Introduction
Social network analysis has been a very active field for about a century, revealing the complex set of relationships that connect individuals1,2. Among the main objects of interest of social network analysis are personal or egonetworks, which consist of the social networks surrounding selected actors3. A very general observation is that human egonetworks show a layered structure where each layer corresponds to relationships of different emotional closeness4-6. These layers have a definite emotional closeness: there is a layer of very close friends, a subsequent one of good friends, and so on. It is convenient to introduce the concept of nested circles, i.e., the sets of all the relationships up to a certain closeness. Typical circles established in the literature contain 5, 15, 50 and 150 individuals—with a scaling ∼3 between a circle and the next one. There is also evidence for a subsequent circle, formed by acquaintances, of about 500-600 people7.
Social networks have also been studied in a diverse array of species, including mammals, birds, fish, amphibians, reptiles and invertebrates8-13. In this context, the study of nonhuman primate social networks is of particular interest in light of the complexity of their societies, the variability between species, and their evolutionary proximity to humans14. Layered structures have been reported in both the distribution of primate social group sizes15 and in groups of mammals living in multilevel social systems (mainly baboons, chimpanzees, elephants, and dolphins)16,17. These results suggest that human social networks (specifically, our ego networks) may be quantitatively different from those of other species, and that a similar structure in terms of layers or circles may be underlying the social networks of many species. However, the available data on non human animals do not allow substantiating this claim of similarity, because they are not about individual ego networks but about group-level social structures. In this paper we use a continuous analysis of nonhuman primate social interactions (specifically, of chimpanzees) to show that, even in ego networks, the corresponding underlying structure is consistent with that of humans, due to inherently limited resources of cognition and time applying to both species alike.
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To fill this gap, in this paper we present strong evidence that chimpanzees organize their relationships very much like humans do by means of a continuous version18 of the theory introduced in19, consisting of a resource allocation model based on two widely accepted assumptions: the capacity that an individual can invest in social relationships is finite, and relationships of a different intensity carry different costs. This mathematical approach allows us to advance our thinking beyond circles and assign a continuum value to a relationship, which is more reflective of real life and can include, for example, frequency of contact20, number of messages exchanged21, or duration of time spent together21. The formalism developed in18 was applied to face-to-face contact time22, number of messages between Facebook users23, and number of phone calls21, showing a structure similar to that arising when intensities are regarded as discrete categories. This implies that it is not necessary to arbitrarily categorize the data to unveil its structure. As a consequence, egonetworks turn out to be characterized by a new universal scale parameter η, which plays the role of (and is consistent with) the scale factor ∼3 typically found in the discrete setup. We here apply the same formalism to grooming data extracted from over four years of observations of four groups of chimpanzees living in the Chimfunshi Wildlife Orphanage in Zambia, taking grooming as a proxy of the effort devoted to pairwise relationships. Grooming behavior is characterized by one individual manually or orally manipulating the hair or skin of another individual. While this behavior does serve a hygienic function, grooming is well-known to facilitate and reflect social bonding between individual chimpanzees24. We used grooming instead of other relevant behaviors because grooming is one of the most essential social commodities in the lives of chimpanzees and also occurs sufficiently frequently for stable patterns to emerge within a reasonable time frame. Moreover, it is a clearly identifiable behavioral phenomenon with a well-defined direction—this is in contrast to e.g., social proximity, which can be instigated by either partner as well as a coincidental occurrence due to non-social factors like food presence or predation risk. This allows us to compare our results to those obtained when analyzing human ego networks, which are always directed, i.e., it is ego who indicates or shows their relation to the alteri, and not viceversa. As we will see below, our results confirm that the time chimpanzees devote to grooming other individuals is well described by the continuous probability distribution predicted by the model, supporting the existence of similar social signatures for both humans and chimpanzees.
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