High variability within pet foods prevents the identification of native species in pet cats' diets using isotopic evaluation.

Brandon W Mcdonald,Jennifer Mcdonald,Holly Cole,Troi Perkins,Roland Kays,Robert S Feranec,Robert R Dunn

doi:10.7717/peerj.8337

Abstract

Domestic cats preying on wildlife is a frequent conservation concern but typical approaches for assessing impacts rely on owner reports of prey returned home, which can be biased by inaccurate reporting or by cats consuming prey instead of bringing it home. Isotopes offer an alternative way to quantify broad differences in animal diets. By obtaining samples of pet food from cat owners we predicted that we would have high power to identify cats feeding on wild birds or mammals, given that pet food is thought to have higher C isotope values, due to the pervasive use of corn and/or corn by-products as food ingredients, than native prey. We worked with citizen scientists to quantify the isotopes of 202 cat hair samples and 239 pet food samples from the US and UK. We also characterized the isotopes of 11 likely native prey species from the southeastern US and used mixing models to assess the diet of 47 cats from the same region. Variation in C and N isotope values for cat food was very high, even within the same brand/flavor, suggesting that pet food manufacturers use a wide range of ingredients, and that these may change over time. Cat food and cat hair from the UK had lower C values than the US, presumably reflecting differences in the amount of corn used in the food chains of the two countries. This high variation in pet food reduced our ability to classify cats as hunters of native prey, such that only 43% of the animals could be confidently assigned. If feral or free ranging cats were considered, this uncertainty would be even higher as pet food types would be unknown. Our results question the general assumption that anthropogenic foods always have high C isotope values, because of the high variability we documented within one product type (cat food) and between countries (US vs. UK), and emphasize the need to test a variety of standards before making conclusions from isotope ecology studies.

Highlights

While the domestic cat (Felis catus) has enjoyed a status as one of the most popular companion animals around the world, they have made a detrimental impact on wildlife (Medina et al, 2011)
Cats were categorized into those that likely consumed prey, those that likely ate only cat food, those that had an uncertain diet, and those that had isotope values outside range of food sources. This type of analysis works regionally in areas where the vegetation relies on mostly C3 photosynthesis, and pet food is mostly composed from ingredients that come from C4 plants
We found that the high variability of isotope values for pet foods, across and within brands, not to mention across countries, makes it extremely difficult to determine the diet of individual cats

Summary

Introduction

While the domestic cat (Felis catus) has enjoyed a status as one of the most popular companion animals around the world, they have made a detrimental impact on wildlife (Medina et al, 2011). Cats have caused intense conservation problems on island systems where there are 33 examples of cats being directly responsible for the extirpation of island species, and another 37 cases in which domestic cats had serious impacts resulting in drastic reductions in populations (Medina et al, 2011). This loss or reduction of endemic populations can have alarming impacts on local diversity beyond direct predation pressures. The problem continues today, with cats impacting 120 islands throughout the world, resulting in 175 threatened species suffering negative impacts of island cats (IUCN, 2008)

Methods

Results

Discussion

Conclusion