Outdoor domestic cats are a conservation concern as invasive and effective predators that prey on wildlife around the world. Introduced cats are responsible for the extinction of at least 63 bird, reptile and mammal species in the past 500 years.
Scientists had hoped to study the impact of cats on their environment by studying their diet with isotope analysis, a frequently used tool for such a task. But the high variation in pet food made the task nearly impossible, stymieing their ability to understand cats’ effects on the environment in a new study.
“We got a result that was not what we were expecting—it didn’t work very well,” Roland Kays, one of the study’s authors and head of the Biodiversity Research Lab at the North Carolina Museum of Natural Sciences, told Gizmodo. “This is important to show because other people have wanted to use this method to say whether an animal is domestic or wild.”
The researchers hoped to rely on isotope analysis due to the unreliable nature of owners reporting what kinds of animals their cats have been eating away from the house. Isotope analyses compare the different isotopes of certain elements in a sample. Different isotopes represent the same element but with different numbers of neutrons—the subatomic particle—in their nuclei. Pet foods typically contain corn or livestock fed on corn, which would have higher ratios of the isotope carbon-13 than native prey. These differing ratios would then be reflected in the cat’s hair as a kind of “you are what you eat” scenario.
With the help of citizen scientists, the team gathered 202 samples of cat hair and 239 pet food samples and characterised the carbon and nitrogen isotope composition. They also analysed the composition for 11 species of native prey that the cats would be likely to eat.
Researchers then separated the cats into four categories: Hunters that were likely to have eaten lots of native prey, non-hunters that probably ate solely cat food, cats whose habits were uncertain, and those with heavy isotope values higher than would be expected from food alone.
But even though pet cats are typically only fed one kind of food, the analysis revealed that pet food couldn’t serve as a useful control in the experiment. Pet foods contained too much variability in their isotope values, both between brands, between flavours, and even between boxes of the same flavour. The food also differed between the US and the UK.
“Therefore, counter to our expectations, we found that isotope analysis of cat hair has little value in determining if a cat has been eating wild prey,” the researchers wrote in the paper published in PeerJ, “as a result of both high variability in the carbon and nitrogen values of pet foods and the hunting habits of cats.”
Though discouraging, Kays told Gizmodo that it led to another interesting question—how and why cat food is so variable, and whether there are other ways to use it as a chemical signature for domestication.
This study has other important consequences. Yes, it shows that the composition of your cat’s food is likely less consistent than you thought, even if you’re using the same kind. But more importantly, null results are important signposts to help other scientists arrive at some broader truth. This paper shows that studying cats’ effects on the environment is going to be more difficult that simple chemical analyses like these. It will require different and more complex approaches, especially given that not all prey is eaten at home in front of the owner.
Kays told Gizmodo that his team is now working on high resolution cat trackers with accelerometers that can show them where outdoor cats are without following around them all the time in order to monitor their behaviour.