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September 8, 2017

In evolutionary terms, the domestic chicken is one of the most successful species on Earth. There are an estimated 20 billion chickens alive right now—almost three times as many as there are humans. Apparently, “tasting good to people so they’ll protect you” is a pretty effective evolutionary strategy—aside from that whole “they eat you” thing. Statistically, every human on the planet eats the equivalent of 27 individual chickens every year. The only other terrestrial vertebrate that might rival the chicken in sheer numbers is the Norway rat.

More than just a food source, though, the humble chicken has taught us some important lessons. Through the careful observation of a backyard chicken flock, one of the most important principles of social biology was uncovered—one that applies equally well to humans.

Norwegian Knowhow

In 1904, 10-year old Thorleif Schjelderup-Ebbe was placed in charge of caring for his family’s flock of chickens in Kristiania, Norway (which is now the city of Oslo). Schjelderup-Ebbe had a keen interest in nature and animals, and as he began to carefully watch the behavior of the chickens, he wrote down detailed descriptions and observations in a daily journal.

For more than 4,000 years, since the red jungle-fowl was first domesticated in Southern Asia, farmers had noticed that a flock of hens was a very orderly group. At feeding time, the dominant birds in the flock would eat first, picking out the best morsels. Then, the more submissive birds got their chance, and finally, the least dominant got whatever was left. Farmers knew that if anything happened to disrupt this order—introducing a new bird to the flock or removing one of the dominant birds—there would be a brief period of discord as birds fought with each other to re-establish dominance. Then, peace would reign once again.

But it wasn’t until young Schjelderup-Ebbe began his years of observations, however, that anyone finally began to get a clear picture of how this flock order was established and maintained. Through years of recorded data, he realized that there was a hierarchy within the flock.

The dominant hen at the top got first choice at all the food and the best roosting spot, and if any other bird tried to infringe on those prerogatives, she would quickly peck the usurper into submission. And this was not based solely on size: The older, savier hens were often able to dominate even bigger, naive birds. The second-ranking bird, on the other hand, was able to peck any subordinate but dared not try to assert herself against the dominant bird. And so it went down the line, with each hen pecking the birds ranked below her and in turn being pecked by those above her.

When Schjelderup-Ebbe became a zoology student at the University of Oslo and wrote his Ph.D. thesis in 1921 on the social structures of birds, he called this the pecking order. The concept of the dominance hierarchy and pecking order was quickly applied to social animal groups ranging from fish to wolves to humans.

Picking The Order

For the most part, the resulting social order is peaceful: Each chicken knows her place and stays in it. It’s a rare thing to see open combat over dominance in any well-established flock, unless there is some sort of disruption. Though the pecking order itself can occasionally change if older birds grow too weak to defend their position or younger birds become more experienced and move up in rank.

Schjelderup-Ebbe was able to see enough examples of fighting to detect the pattern that lay beneath them but only after several years. Later experiments by other researchers established that the chickens in a flock need not even engage in actual combat to determine who was dominant to whom; the birds were able to learn their own place by watching the results of fights with others.

If Chicken No. 2 beats Chicken No. 3 in a fight, for instance, Chicken No. 4 will learn that she can’t mess with Chicken No. 2, even if they have never fought. In fact, researchers found that Chicken No. 4 will be submissive to Chicken No. 2 even if they had never seen each other before and remain so even if they are then separated for months or years and reintroduced.

Far from being the dumb-as-a-rock chicken brain of popular image, barnyard chickens were capable of understanding and retaining an enormous amount of information about their social surroundings. But Schjelderup-Ebbe realized that the pecking order was not a simple linear ladder, which fell neatly into a rigid set of rungs. Many times, he had observed instances where Chicken No. 1 would dominate Chicken No. 2, Chicken No. 2 would peck Chicken No. 3, but Chicken No. 3 was often able to assert herself against Chicken No.1. Schjelderup-Ebbe called these situations “triangles.”

Numbers Game

Another discovery made by Schjelderup-Ebbe in his parents’ chicken coop, however, also had unexpected relevance to human evolution. He found that if the size of the flock grew above 30 birds or so, the chickens were unable to remember all the social relationships, and their pecking order completely broke down. Now, instead of the orderly peaceful social group they had been living under, no bird knew its place, and every bird tended to occasionally try to assert dominance over another, at random.

Over the centuries, chickens have been bred for size, egg production and meatiness, but they have not been selectively bred for disposition. In the absence of a social hierarchy to keep them in check, chickens are naturally aggressive and quarrelsome birds; after all, they were originally used by humans for cockfighting, not as food animals. In very large flocks, then—such as the thousands of birds in a typical poultry factory farm—bird-on-bird violence is constant and never-ending.

To add to the problem, chickens are naturally triggered by bright-red colors, which is why roosters have bright-red combs at breeding time. Once a hen in a flock pecks another hen and draws blood, the wound then attracts the rest of the flock who peck at it also, usually causing fatal damage.

Some farms attempt to stop this by keeping the birds under constant red light bulbs, which makes red look duller and lessens the triggered aggression. But by far the most common method of preventing injury and death in the large commercial flock is to de-beak all of the chickens, by cutting off the sharp end of their beaks with an electrically heated trimmer so they can’t peck each other—barbaric at best.

From Poultry To People

In 2003, British evolutionary psychologist Robin Dunbar randomly selected a number of English families and made a simple measure: How many Christmas cards did they send out every year to friends, family and acquaintances? What Dunbar found was exactly what he had already expected to find: Nearly all of his test subjects, no matter where they lived or what their profession or level of education or income, sent out roughly 150 Christmas cards.

How had Dunbar made that prediction? After mathematical sociologist H.G. Landau’s studies on chicken societies (see “The King Chicken Theorem” on page 55), biologists had also began studying primate societies. One of the leading figures in this effort was paleo-
anthropologist Louis Leaky, who dispatched three of his students to study apes in the wild, hoping it would provide insight into how early hominid societies functioned. It was soon discovered that there seemed to be a mathematical relationship between the size of a primate brain’s cortex—the part of the brain that handles conscious thought, memory and learning—and the number of individual members in that species’ typical band. The bigger the brain was, the bigger the troop size was.

In 1992, Dunbar applied this mathematical relationship to humans, and he found that a typical human troop should contain about 150 individuals. This became known as Dunbar’s Number, and it relates directly to the chickens. (See “Dunbar’s Number” on page 57 for examples.)

Just as the chicken brain can only remember and process about 30 different social relationships, the human brain can only handle about 150. Like the chickens, if you exceed the rough maximum number of social relationships that the human brain can handle at once, the whole system collapses. While you might live in a city of 15 million people or have a million friends on the Internet, you can’t actually know more than 150 of them; everyone after that is just a name, no different than a stranger.

In primate groups, bigger numbers mean better protection, as the larger group is more effective at detecting and fighting off predators and defending the group’s territory from incursions by neighboring groups. But more social members also means more social relationships to keep track of, and as primate society became more complex—unlike chickens, primates are able to form social alliances and friendships that allow them much more power and flexibility inside the dominance hierarchy—the number of possible social permutations grows exponentially.

The human version of The King Chicken Theorem is enormously complex. The result was a ratcheting effect during hominid evolution: Bigger and more complex social groups led to bigger and better brains to process all that new social information, which in turn allowed larger social groups to form.

Our big brains, which allow us to ponder the universe, evolved so we could keep track of who we could pick on in the social pecking order, and who could pick on us. In the end, chicken politics and human politics aren’t that much different from each other.


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