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

Abstract

AbstractSurvival of ungulates can vary seasonally due to changing environmental conditions, e.g. weather or predation pressure. The wild forest reindeer (Rangifer tarandus fennicus, “WFR”) migrate between calving and wintering grounds. The annual cycle of female WFR includes four main seasons (wintering, calving, rutting and autumn migration) during which they are subject to different conditions, but almost nothing is known about their survival or mortality patterns. We measured survival in 305 GPS-tagged female WFR in two subpopulations in Finland (2010–2022). Data were analysed for inter-annual and seasonal patterns in survival and cause-specific mortality (predation, traffic, accidents and unknown causes) with known-fate models. Inter-annual survival in Suomenselkä (mean 0.90) showed an increasing trend during the study and was higher than in Kainuu (mean 0.84) which showed a declining trend. Seasonal variation in survival was population dependent. Survival was lowest in Kainuu during the winter while in Suomenselkä, it was lowest during autumn migration. Concerning cause-specific mortality, seasonal variation mirrored variation in predation that was the primary cause of mortality (55% of cases). Wolves (Canis lupus) killed the majority of predated individuals (58%). Predation rates were two times higher in Kainuu where the wolf density was higher. We provide the first adult survival estimates for WFR and show that the seasonal variation in survival is clearly different between these two northern ungulate populations. This indicates that seasonal variation in survival is determined by site-dependent factors and cannot be generalised across populations.