A US study that set out to measure how much wildlife domestic cats eat to supplement the food they are given by their owners was unsuccessful due to an unexpectedly high variability in cat food ingredients. This accidental discovery suggests that some cat food manufacturers regularly change ingredient composition, even within the same flavoured cat food.
Feral cats are responsible for several native wildlife declines, but the impact of pet cats on urban wildlife isn’t well understood.
This inspired a collaborative study—published in the journal PeerJ—led by researchers at North Carolina State University to directly measure how often pet cats eat outside of their food bowls.
A common way to understand the composition of animal diets is to collect samples of fur, nails, or blood from an animal and analyse its carbon and nitrogen isotopes. All organic materials contain isotopes of elements that get locked into body tissues, following the basic principle that you are what you eat.
For this study, researchers collected isotopes from things a cat might eat, including different brands and flavours of cat foods. They predicted cats that only ate from their food bowls would have an identical isotopic match to the food, while differences between cat and pet food would indicate a cat supplementing its diet with wild prey.
“We really thought this was going to be an ideal application of the isotope methodology,” co-author Roland Kays said.
“Usually these studies are complicated by the variety of food a wild animal eats, but here we had the exact pet food people were giving their cats.”
This assumes that cat food producers use consistent types and amounts of ingredients. As it turns out, that is not the case.
The carbon and nitrogen isotopes in cat foods varied widely—even between foods that were the same flavour and from the same brand.
The only clear relationship found was that the least expensive cat foods had higher carbon values, indicating a strong presence of corn product in inexpensive cat food. In addition, pet foods sampled from the UK had lower carbon values, suggesting less input from corn products.
“In short, at the end of this study we are still ignorant about why some cats kill more wildlife than others, and we have also found we are ignorant about something else—the shifting dynamics of ‘Big Pet Food’”, co-author Rob Dunn said.