Dispersal patterns in a medium‐density Irish badger population: Implications for understanding the dynamics of tuberculosis transmission

Abstract European badgers (Meles meles) are group‐living mustelids implicated in the spread of bovine tuberculosis (TB) to cattle and act as a wildlife reservoir for the disease. In badgers, only a minority of individuals disperse from their natal social group. However, dispersal may be extremely important for the spread of TB, as dispersers could act as hubs for disease transmission. We monitored a population of 139 wild badgers over 7 years in a medium‐density population (1.8 individuals/km2). GPS tracking collars were applied to 80 different individuals. Of these, we identified 25 dispersers, 14 of which were wearing collars as they dispersed. This allowed us to record the process of dispersal in much greater detail than ever before. We show that dispersal is an extremely complex process, and measurements of straight‐line distance between old and new social groups can severely underestimate how far dispersers travel. Assumptions of straight‐line travel can also underestimate direct and indirect interactions and the potential for disease transmission. For example, one female disperser which eventually settled 1.5 km from her natal territory traveled 308 km and passed through 22 different territories during dispersal. Knowledge of badgers' ranging behavior during dispersal is crucial to understanding the dynamics of TB transmission, and for designing appropriate interventions, such as vaccination.

Filled circles represent GPS locations, with different colours indicating different nights. Thin grey lines join consecutive GPS locations. The thick red line outlines the natal territory boundary, and thin black lines outline other territory boundaries. The average distance between main setts in the study area was 1.3 km2. A grid of 1.3 km2 is used as a proxy for social group territories outside of the study area. excursions before leaving her natal territory permanently on 16th of January. She appeared to explore before settling down in February. In this time, she travelled 75 km through an area of approximately 6 km2, passing through approximately 9 social groups' territories before settling down.
Filled circles represent GPS locations, with different colours indicating different nights. Thin grey lines join consecutive GPS locations. The thick red line outlines the natal territory boundary and thin black lines outline other territory boundaries. The average distance between main setts in the study area was 1.3 km2. A grid of 1.3 km2 is used as a proxy for social group territories outside of the study area.
Quadrants with GPS locations are highlighted using a hatched pattern. to the south, outside of the study area. Exploratory forays were made over multiple nights where she did not return to the natal social group but spent the day in the setts of other social groups. During this period, she travelled at least 147 km through an area of approximately 26 km 2 , passing through approximately 19 social groups' territories. However, her collar was only working intermittently, so this is an underestimate. She recorded one round-trip of 21 km, where she spent at least two full nights away from her natal group. She was found dead before completing dispersal.
Filled circles represent GPS locations, with different colours indicating different nights. Thin grey lines join consecutive GPS locations. The thick red line outlines the natal territory boundary and thin black lines outline other territory boundaries. The average distance between main setts in the study area was 1.3km 2 . A grid of 1.3km 2 is used as a proxy for social group territories outside of the study area. The red arrow indicates where she was found dead. During this time, F11 wandered widely, and travelled 307 km through an area of approximately 25 km 2 , passing through approximately 22 social groups' territories, before she moved into her new territory following the death of the resident female.
Filled circles represent GPS locations, with different colours indicating different nights. Thin grey lines join consecutive GPS locations. The thick red line outlines the natal territory boundary and thin black lines outline other territory boundaries. The stippled area represents the territory she dispersed to. The average distance between main setts in the study area was 1.3km 2 . A grid of 1.3km 2 is used as a proxy for social group territories outside of the study area. Figure 2.6 F12's dispersal trajectory. January and February 2016. Exploratory forays preceding an eventual move to a non-adjacent social group to the west (black dotted polygon, collar died before she settled). ETEs were made over multiple nights where F12 did not return to the natal social group but spent the day in the setts of other social groups. During this time, F12 travelled 66 km through an area of 40 km 2 , passing through approximately 20 social groups' territories. However, her GPS collar failed before she completed dispersal, so this is an underestimate.
Filled circles represent GPS locations, with different colours indicating different nights. Thin grey lines join consecutive GPS locations. The thick red line outlines the natal territory boundary and thin black lines outline other territory boundaries. The stippled area represents the territory she dispersed to. The average distance between main setts in the study area was 1.3 km 2 . A grid of 1.3 km 2 is used as a proxy for social group territories outside of the study area. The red arrow indicates the last GPS location received before collar failure.    Filled circles represent GPS locations, with different colours indicating different nights. Thin grey lines join consecutive GPS locations. The thick red line outlines the natal territory boundary, the thick black line represents the new territory boundary, and thin black lines outline other territory boundaries. The average distance between main setts in the study area was 1.3 km 2 . A grid of 1.3 km 2 is used as a proxy for social group territories outside of the study area.  In this period, he travelled 110 km through an area of approximately 8.5 km 2 , passing through approximately 11 social groups' territories before we lost contact. This is an underestimate as his collar was faulty and functioned intermittently, before failing.
Filled circles represent GPS locations, with different colours indicating different nights. Thin grey lines join consecutive GPS locations. The thick red line outlines the territory boundary in which he was trapped. Thin black lines outline other territory boundaries. The average distance between main setts in the study area was 1.3 km 2 . A grid of 1.3 km 2 is used as a proxy for social group territories outside of the study area. Quadrants with GPS locations are highlighted using a hatched pattern.