Individual variation creates diverse migratory portfolios in native populations of a mountain ungulate

Migratory behaviours are risk-sensitive to physiological state in an elevational migrant

Accretion of body fat by animals is an important physiological adaptation that may underpin seasonal behaviours, especially where it modulates risk associated with a particular behaviour. Using movement data from male Sierra Nevada bighorn sheep (Ovis canadensis sierrae), we tested the hypothesis that migratory behaviours were risk-sensitive to physiological state (indexed by body fat). Sierra bighorn face severe winter conditions at high elevations and higher predation risk at lower elevations. Given that large body fat stores ameliorate starvation risk, we predicted that having small body fat stores would force animals to migrate to lower elevations with more abundant food supplies. We also predicted that body fat stores would influence how far animals migrate, with the skinniest animals migrating the furthest down in elevation (to access the most abundant food supplies at that time of year). Lastly, we predicted that population-level rates of switching between migratory tactics would be inversely related to body fat levels because as body fat levels decrease, animals exhibiting migratory plasticity should modulate their risk of starvation by switching migratory tactics. Consistent with our predictions, probability of migration and elevational distance migrated increased with decreasing body fat, but effects differed amongst metapopulations. Population-level switching rates also were inversely related to population-level measures of body fat prior to migration. Collectively, our findings suggest migration was risk-sensitive to physiological state, and failure to accrete adequate fat may force animals to make trade-offs between starvation and predation risk. In complex seasonal environments, risk-sensitive migration yields a layer of flexibility that should aid long-term persistence of animals that can best modulate their risk by attuning behaviour to physiological state.

Snow avalanches are a primary climate-linked driver of mountain ungulate populations

Snow is a major, climate-sensitive feature of the Earth's surface and catalyst of fundamentally important ecosystem processes. Understanding how snow influences sentinel species in rapidly changing mountain ecosystems is particularly critical. Whereas effects of snow on food availability, energy expenditure, and predation are well documented, we report how avalanches exert major impacts on an ecologically significant mountain ungulate - the coastal Alaskan mountain goat (Oreamnos americanus). Using long-term GPS data and field observations across four populations (421 individuals over 17 years), we show that avalanches caused 23-65% of all mortality, depending on area. Deaths varied seasonally and were directly linked to spatial movement patterns and avalanche terrain use. Population-level avalanche mortality, 61% of which comprised reproductively important prime-aged individuals, averaged 8% annually and exceeded 22% when avalanche conditions were severe. Our findings reveal a widespread but previously undescribed pathway by which snow can elicit major population-level impacts and shape demographic characteristics of slow-growing populations of mountain-adapted animals.

Diverse migration patterns and seasonal habitat use of Stone's sheep (Ovis dalli stonei)

We describe temporal and spatial patterns of seasonal space-use and migration by 16 GPS-collared Stone's sheep (Ovis dalli stonei) from nine bands in the Cassiar Mountains of northern British Columbia, Canada. Our objectives were to identify the timing of spring and fall migrations, characterize summer and winter ranges, map and describe migration routes and use of stopover sites, and document altitudinal change across seasons. Our last objective was to assess individual migration strategies based on patterns of geographic migration, altitudinal migration, or residency. Median start and end dates of the spring migration were 12 and 17 Jun (range: 20 May to 05 Aug), and of the fall migration were 30 Aug and 22 Sep (range: 21 Aug to 07 Jan). The median area of winter and summer ranges for geographic migrants were 630.8 ha and 2,829.0 ha, respectively, with a broad range from about 233.6 to 10,196.2 ha. Individuals showed high fidelity to winter ranges over the limited duration of the study. The winter and summer ranges of most individuals (n = 15) were at moderate to high elevations with a median summer elevation of 1,709 m (1,563-1,827 m) and 1,673 m (1,478-1,751 m) that varied <150 m between ranges. Almost all collared females (n = 14) exhibited changes in elevation use that coincide with abbreviated altitudinal migration. Specifically, these females descended to lower spring elevations from their winter range (Δ > 150 m), and then gradually moved up to higher-elevation summer ranges (Δ > 150 m). In the fall, they descended to lower elevations (Δ > 100 m) before returning to their higher winter ranges. The median distance travelled along geographic migration routes was 16.3 km (range: 7.6-47.4 km). During the spring migration, most geographic migrants (n = 8) used at least one stopover site (median = 1.5, range: 0-4), while almost all migrants (n = 11) used stopover sites more frequently in the fall (median = 2.5, range: 0-6). Of the 13 migratory individuals that had at least one other collared individual in their band, most migrated at about the same time, occupied the same summer and winter ranges, used similar migration routes and stopover sites, and exhibited the same migration strategy. We found collared females exhibited four different migration strategies which mostly varied across bands. Migration strategies included long-distance geographic migrants (n = 5), short-distance geographic migrants (n = 5), vacillating migrants (n = 2), and abbreviated altitudinal migrants (n = 4). Different migratory strategies occurred within one band where one collared individual migrated and two did not. We conclude that female Stone's sheep in the Cassiar Mountains displayed a diverse assemblage of seasonal habitat use and migratory behaviors. By delineating seasonal ranges, migration routes and stopover sites, we identify potential areas of priority that can help inform land-use planning and preserve the native migrations of Stone's sheep in the region.

Seeing Is Be-Leaving: Perception Informs Migratory Decisions in Sierra Nevada Bighorn Sheep (Ovis canadensis sierrae)

Seasonal migration is a behavioral response to predictable variation in environmental resources, risks, and conditions. In behaviorally plastic migrants, migration is a conditional strategy that depends, in part, on an individual’s informational state. The cognitive processes that underlie how facultative migrants understand and respond to their environment are not well understood. We compared perception of the present environment to memory and omniscience as competing cognitive mechanisms driving altitudinal migratory decisions in an endangered ungulate, the Sierra Nevada bighorn sheep (Ovis canadensis sierrae) using 1,298 animal years of data, encompassing 460 unique individuals. We built a suite of statistical models to partition variation in fall migratory status explained by cognitive predictors, while controlling for non-cognitive drivers. To approximate attribute memory, we included lagged attributes of the range an individual experienced in the previous year. We quantified perception by limiting an individual’s knowledge of migratory range to the area and attributes visible from its summer range, prior to migrating. Our results show that perception, in addition to the migratory propensity of an individual’s social group, and an individual’s migratory history are the best predictors of migration in our system. Our findings suggest that short-distance altitudinal migration is, in part, a response to an individual’s perception of conditions on alterative winter range. In long-distance partial migrants, exploration of migratory decision-making has been limited, but it is unlikely that migratory decisions would be based on sensory cues from a remote target range. Differing cognitive mechanisms underpinning short and long-distance migratory decisions will result in differing levels of behavioral plasticity in response to global climate change and anthropogenic disturbance, with important implications for management and conservation of migratory species.

Causes, Consequences, and Conservation of Ungulate Migration

Our understanding of ungulate migration is advancing rapidly due to innovations in modern animal tracking. Herein, we review and synthesize nearly seven decades of work on migration and other long-distance movements of wild ungulates. Although it has long been appreciated that ungulates migrate to enhance access to forage, recent contributions demonstrate that their movements are fine tuned to dynamic landscapes where forage, snow, and drought change seasonally. Researchers are beginning to understand how ungulates navigate migrations, with the emerging view that animals blend gradient tracking with spatial memory, some of which is socially learned. Although migration often promotes abundant populations—with broad effects on ecosystems—many migrations around the world have been lost or are currently threatened by habitat fragmentation, climate change, and barriers to movement. Fortunately, new efforts that use empirical tracking data to map migrations in detail are facilitating effective conservation measures to maintain ungulate migration.