Cats – unlike humans – are true carnivores: they must eat meat to survive because their bodies can’t draw some essential nutrients from plants. By looking at tissues, researchers can get a good understanding of what foods animals ate.
Now, researchers at the Department of Microbiology and Ecosystem Science at the University of Vienna measured nitrogen and its stable isotopes 15 N and 14 N in the fur and whiskers of domestic cats as well as in cat food available in supermarkets. They then compared nitrogen isotope values to those found in the hair of humans who ate different diets. The results were published in Frontiers in Ecology and Evolution .
“When we tested cats’ hair for nitrogen isotopes, the results made them look like they eat mostly plants,” said first co-author Viktoria Zechner. “This means that looking isotopically at animal hair alone can sometimes be misleading about their diet.”
From food to fur
Nitrogen has two main stable isotopes, 14 N (lighter) and 15 N (heavier). The ratio between them is expressed by δ 15 N. It can be used to determine an animal’s position in the food web, with higher δ¹⁵N values indicating a higher position. However, nitrogen isotope values in animals’ food and tissues usually aren’t identical. This difference is described by a number known as the trophic discrimination factor (TDF). A low TDF means the isotope signatures in an animal’s tissues are very similar to the isotope signatures of their food source. On average, animals show a TDF of around 3 to 5‰ (parts per thousand).
The team used hair from 35 domestic cats who were kept indoors and fed commercially available food. Whisker samples were taken from 14 cats. In a parallel study, human scalp hair from 653 people eating different diets was collected. The average isotopic value of all commercial cat foods included in the study was determined. For humans, the team determined isotopic values by diet: vegans, vegetarians, and omnivores, respectively. The team then compared isotope differences between the hair of cats and humans.
‘Vegan’ meat-eaters
In human hair, δ 15 N values differed significantly by diet: the omnivores’ value was around 8.8‰, displaying much higher δ 15 N signatures than vegetarians (around 8.2‰) and vegans (7.2‰). In cats’ hair and whiskers, δ 15 N values were closer to the values of vegans, lying at around 6.6‰ and 6.5‰, respectively. Average δ 15 N values of a mixed human diet lay at 4.1‰ and in cat food at around 5.0‰.
Therefore, when considering differences in 15 N enrichment in human food and tissue and cat food and tissue, the researchers found a substantially lower TDF of around 1.6‰ in cats than in human omnivores (around 4.7‰).
“In this sense, the old saying ‘you are what you eat’ might apply to cats more than we thought,” said Maryna Tiutiunnyk, first co-author of the study. The reason for this unexpectedly low TDF could have to do with how cats’ bodies process nitrogen and its stable isotopes. Cats are efficient in using the proteins from their meat diet. “Because they eat high-quality meat protein that closely matches their own bodies’ amino acid makeup, they can channel those dietary amino acids straight into their hair protein (keratin) with only minimal isotopic changes,” Tiutiunnyk explained. Accordingly, their hair does not reflect many of the chemical and isotopic changes that a meat meal induces. Other animals and humans, both meat-eaters and vegans, process dietary protein far less efficiently, leading to higher nitrogen isotope enrichment in their hair.
Treacherous signatures
“This does not mean that cats eat like vegans,” first co-author Hannah Riedmüller pointed out. “But it overturns long-standing assumptions about carnivore isotope signatures.” It also means that low δ 15 N values are not always proof of a plant-based diet but can reflect other things such as diet composition or metabolism efficiency.
“It therefore is imperative to find valid proxies that can be measured relatively easily to define diet quality relative to organismal demand and body composition, to better predict trophic level enrichment,” senior author Dr Wolfgang Wanek explained.
Despite these findings, the physiological mechanisms underlying the low trophic 15 N enrichment and therefore TDF in cats remain unknown. The study also focused exclusively on hair and whiskers, and it is yet to be determined if similarly low TDFs occur in other tissues that are frequently analyzed in isotopic studies, such as blood, muscles, or bones.
Frontiers in Ecology and Evolution
Experimental study
Animals
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The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.