Only a relatively small number of animals have been brought into the lives of humans to serve a purpose of some sort. One of the earliest domesticated animals, the sheep, was an excellent choice for its job – providing wool and food. But why did we manage to tame it at all?
According to evolutionary psychologist Jared Diamond, “domesticable animals are all alike; every undomesticable animal is undomesticable in its own way.”1 He calls it the Anna Karenina principle. Diamond argues that there are six criteria an animal should meet in order to be successfully domesticated. These criteria involve diet, growth rate, breeding in captivity, disposition, tendency to panic, and a social structure. Sheep, one of the major five large mammals that humans ever domesticated, perform admirably against these criteria.
In terms of diet, the animal needs to be an efficient omnivore, willing to feed on what humans have at hand. A picky eater, such as a panda or a koala, would not be a practical animal to keep around. Sheep, on the other hand, are happy grazers.
The growth rate of the animal needs to be fast, otherwise humans would find it difficult to wait around for many years until their livestock reaches maturity. Sheep mature in less than a year.
The animal also needs to be able to breed in captivity. Some of them have such elaborate mating rituals, captivity just doesn’t cut it. But sheep, like other domesticated animals, just don’t care that much about an audience.
Nasty disposition is a big hurdle for successful domestication. This is the reason why humans have been able to take in certain species of horses, but not zebras, who bite, kick, and are downright unpredictable. Sheep, on the other hand… they’re pretty docile.
Tendency to panic is another hurdle – a species that is terribly nervous, fast, and programmed for instant flight, won’t do very well when put in a cage. Think gazelles. Sheep seek protection by huddling together and standing their ground; they won’t run unless it’s absolutely necessary.
The social structure of domesticated animals follows a certain pattern – they live in herds and don’t mind bunching up and sharing a crowded space; they maintain a dominance hierarchy; and they aren’t exclusively territorial. A human can easily hijack such a structure by assuming the role of the head of the pack. A sheep herd will gladly follow its leader, even if it doesn’t look much like a sheep.
There is a caveat, however. This list of criteria was created by analysing the domestication of large mammal herbivore species, such as cattle, sheep, horses and the like.
After all, when we think about the spectrum of animals kept in human vicinity, there are plenty exceptions to this list of criteria. Cats, for example, are solitary and territorial animals. Bunnies most definitely have a tendency to panic. Dogs can be pretty vicious.
A most notable exception is the species that isn’t even a mammal to begin with. It was domesticated in China over 5000 years ago, and has since become entirely reliant on human support for survival, and does not exist in the wild. It is also the only fully domesticated insect.2
At first glance, the larva of the Bombyx mori moth, the domesticated silkworm, is nothing like a sheep. It doesn’t live in herds, has no dominance hierarchy, and has the definition of a picky diet, because it spends its life munching on an almost exclusive diet of mulberry leaves.
However, that life is short – the silkworms grow and cocoon in 6-8 weeks. They also do not have a nasty disposition or even any weapons, like some colourful and venomous caterpillars do. A moth larva also isn’t known for its tendency to panic and flee in dangerous situations, and it definitely doesn’t have trouble performing with a mate, as long as it is allowed to mature into a moth at all.
Amongst all the species that humans have successfully befriended, tamed, and put to good use, the silkworm might seem like the odd one out. However, it has more in common with sheep than we realise.