Movement-based methods to infer parturition events in migratory ungulates

Abstract

Long-distance migrations by ungulate species are a globally imperiled natural phenomenon and conservation of them requires monitoring population vital rates. Satellite telemetry tracking is widely used for understanding the spatial distribution and movement of animals, especially migratory animals in remote environments. Recently, analytical methods have been developed to infer parturition events from movement data in multiple species that calve in isolation, but to date such methods have not been tested on animals that both migrate and spatially aggregate during calving. We applied two movement-based methods developed to infer parturition in nonmigratory woodland caribou (Rangifer tarandus caribou (Gmelin, 1788)) to 241 reproductive seasons spanning 6 years of GPS data from migratory barren-ground caribou (Rangifer tarandus granti J.A. Allen, 1902). We compared results from both methods to data from aerial surveys of collared females during the calving period. We found that each movement-based method had ∼80% overall accuracy to identify calving events, with interannual variation ranging from 61% to 100%. When we considered instances when the two analytical methods agreed on parturition outcome, the accuracy increased to 89% with an annual range of 73%–100%. Using these methods, we identified marked interannual differences in peak calving dates and higher parturition rates than previously reported for this caribou herd. The successful application of these analyses to a migratory, gregarious ungulate suggests a broader applicability of the methodology.

Tactical departures and strategic arrivals: Divergent effects of climate and weather on caribou spring migrations

“…We compared arrival times estimated in this study with independently estimated and validated parturition dates for the Western Arctic herd caribou (Cameron et al. , M. Cameron, E. Gurarie, K. Joly, unpublished data ), and found that parturition occurred on average 4.0 d (SD 3.4, n = 100) after the individual mean posterior arrival date. Similarly, Gunn and Poole () identified peak calving times roughly within a week of our estimated arrival times for the Bathurst herd, with 2005 identified as the latest peak calving period (14–20 June), corresponding to the latest estimated mean arrival time in our analysis (10 June).…”

“…, Cameron et al. ). As the quantity of individually tracked movement data increases, hierarchical approaches to movement modeling will be increasingly relevant.…”

“…, Cameron et al. ) and parturition success. If this relationship holds, remotely tracked animal movement data could be used as an indicator of female body condition—and perhaps reproductive potential—on a large scale.…”

“…It is unclear whether caribou are more similar to green wave surfers or green wave jumpers, that is, whether they are responding to immediate conditions or are driven by longer-term goals. While caribou migrations coincide, roughly, with that of many temperate ungulates, beginning in April, and calving in late May or early June (Couturier et al 2009, Parker et al 2009, Cameron et al 2018, there is typically very little spring vegetation growth on the northern taiga or tundra during the migration period, and Arctic barren-ground migrations can ❖ www.esajournals.org take place over largely snow-covered landscapes (Boelman et al 2019). While there is consequently no green wave to surf, caribou may lead or pursue the front of melting snow in a "surfing the snow edge" Arctic analogue.…”

“…, Cameron et al. ), there is typically very little spring vegetation growth on the northern taiga or tundra during the migration period, and Arctic barren‐ground migrations can take place over largely snow‐covered landscapes (Boelman et al. ).…”