Dead-reckoning elucidates fine-scale habitat use by European badgers Meles meles

Abstract

Background: Recent developments in both hardware and software of animal-borne data loggers now enable large amounts of data to be collected on both animal movement and behaviour. In particular, the combined use of tri-axial accelerometers, tri-axial magnetometers and GPS loggers enables animal tracks to be elucidated using a procedure of 'dead-reckoning'. Although this approach was first suggested 30 years ago by Wilson et al. (1991), surprisingly few measurements have been made in free-ranging terrestrial animals. The current study examines movements, interactions with habitat features, and home-ranges calculated from just GPS data and also from dead-reckoned data in a model terrestrial mammal, the European badger (Meles meles).

Methods: Research was undertaken in farmland in Northern Ireland. Two badgers (one male, one female) were live-trapped and fitted with a GPS logger, a tri-axial accelerometer, and a tri-axial magnetometer. Thereafter, the badgers' movement paths over 2 weeks were elucidated using just GPS data and GPS-enabled dead-reckoned data, respectively.

Results: Badgers travelled further using data from dead-reckoned calculations than using the data from only GPS data. Whilst once-hourly GPS data could only be represented by straight-line movements between sequential points, the sub-second resolution dead-reckoned tracks were more tortuous. Although there were no differences in Minimum Convex Polygon determinations between GPS- and dead-reckoned data, Kernel Utilisation Distribution determinations of home-range size were larger using the former method. This was because dead-reckoned data more accurately described the particular parts of landscape constituting most-visited core areas, effectively narrowing the calculation of habitat use. Finally, the dead-reckoned data showed badgers spent more time near to field margins and hedges than simple GPS data would suggest.

Conclusion: Significant differences emerge when analyses of habitat use and movements are compared between calculations made using just GPS data or GPS-enabled dead-reckoned data. In particular, use of dead-reckoned data showed that animals moved 2.2 times farther, had better-defined use of the habitat (revealing clear core areas), and made more use of certain habitats (field margins, hedges). Use of dead-reckoning to provide detailed accounts of animal movement and highlight the minutiae of interactions with the environment should be considered an important technique in the ecologist's toolkit.

Keywords: Accelerometer; Badger; Dead-reckoning; Distance travelled; GPS; Home-range; Logger; Movement; Tuberculosis.

Fig. 1

Activity measured as vectorial dynamic body acceleration (VeDBA, g) and ambient temperature (^oC) measured when a badger was outside and inside its sett. The badger entered the sett at approximately 22:05 and exited again at 23:06. The shaded area represents period the badger spent inside the sett. The solid line indicates VeDBA and the dashed line indicates ambient temperature. Note the slight delay in the temperature sensor recording lower values when the animal exited the sett

Fig. 2

Periods spent outside the sett represented by black bars for a female (top) and male (lower) badger for 7 days after deployment of loggers

Fig. 3

A Example of a GPS-enabled dead-reckoned movement path by a female badger over the course of a nightly outing. B, C Show expanded sections illustrating movements when the badger visited a field and interacted with a hedgerow, respectively. GPS fixes are shown as white dots. Straight-line movements between GPS fixes are shown by dashed lines. The GPS-enabled dead-reckoned path is shown by a black continuous line

Fig. 4

Movement paths and home-ranges of a female badger, measured using GPS (white circles) and GPS-enabled dead-reckoning (solid black lines), over seven consecutive nights of activity. Panel A shows the home-range calculated using the 95% minimum convex polygon method (MCP95) and panel B shows the home-range calculated using the 95% kernel utilisation distribution method (KD95). Grey shaded area denotes the home-range calculated using just the hourly GPS data. The black cross-hatched area indicates the home-range calculated using the GPS-enabled dead-reckoned data at 40 Hz

Fig. 5

Proportion of time spent within various landscape features by two badgers according to whether the positional data were derived from GPS or GPS-enabled dead-reckoned data, labelled “GPS” and “DR”, respectively

Fig. 6

Proportion of time spent within various landscape features by two different badgers (top panel A = female; lower panel B = male) according to whether the positional data were derived from GPS or GPS-enabled dead-reckoned data, labelled “GPS” and “DR”, respectively