Red foxes were introduced into Australia from Europe and have become established over the continent over the last 100 years (Saunders et al. 1995). The red fox has contributed to the extinction of more than 25 Australian mammal species (reviewed by Saunders, Gentle & Dickman 2010, Woinarski et al. 2014).
The Red Card for Rabbits and Foxes is a community-based feral animal control program that has been operating in agricultural areas of southwest Western Australia since 2004. RCRF brings together over 1000 landholders for a targeted, landscape-scale approach to pest control.
The instigation of broad scale fox control in Australia in the 1980s has arguably prevented the extinction of many native species (Abbott 2008). In 2010, we joined a coordinated group of hunters, landholders and local facilitators in the annual Red Card for Rabbits and Foxes cull of invasive and pest species across southwest Western Australia.
We collected samples from 540 red foxes culled over two weekends. This included intestines to identify what parasite species they carried (Dybing, Fleming & Adams 2013). Fleas collected from these animals revealed zoonotic Bartonella species, which are the causes of cat scratch disease and endocarditis in humans (Kaewmongkol et al. 2011).
We also collected their stomachs to identify what they had been eating, skulls to age each animal and to compare morphology. This involved a lot of sticky, smelly work in the field, followed by more long hours in the laboratory.
Over half (57%) of the foxes we collected were juvenile dispersers, in their first year out from their dens. This finding correlates with that of other studies in Europe, UK and the USA that have found juveniles have a dispersal period of ~3 months, during which they move widely across the landscape in an attempt to establish territories. Studies have also confirmed that 75% of foxes die within their first year, and that only 1 in 10,000 foxes will survive up to the maximum recorded age of 9 years (Macdonald & Reynolds 2004). Despite high fox mortality during the juvenile dispersal period, 43% of the animals we processed were breeding adults, the oldest animals we sampled were 8 years of age. A large adult population may indicate that in southwest WA foxes are surviving well into adulthood, which allows them to develop increased bite strength and possibly hone hunting skills that will impact native species.
We calculated bite force from skull anatomy. Male foxes are heavier and also have larger skulls than females. Young foxes start their lives with sharp teeth and elongated skulls that do not have great bite force. As they age into adults, their skull broadens out to accommodate their chewing muscles, and their teeth wear down. This change is likely to reflect the demands of an increasing proportion of meat in their diet, which would require additional biting and chewing.
We also found marked sex and age differences in fox diets. Foxes taken as part of the culls were largely eating sheep carrion (47–65% of diet volume). Despite their lower bite force compared with adults, even the juveniles were eating a large volume of sheep meat and their diet was similar to that of adult males. Sheep carrion appears to be a readily available resource for foxes during the late summer-autumn months in southwest Western Australia.
Adult females ate less sheep but had more mice and invertebrates (particularly beetles and centipedes) in their diet. This difference (compared with juveniles and adult males) suggests that adult female foxes have different hunting behaviour compared with adult males.
Our paper, published this week (2017 Forbes-Harper et al fox skull ecomorphology), identifies how bite force, calculated from skull anatomy, is not directly related to diet in these animals.
Abbott, I. (2008) Historical perspectives of the ecology of some conspicuous vertebrate species in south-west Western Australia. Conservation Science Western Australia, 6, 1.
Dawson, S. J., H. Crawford, R. Huston, P. J. Adams & P. A. Fleming (2017) How to catch red foxes red handed: identifying predation of freshwater turtles and nests. Wildl. Res., In Press.
Dybing, N. A., P. A. Fleming & P. J. Adams (2013) Environmental conditions predict helminth prevalence in red foxes in Western Australia. International Journal for Parasitology: Parasites and Wildlife, 2, 165.
Fleming, P. A., S. J. Dundas, Y. Y. W. Lau & J. R. Pluske (2016) Predation by red foxes (Vulpes vulpes) at an outdoor piggery. Animals, 6, 60.
Forbes-Harper, J. L., H. M. Crawford, S. J. Dundas, N. M. Warburton, P. J. Adams, P. W. Bateman, M. C. Calver & P. A. Fleming (2017) Diet and bite force in red foxes: ontogenetic and sex differences in an invasive carnivore. Journal of Zoology, In press.
Kaewmongkol, G., S. Kaewmongkol, P. A. Fleming, P. J. Adams, U. Ryan, P. J. Irwin & S. G. Fenwick (2011) Zoonotic Bartonella species in fleas and blood from red foxes in Australia. Vector-Borne and Zoonotic Diseases, 11, 1549.
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Woinarski, J. C. Z., A. A. Burbidge & P. L. Harrison (2014) The Action Plan for Australian Mammals 2012. Collingwood, Victoria: CSIRO Publishing.