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

Survival 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.