Pecking order and the struggle of group living.

At the age of ten, Thorleif Schjelderup-Ebbe, a Norwegian zoologist and psychologist, started to collect systematic data on patterns in aggressive interactions among domestic chickens.

At this young age, he saw past the chickens' outwardly similar appearance, and by learning to identify each individual, he became captivated by the differences in behavior among them. He noticed that although the chickens frequently pecked each other during feeding competition, not all chickens were pecked equally. Instead, certain hens were the ‘despots’, able to peck all others, whereas other hens were pecked by all others. In fact, through careful observation of which hens were pecking, Schjelderup-Ebbe found that members of his chicken societies were highly variable in their tendency to peck and be pecked and could be arranged in order of their ability to peck others.?

At nineteen, he published these observations from his childhood data notebooks in his first scientific paper describing what he called the pecking order.

In 1922, by the age of twenty-seven, in his doctoral thesis, he concluded that these social orders were not necessarily dependent on the hens' strength or age but on their individual personalities. His work revealed that hens established a social order determining who could peck whom in a fight. Remarkably, he even observed non-linear hierarchies, such as dominance triangles.

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Even today, chickens continue to be among the most studied models for collecting data on social dynamics and dominance hierarchies, even though in Italy we say "chicken brain" to indicate a stupid person.?

In some cases, research aimed at improving animal welfare in farms can provide surprising insights for understanding stress and relational difficulties among humans.

The cover picture of this article refers to a recent study aiming to analyze the impact of physical and social stress on laying hens kept in different housing systems: battery cages (a), small littered ground pens (b), and free-range (c) systems.

Despite the deplorable conditions in which battery-caged chickens live, confined to the space of an A4 sheet, even free-range chickens do not seem to fare much better, as evidenced by their damaged plumage from constant pecking.

Free-range chickens are freer to move but must cohabit with another 3,000 hens, which also causes a very high level of stress.

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Modern domestic chickens derive from the red jungle fowl, a wild species from India that lived in groups of about ten to thirty animals, where a strict dominance hierarchy based on individual recognition prevailed.

Although the productivity of domestic chickens has increased significantly, from the prehistoric hens laying 4-8 eggs per year to the modern hens laying 200-300 eggs per year, their brain capabilities have remained roughly the same as their ancestral species. Living in groups of 3,000 individuals makes it impossible to form stable hierarchies: the number of conspecifics they encounter far exceeds their memory limits for individual recognition. The animals perceive each other as complete strangers and are forced to constantly reestablish their own and others' positions in the social hierarchy, leading to continual pecking.

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Dominance hierarchies are a form of social organization that evolved to limit the number of conflicts so that not everyone has to fight everyone else.

For example, if I am the hen number three in the rank and a new hen arrives and challenges hen number two and wins, my rank position will drop from three to four. But, if the new hen loses to hen number two but wins against hen number four, my position as hen number three remains unchanged.?

Therefore, all animals in social organizations based on dominance hierarchies must also be capable of transitive inference calculations to reestablish their position in the group continually.

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The story of stressed chickens invites us to reflect on how the increase in productivity that humans have also experienced from prehistory to the present day does not correspond to a similar increase in cognitive abilities, which have remained largely unchanged in Homo sapiens.

Even though we have built metropolises, we continue to live in clustered networks of relatives and friends whose number fits within the limits of our individual recognition capabilities. This strategy allows us to maintain a certain control over our social position and this is the reason why we care so much about all social signals, like cars or job titles, that facilitate to assess (and hopefully increase...) our position within the group.

Main references:

·????? G. Vallortigara – Cervello di gallina: visite (guidate) tra etologia e neuroscienze (2005)

·????? E.? D. Strauss et al. - The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies (2022)

·????? E.D. Jarvis et al. - Avian brains and a new understanding of vertebrate brain evolution (2005)