Stratified societies are among the most complex societies known in nature. They are organized like Russian nesting dolls—individuals belong to family groups, to clans, to clans.
At each level, the relationships between these social units (individuals, families, clans, and clans) are stable and predictable.
Such a social structure, which has been described in some primates, whales, elephants, and more recently in birds, has likely characterized much of human evolution. In fact, it is still common in many hunter-gatherer societies around the world.
Although stratified societies are documented throughout the animal world, it is not entirely clear what their benefits are.
One hypothesis, based on observations of two populations of modern human hunter-gatherers, is that living in a multi-level society allows people to simultaneously have different types (levels) of cooperative relationships.
In our research, published on March 9 at Current Biologywe tested this hypothesis in a wild population of great wagtails, a familiar songbird in the parks and gardens of southeastern Australia.
Living together means helping each other
Great Faeries live in multi-level societies in which breeding groups – between two and six birds – represent the lowest social level, with close social bonds between individuals.
During the non-breeding season, neighboring breeding groups become closely associated with a few other breeding groups, and these “supergroups” then connect to form communities (the highest social level). As a result, these birds develop social relationships of varying levels of intensity.
To make it possible to track these complex relationships, we attached different colored leg bands to great nymphs in our study population so that we could identify all individuals through binoculars. While banding them, we recorded any birds that made distress calls, characteristic calls that individuals use to seek help when they are in immediate danger, for example from a predator.
Other prey usually respond to such calls and try to help, for example by approaching the predator and giving alarm calls. They may also use a distraction tactic called “rodent running.” To do this, the birds approach the threat in range, assume a crouched posture, and run back and forth like a mouse. This distracts the predator and this “altruistic display of distraction” puts the bird performing it at high risk.
Here we tested whether altruistic responses to calls for help vary across different social levels of society, similar to food sharing among hunter-gatherers, but with much higher stakes.
To simulate a predator threatening a colleague, we presented a stuffed kookaburra—a ferocious predator of small birds, including puffins—while playing a distress call recorded by a local fairy. We then recorded the responses of all the witnesses.
For each breeding group, we tested whether social relatedness affected how willing birds were to help another in distress. We reproduced, on different occasions, a distress call from an individual in the same breeding group, one from the same community, or one from an unknown individual outside the community.
We found that great wagtails were more likely to heed calls for help from birds of the same breeding group. They responded less fervently, took fewer risks, and never carried out rodents when a merely familiar wrecker—from the same community—called for help.
As for foreigners? They were completely ignored. Thus, being part of a complex society allows birds to carefully “dosage” their cooperative assistance.
Like birds, like people
This pattern mirrors that previously found in hunter-gatherers. Here, food is mostly shared by people from the same household, followed by members of the same household group. The least sharing occurs between members of the same camp—the highest social level of their multi-level society.
Similarly, living in a multi-layered society helps fruit to discern with whom to cooperate and how much. Cooperation at different social levels probably also has different social functions.
For example, cooperative relationships among breeding group members may increase group cohesion, survival, and reproduction. At the community level, alliances between neighboring breeding groups are likely to help birds better defend themselves against predators and have less aggression between groups.
Humans and lovely fairies belong to very distant branches in the tree of life (our common ancestor lived at least 200 million years ago. However, the pattern of cooperative behavior shown by these little songbirds is surprisingly similar to our own .
This suggests that the complex patterns of cooperation we see in our own society may have emerged independently multiple times in different species and first appeared millions of years before we ever set foot on this planet.
Ettore Camerlenghi et al, Multilevel social structure predicts individual helping responses in a songbird, Current Biology (2023). DOI: 10.1016/j.cub.2023.02.050
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Reference: Fairies are more likely to help their closest friends but not strangers, as are humans (2023, March 10) Retrieved March 12, 2023, from https://phys.org/news/2023-03-fairy -wrens-closest-friends -strangers.html
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