certain patterns of plant phenology (22,56). For instance, the progression across the landscape of the instantaneous rate of green-up, a proxy of “springness” (26) can be compared to individual locations to evaluate whether animals are maximizing the use of newly emerged vegetation by surfing or jumping green waves (56,56–58). However, we found that these wave-like patterns are not representative of the phenology in our study area. The Patagonian dry shrub and grass steppes exhibit a complex relationship between precipitation, temperature and annual variations in NDVI, with lower elevation areas presenting a lagged vegetative green-up that occurs in early winter (59,60). This corresponds with the pattern we detected in our study area, where higher elevations had wave like patterns with summer peaks while lower elevations had more unpredictable patterns with late fall and winter peaks. The particularities of the phenology along with the short migratory distances, therefore, likely hamper the ability of guanacos to surf green waves. We did, however, detect higher primary productivity levels in the greenscape experienced by migratory guanacos compared to the available greenscapes corresponding to their summer and winter ranges had they remained resident. Thus, although guanacos did not appear to surf green-waves, migratory guanacos did track general patterns of vegetation green-up.
Although less common than vegetation phenology, snow is also a major driver of ungulates migrations (58,61–63). Ungulates tend to avoid snow because increasing snow depth reduces mobility, exacts additional energetic costs, and hinders the search for forage (31). As predicted, guanacos’ migration was associated to snow cover as they appeared to avoid “white waves”: migratory individuals occupied high altitude areas in spring and summer when snow was absent, but moved into low altitude areas featuring less now, during late fall and winter. Indeed, migratory individuals experienced less snow compared to what they would have experienced in their summer range. Having remained in their winter ranges in some cases however, would have meant even less exposure to snow, which could indicate that a synergy of both vegetation and snow phenology are driving guanacos’ migrations. In fact, given the strong correlation between these two variables, in which snow cover mediates vegetation green-up dynamics (64,65), it is difficult to disentangle their effects at such spatial and temporal resolutions. This was the case in our study area, particularly so for high altitudes where NDSI reached higher values and snow cover was present for extended periods of time (Figure S5).
Snow has also been related to smaller winter home ranges given the constraints it can have on animal movement (62,66). While we found that guanacos moved slower during the winter season, they had winter ranges that were larger than summer ones. This is uncommon among migratory ungulates and could be related to snow depth in winter ranges being insufficient to constrain movement, or to a combination of factors including the fact that winter ranges are shared between migratory and resident individuals, as well as the fact that guanacos form larger social groups during this season (36) which could limit vegetation availability and force individuals to forage in larger areas. Reduced forage due to snow cover could also be a factor influencing winter home range size, as 2020 was both the year with higher snow presence cover and larger winter home ranges. Additionally, displaying of male guanacos during the mating season might constrain movements and prevent access to certain areas leading to a reduction of the space effectively available in summer months (53).
While migration occurs at a landscape and seasonal level, driven by broad scale variables, such as snow and vegetation phenology, within seasonal ranges, habitat selection occurs at finer scales, driven by the daily need to maximize foraging while reducing predation risk and minimizing competition (2,67). We found that more than half of the collared guanacos exhibited daily micro-migrations nested within their seasonal ones. Such behaviors were more common during summer, and among residents (6/7 residents vs 12/18 migrants) and resulted in small home ranges. These daily movements are possibly driven by local trade-offs between resource availability and perceived risk, as it has been found previously among vicuñas (68) that tended to visit highly productive but also high risk foraging sites during the day, while moving into lower productivity yet safer sites at night.
Guanacos exhibit a mixture of both fidelity and plasticity regarding migration, which might prove to be critical for the continuity of this strategy in the face of rapid environmental change. Climate change could negatively impact migration by creating a mismatch between climatic conditions, vegetation green-up and migratory movements. This in turn could decrease the benefits of migratory behaviors, which has been observed in other species of migratory ungulates (69). Similarly, land use change and habitat fragmentation, such as those caused by roads and fences, may impact migratory movements by altering connectivity between seasonal ranges. Individual variations in temporal and spatial migration characteristics, including the ones we described for this guanaco population, could provide the necessary plasticity for this species to adapt to changing environmental conditions.