Survival and cause-specific mortality in adult females of a northern migratory ungulate

Abstract

Abstract Survival of migratory species, such as ungulates, can vary seasonally due to changing environmental conditions, e.g., weather or predation pressure. Female wild forest reindeer (Rangifer tarandus fennicus, ‘WFR’) migrate between calving and wintering grounds. Their annual cycle includes four main seasons (wintering, calving, rut and autumn migration) during which they are subject to different conditions, but nothing is known about their survival or mortality patterns. We tracked two separated WFR subpopulations in Kainuu and Suomenselkä in Finland with GPS-collars. Using histories from 271 WFR females followed during 2009–2020, we examined annual and seasonal patterns in survival and cause-specific mortality (predation, traffic and unknown causes) with known-fate models. Annual, seasonal and population variation in survival were negligible (annual mean = 0.85). Predation was the primary cause of mortality (56% of all cases) and wolves (Canis lupus) killed the majority of predated individuals (59% of predation mortality). Importantly, predation rates were higher in Kainuu where the wolf density was 3.7 times higher. In line with this, seasonal variation was not clear in Suomenselkä whereas predation varied across the annual cycle in Kainuu being lower during autumn migration (1.4% / 2 months) and calving (1.7% / 4 months) but higher during rutting (2.8% / 2 months) and wintering (5.3% / 4 months) season. Our results warrant research on the mechanisms (climate, habitat, movement patterns etc.) affecting predation rates during these periods of higher mortality.