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Scientists at Nottingham Trent University found that food from people—directly or indirectly—contributes an estimated 35% of the urban fox diet, compared to just 6% for their rural counterparts.

Foxes are omnivorous; naturally consuming prey such as small mammals, birds, and invertebrates, while also eating fruit and carrion (dead animals).

They can exploit urban areas due to the abundance and accessibility of food, including discarded food waste or that which is intentionally left out for wildlife, such as meal leftovers, pet food and bird food.

The researchers argue it is possible that processed foods, which are increasingly common in the British diet, may enter the fox diet and lead to poor health. These foods typically contain high levels of saturated fats, refined sugars and grains, while low in fiber and micronutrients.

They say that more needs to be done to understand the potential health impact on foxes, particularly in urban areas, as they eat more human-derived food and less natural food.

As part of the study, published in Ecology and Evolution, the team also found that female foxes may consume more human-derived food than males, which they suggest may be a result of adult females utilizing more abundant human resources while raising their offspring.

The research involved carrying out "stable isotope analysis"—a diet tracing method based on the ratio of light and heavy isotopes in a sample—on the whiskers of almost 100 foxes in rural and urban areas across Britain.

Rather than being a snapshot of dietary information, as is the case with analyzing fecal or stomach content, the isotope ratios of foods are reflected in the biological tissues of their consumers.

Since whiskers grow over a period of weeks to months, the team was able to determine the diet of foxes over longer time periods by examining the carbon and nitrogen isotopes within them.

The study showed that human food made up 16% of the urban fox diet, while pet food accounted for 19%. This compares to just 2.5% human food and 3.5% pet food for rural foxes.

The data revealed that birds were the largest dietary source for both rural and urban foxes, accounting for almost a third of the rural fox diet overall (32%) and about a fifth of the urban fox diet (22%).

Mammals (32%), fruit (26%) and invertebrates (4%) were the next most common sources among rural foxes before human food and pet food, which were the least consumed.

In urban foxes, however, pet food was the second most consumed item (19%), followed by fruit (17%), mammals (16%), human food (16%) and invertebrates (10%).

"Human-derived or 'anthropogenic' foods have been implicated in poor health outcomes for species that consume them, so it is important we examine the extent of such foods in the diet of British foxes," said lead author Dr. Jonathan Fletcher, a researcher in Nottingham Trent University's School of Animal, Rural and Environmental Sciences.

He said, "A diet high in anthropogenic food may set them up for failure if such foods do not allow them to achieve their nutritional requirements, therefore impacting their body condition and perhaps making them more susceptible to disease."

"More work needs to be done to better understand the potential health impacts of this, but any consequences would disproportionately affect urban-dwelling foxes and females."

Nottingham Trent University's Professor Dawn Scott, senior author on the study, said, "Although the urban fox diet does include a range of natural food items, we found less natural food along with a higher anthropogenic diet which could include ultra-processed foods, leftovers or commercial foods not intended for wildlife."

"This is not to say that all anthropogenic food will be nutritionally inadequate, but since processed foods can be high in saturated fats and refined sugar, it would not be unreasonable to suggest that a degree of this food source is likely to be nutritionally poor for wildlife."

The research also involved the Animal and Plant Health Agency, the University of Brighton and the University of Glasgow.

Dr. Ruth Cox of the Animal and Plant Health Agency said, "By using our existing networks to collect a large number of samples over a long time period for this study, we were able to quantify the contribution of human-derived food to the fox diet. The results suggest that urban foxes in particular could be at risk of health-related impacts.

"This research contributes to our understanding of the impact of human-derived food on wildlife health."