Determinants of winter kill rates of wolves in Scandinavia

Abstract: Winter wolf (Canis lupus) kill rates on moose (Alces alces) on the Scandinavian Peninsula are high and subject to strong variation, compared to other boreal wolf-moose systems. A more detailed understanding of factors influencing this variation is crucial for management and conservation of the Scandinavian wolf population. Although functional response models explain the relationship between prey density and kill rates, few studies quantify and investigate the predator-prey characteristics shaping the mechanisms of predation at a finer scale. I analysed 18 605 hourly GPS locations from 13 wolf packs during 2001-2010. There were 182 usable moose predation events from 17 winter study periods during this time span. With the GPS data I assessed patterns of wolf activity detecting consecutive killsites and reconstructing wolf hunting paths between predation events. I estimated two components of kill rate i.e., searching time (Ts) which is the time spent to detect, chase, and kill prey, and handling time (Th), i.e., the time used to consume and process the prey for each predation event. I tested the difference between two different methods used for defining spatial criteria and evaluated a set of models to test factors potentially affecting Ts and Th. These factors included prey age, pack size, winter period, moose density, and wolf density at two functional levels of scale. I demonstrate that the method used for defining Ts and Th influenced their estimates and model selection. The majority of moose kills occurred during night time and wolves spent 75% of their total time away (>1km) from kill-sites. Wolf search time decreased with an increase in local moose density and pack size. At the pack level, Th increased with pack size and decreased with the number of neighbouring packs highlighting the influence of territoriality and social interactions among wolf packs. Age of moose, i.e., calf (< 1 year), yearling (1 year) and adult (> 2 year) moose, was an important variable for explaining variation in both Ts and Th. This study shows the potential of a research approach using GPS data in time-to-event models and the effectiveness of these models to quantify mechanisms of predation and factors influencing wolf kill rates on moose.