Social cognition and social behaviour in cooperatively breeding Arabian babblers (Turdoides squamiceps) and humans (Homo sapiens) : a case of convergent evolution?

Abstract

In this Ph.D. thesis I investigate the role that cooperative breeding, monopolisation of mating and an “ape-like cognitive system” may have played in the evolution of human social traits. To this end, I have studied two specific socio-cognitive skills (intentional communication, joint attention) and a social behaviour (concealed sex) in a cooperative breeding bird species: the Arabian babbler (Turdoides squamiceps). In the following, I outline the main results of my dissertation.In article one, “Intentional presentation of objects in cooperatively breeding Arabian babblers (Turdoides squamiceps)”, I test whether Arabian babblers are capable of first-order intentional communication, a socio-cognitive skill that is fundamental to human communication. The results show that: (i) Arabian babblers invite each other for copulation by presenting an object; (ii) object presentation is not a costly signal, as objects are not costly to acquire; (iii) birds do not trade food for sex, as presenting edible items is not more likely to result in copulation; and (iv) object presentation by Arabian babblers possesses characteristics that are used to infer first-order intentional communication in pre-linguistic children.In article two, “Joint attention skills in wild Arabian babblers (Turdoides squamiceps): A consequence of cooperative breeding?”, I test the hypothesis that the evolution of joint attention skills requires an ape-like cognitive system and systematic reliance on alloparental care. I present evidence that travel coordination among Arabian babblers fulfils key hallmarks that are used to infer joint attention-based communication in pre-linguistic children: (i) intentional communication, (ii) co-orientation of attention, and (iii) establishment of common ground between interlocutors. The finding of intentional communication and joint attention - two socio-cognitive skills that are considered uniquely human by some scholars – in a cooperatively breeding bird species questions the necessity of an ape-like cognitive system for the evolution of these skills. At the same time, these results support the hypothesis that cooperative breeding facilitates socio-cognitive performances.In article three, “Why hide? Concealed sex in dominant Arabian babblers (Turdoides squamiceps) in the wild”, I show that dominant Arabian babblers use tactical deception to conceal their copulations from the view of their group members. I present evidence that dominant Arabian babblers do not conceal sex, either to avoid predation, as a dominance signal or to avoid social interference. I suggest that concealment of copulations by dominant animals and concealment of socially legitimate sex by humans raise the same question: why do individuals, whose sexual activity is not expected to be interrupted, conceal sex from the sensory perception of conspecifics? I present the "Cooperation-Maintenance" hypothesis, which postulates that concealed sex is a behavioural strategy prevent sexual arousal in observers (proximate explanation), thereby to maintain sexual monopolisation of one’s partner/s and cooperation with other group members (ultimate explanations).In article four, “Why do humans and nonhuman animals conceal sex? The Cooperation-Maintenance Hypothesis”, I analyse zoological and cultural data to (i) provide the first systematic evidence that the preference to conceal sex is widespread across human cultures; (ii) show that current hypotheses about the function of concealed sex in humans are not supported by ethnographic data, nor by comparative data from nonhuman great apes; and (iii) present an integrative framework for studying concealed sex across species and cultures.In article five, “Why do we hide legitimate sex? The relationship between concealed sex and sexual exclusivity in humans”, I present preliminary analyses of the largest ethnographic database exists – the eHRAF - showing that the extent of sexual exclusivity between human pairs is associated with the extent of sex concealment across cultures. These results provide first empirical support for the cooperation-maintenance hypothesis. The findings that a cooperative breeding bird species demonstrates cognitive skills and a complex social behaviour that were long considered human prerogatives, lead to two conclusions: First, the need to balance intensive cooperative versus competitive motives within the social group (e.g., heavy reliance on alloparental care versus monopolisation of all/specific sexual partners) may have had a synergetic effect on the evolution of social traits in Arabian babblers, humans and other species that live in comparable social systems. Second, the presented similarities between humans and a phylogenetically distant species emphasise the need to expand comparative research in evolutionary anthropology to species that experience similar social pressures to humans. In particular, the comparison between humans and cooperative breeding species that live in groups with skewed mating ratio, may shed important light on the evolution of sophisticated social traits.