Weighed down by science: do collar-mounted devices affect domestic cat behaviour and movement?

Context Animals carrying tracking and logging devices are subject to a range of instrument effects that negatively affect survival, reproduction and behaviour. The common recommendation is that device weight should not exceed 5% body mass (BM) for terrestrial species; however, this rule-of-thumb has little empirical basis. Modelling indicates that devices weighing less than 3% BM may still have impacts. Several studies have used telemetry and data loggers on domestic cats (Felis catus) with instruments ranging in weight from 30 g to 125 g, but there has been no quantitative evaluation of instrument effects. In addition, inexpensive GPS tags such as iGotU are increasingly being used to track domestic cats, but often with little acknowledgement of habitat-related location error. Aims We evaluated the impact of wearing devices of different weights on domestic cat movements, and quantified location error across typical suburban habitats. Methods We recorded movements of cats wearing three different GPS collar weights for a week at a time: light, 30 g (<1% BM); medium, 80 g (~2% BM); and heavy, 130 g (~3% BM). Location error (LE) and fix success rates (FSR) were compared between backyard habitats and up- or downward orientation of the GPS tags on collars. Key results Home-range size and distance travelled from home were smaller when cats wore the heaviest collar. LE was lower and FSR higher for GPS tags with direct-line-of-sight to satellites (e.g. on lawns), but there was no difference between tags placed in dense vegetation (hedges) or more open vegetation (trees), or tags oriented up or down. Conclusions Collars carrying instruments on cats should be no more than 2% BM (medium-weight collar). LE was large relative to typical urban habitat size, indicating that misclassification of locations into habitats could easily occur in habitat-selection studies. Implications Some published accounts of cat home-range sizes may be underestimates, resulting in underestimates of the extent of impacts on prey species. Habitat-use studies should acknowledge the error associated with GPS tags and incorporate it into analyses using techniques such as Brownian Bridges.