Cats and Isotopes

A domestic cat in a bag.

A domestic cat in a bag.

Cats. The very mention of them has the power to generate innumerable lazy hits on a blog post. If one were to do an anthropological study of cats using only the Internet as source material, one might be lead to believe that we worship them as deities.

We wouldn’t be the first culture, either. The ancient Egyptians held them in pretty high regard. Their goddess of justice and execution, Mafdet, was a feline-headed creature who protected against snakes and scorpions. Baset, another feline goddess, represented protection, fertility, and motherhood.

Despite the high regard humans have had for cats since at least the dawn of written history, we know very little about how or when they became domesticated. We are pretty sure that the housecat is descended from the African wildcat (Felis silvestris lybica), and people generally assume the process involved a mutually beneficial relationship between farmers and felines in which the cats protected the farmer’s grain from vermin, and the grain provided for a steady supply of vermin for them to eat.

But it is really hard to figure out when that would have happened, and even harder to figure out if that general picture, which makes a great deal of heuristic sense, is accurate.  A recent study on this subject published by a team of archeologists got a great deal of press. They found a small Chinese farming village dated to about 5300 years ago with cat and rodent fossils (among others) discovered at the site.

The basic gist of the study was that chemical analysis of the animal bones found at the site revealed that the rodents ate grain, and that the cats ate those rodents or the grain products directly—suggesting a mutually tolerant relationship between human and cat. Other wild animals found at the site, like deer and hares, didn’t seem to eat any grain, suggesting their food web was independent of any human influence. It’s a fascinating and impressive result, and it seems to be consistent with the generally accepted theory of cat domestication.

But this post isn’t really about cats. It’s about isotopes—different forms of the same element whose variations allow scientists to piece together a cool stories like this one in the first place. Am I pulling a bait and switch? Kind of, but you’ll thank me later.

Photo of an African Wildcat

The African Wildcat (Felis silvestris lybica). Credt: Sonelle.

The story that these archeologists put together was dependent on their ability to construct a food web of the creatures in this ancient village.  Essentially, they had to figure out where everything was getting their food. To do this, they used variations in the isotopes of two elements: nitrogen and carbon.

An isotope is an atom that has the same number of protons as a certain element (thereby making it chemically the same element) but a differing number of neutrons (giving it a slightly different atomic mass.) Isotopes occur naturally, and nature handles each isotope of an element just a little a bit differently. This fact is what makes them so immensely useful.

Nitrogen’s most common atomic form is N14. It has seven neutrons and seven protons. The only other form of nitrogen that isn’t radioactive has 8 neutrons and 7 protons—known as N15. Carbon, similarly, has two stable species as well—C12 (six protons and six neutrons) and C13 (seven neutrons and six protons). Subtle variations in the ratio of C13 to C12 and N15 to N14 found in the bones at this site were the primary clues the archeologists used to puzzle out the feline food web in ancient China.

They basically had two questions. The first was how much human-produced food made up each animal’s diet. The second question concerned from how far up the food chain the source of the food originated. Was an animal eating grain, or was it eating an animal that had eaten grain? Carbon isotopes can answer the first question, nitrogen isotopes can answer the second.

First the carbon. The base of the food chain is generally made up of plants. All plants get their carbon from the atmosphere using photosynthesis—a process that converts CO2 into sugars using sunlight and water. This process prefers regular C12 over carbon C13.

But, there’s a catch. Not all plants incorporate this carbon in exactly the same way. Most plants are known as C3 plants, named after the number of carbon in an intermediate step of the process. But a small subset of plants are C4 plants. The important and relevant thing about C4 plants is that they are much less picky when it comes to the carbon they take in. That means if you analyze the carbon in a C4 plants versus a C3 plant, the C4 plant will have a lot more C13.

photo of foxtail millet.

Foxtail millet. Credit: STRONGlk7.

This is helpful because the only C4 plant that existed in this area of China 5300 years ago was millet—the main agricultural crop of the region. The amount of C13 in the fossil bones, therefore, can serve as a tracer for the rough amount of agriculturally produced millet in the food.

Second, the nitrogen. Because nature likes to use N14 over N15, N15 will generally remain in an animal, while their waste will be concentrated in N14. So when a predator eats that animal, it will be getting nitrogen that has a higher in N15 relative to the base of the food chain. Every time you go up in the food chain, the amount of N15 relative to N14 increases.

So if you had an animal bone that was super low in C13 and N15, you could reasonably conclude that it was from an herbivore that ate native (non-agricultural) plants. If it had low C13 but high N15, you could conclude that it was an animal that ate those herbivores.

Conversely, if you had an animal bone with high C13 and low N15, you could conclude that it was from a creature that ate a lot of millet, but did not eat much meat. An animal with high C13 and high N15 is one that ate a lot of animals that were fed millet.

Using this rationale, the archeologists studied the bones of all kinds of animals from their research area. Deer and hare fell into the low C13 low N15 camp—herbivores that stayed away from human-produced food. Livestock like pigs and domesticated animals like dogs fell into the high C13 low N15 group, suggesting they were fed millet and human food waste products. Humans, not surprisingly, fell into the high C13 high N15 group. This suggests they ate animals that were fed millet—perhaps those pigs they raised.

The real story, though, comes from bones of zokors (which are a kind of rodent) and the cats. The zokors had an isotopic composition very similar to the livestock and dogs. It’s unlikely the farmers were intentionally raising these vermin, so one can pretty safely conclude that they acted a lot like rodents today—stealing whatever they could find from humans. Presumably, these farmers would seek to avoid such a scenario.  Grain storage that protected against rodents found at this site corroborates this assumption.

Wild cats, being predators, might be expected to eat a great deal of those hares scurrying about—if they were truly wild. That would give them a high N15 content and a low C13 content. Instead, the researchers found that the cats had a much higher C13 content than expected. This, in their view, makes it clear that the cats were, in their words, “feeding within the human food web.”

figure from the study

Isotopic values of bones found at the research site. X-axis shows carbon ratios, Y-axis shows nitrogen. Credit: Yaowu Hua et al. 2014, PNAS.

One cat that they analyzed stood out. It lived to a very old age and appears to have had  a diet very high in millet and bones with low nitrogen values. This could mean that the cat relied much more on millet, potentially provided to the cat by the humans. Perhaps, they suggest, the cat was unable to scavenge for rodents. Far from conclusive, the findings are nevertheless consistent with the idea that cats were not only tolerated, but an actively encouraged in this part of  this village.

If the archeologists’ interpretation is correct, then they have found the earliest evidence of cat domestication in China. That headline is certainly an attention grabber. But the science behind that headline is fascinating too. The fact that barely perceptible changes on the atomic scale, miraculously preserved for thousands of years, can be dug up to recreate the food web of an entire ancient village merits a headline of its own.

Thanks for sticking it out with the isotopes—here’s an adorable cat video as a reward:


2 Comments on “Cats and Isotopes”

  1. @abofysikss says:

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