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Abstract

Lately, Emery et al. developed a bird-specific modification of the "social brain hypothesis", termed "relationship intelligence hypothesis". Although the idea may be valuable, we doubt that it is supported by sufficient evidence and critically discuss some of the arguments raised by the authors in favour of their new idea.
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... For example, certain affiliative tactile behaviours, such as feeding or grooming others, are often used as indicators of close bonds between individuals and are expressed similarly in altricial and precocial mammals [75], but are, in contrast to altricial birds, uncommon or absent in many precocial birds [76]. However, both altricial and precocial species express social bonds in a variety of other ways, including vocal and visual displays ( [76][77][78][79][80][81] for a mammalian review) and chemical [82] cues, increased tolerance and spatial proximity [83][84][85]. In particular, the spatial association between individuals is often used as a proxy for determining social relationships ( [86][87][88], but see [89]). ...
... In sum, both altricial and precocial birds and mammals resort to a large variety of displaying affiliative bonds. The lack of any one of these above indicators of social bonds, however, does not necessarily infer weak and/or low quality affiliative relationships between precocial or altricial mammals or birds, since other forms of expressing relationships may be in place [85]. ...
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Various types of long-term stable relationships that individuals uphold, including cooperation and competition between group members, define social complexity in vertebrates. Numerous life history, physiological and cognitive traits have been shown to affect, or to be affected by, such social relationships. As such, differences in developmental modes, i.e. the 'altricial-precocial' spectrum, may play an important role in understanding the interspecific variation in occurrence of social interactions, but to what extent this is the case is unclear because the role of the developmental mode has not been studied directly in across-species studies of sociality. In other words, although there are studies on the effects of developmental mode on brain size, on the effects of brain size on cognition, and on the effects of cognition on social complexity, there are no studies directly investigating the link between developmental mode and social complexity. This is surprising because developmental differences play a significant role in the evolution of, for example, brain size, which is in turn considered an essential building block with respect to social complexity. Here, we compiled an overview of studies on various aspects of the complexity of social systems in altricial and precocial mammals and birds. Although systematic studies are scarce and do not allow for a quantitative comparison, we show that several forms of social relationships and cognitive abilities occur in species along the entire developmental spectrum. Based on the existing evidence it seems that differences in developmental modes play a minor role in whether or not individuals or species are able to meet the cognitive capabilities and requirements for maintaining complex social relationships. Given the scarcity of comparative studies and potential subtle differences, however, we suggest that future studies should consider developmental differences to determine whether our finding is general or whether some of the vast variation in social complexity across species can be explained by developmental mode. This would allow a more detailed assessment of the relative importance of developmental mode in the evolution of vertebrate social systems.
... In addition, the nature of social relationships seems to highly impact upon brain size, irrespective of group stability and social dynamics. For instance, species forming long-term relationships and/or partnerships tend to have bigger brains than species engaging in short-term or seasonal relationships only ( 17 , but see 29 ). Hence, interacting with particular individuals over time-rather than interacting with many individuals and/or over limited periods only-seems to be a highly cognitively challenging enterprise 30 . ...
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Human children show unique cognitive skills for dealing with the social world but their cognitive performance is paralleled by great apes in many tasks dealing with the physical world. Recent studies suggested that members of a songbird family—corvids—also evolved complex cognitive skills but a detailed understanding of the full scope of their cognition was, until now, not existent. Furthermore, relatively little is known about their cognitive development. Here, we conducted the first systematic, quantitative large-scale assessment of physical and social cognitive performance of common ravens with a special focus on development. To do so, we fine-tuned one of the most comprehensive experimental test-batteries, the Primate Cognition Test Battery (PCTB), to raven features enabling also a direct, quantitative comparison with the cognitive performance of two great ape species. Full-blown cognitive skills were already present at the age of four months with subadult ravens’ cognitive performance appearing very similar to that of adult apes in tasks of physical (quantities, and causality) and social cognition (social learning, communication, and theory of mind). These unprecedented findings strengthen recent assessments of ravens’ general intelligence, and aid to the growing evidence that the lack of a specific cortical architecture does not hinder advanced cognitive skills. Difficulties in certain cognitive scales further emphasize the quest to develop comparative test batteries that tap into true species rather than human specific cognitive skills, and suggest that socialization of test individuals may play a crucial role. We conclude to pay more attention to the impact of personality on cognitive output, and a currently neglected topic in Animal Cognition—the linkage between ontogeny and cognitive performance.
... By examining the relationship quality of some of these species, notably rooks and jackdaws, they suggested that some birds face an adaptive pressure to cooperate and coordinate their behaviour with a partner to raise altricial young, and that this may be underpinned by flexible cognitive abilities. Scheiber et al. (2008) critique many elements of this theory -including the assertion that corvids display more complex pair-bond relationships than other birds -noting that many 'primate-like' features of relationship complexity are also found in greylag geese, which have far smaller brains. However, similar links between brain size and pairbonds have been demonstrated by Shultz and Dunbar and colleagues across a range of species. ...
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New Caledonian crows are an exceptional species of bird, known to manufacture and use complex tools in the wild. In captivity, they appear to possess a causal understanding of many elements of tool use and to demonstrate a number of advanced cognitive abilities. As a member of the large-brained corvid family, these birds have been collectively referred to as 'feathered apes'. However, to what extent are the cognitive abilities of New Caledonian crows similar to those of the great apes? Is it the case that these crows possess a suite of generalised cognitive abilities, comparable in breadth and flexibility to those of our closest primate relatives? Or, alternatively, have these birds evolved more specialised cognitive adaptions, perhaps in response to the evolutionary pressures of tool manufacture and use? In this thesis, I investigate the cognitive abilities of New Caledonian crows across a broad range of domains. I report the results of five experimental studies designed to tap different aspects of New Caledonian crow cognition, spanning the fields of causal reasoning, reasoning by exclusion, cooperation, self-control, and the cultural transmission of tool designs. From the results of this work I argue that New Caledonian crows could be characterised as possessing a portfolio of advanced physical cognition and reasoning abilities, but more limited social cognition. This body of work therefore highlights a number of similarities, but also a number of differences, between the cognitive abilities of New Caledonian crows and the great apes, and provides a window into the ways in which intelligence evolves.
... Studies on the understanding of social relationships in birds are underrepresented (Emery et al., 2007), which is surprising since some groups such as corvids show striking cognitive abilities both in the physical and social domain (review in Emery, 2006). A possible reason could be that most species do not live in stable groups, but are characterised by a relatively high degree of fission-fusion -182 -dynamics, with the pair-bond being the main stable unit (Emery et al., 2007; but see Scheiber et al., 2008). However, many species show seasonal differences in sociality (they are more solitary during the breeding season) or may vary social tendencies across developmental periods. ...
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The social intelligence hypothesis, originally developed for primates to explain their high intelligence and large relative brain size, assumes that challenges posed by social life in complex societies with many group members lead to the evolution of advanced cognitive abilities. In birds, pair-bonded species have larger brains than non-pair bonded species, indicating that the quality of social relationships better predicts social complexity than group size. Ravens are a long-term monogamous and territorial species, renowned for their sophisticated socio-cognitive skills and complex social relationships. Notably, during their early years they live in fission-fusion-like non-breeder societies in which social relationships could be of particular importance. Here we observed the development of dominance and affiliative relationships in 12 hand-raised captive ravens, examining the influence of age, sex and kinship on social interactions. Furthermore, we investigated at which developmental step a stable hierarchy emerged, whether third-party interventions played a role and how selectively birds intervened in others' conflicts. At 4-5 months post-fledging, we found an increase in socio-positive behaviour and a decrease in aggression, along with the establishment of a linear dominance rank hierarchy. In line with kin selection theory, siblings exhibited a greater degree of tolerance and engaged in more socio-positive behaviour. In their first few months, ravens frequently intervened in others' conflicts but supported mainly the aggressor; later on, their support became more selective towards kin and close social partners. These findings indicate that ravens engage in sophisticated social behaviours and form stable relationships already in their first year of life.
... Many hypotheses on the cognitive factors that play a role in the evolution of the size and the composition of brains have been proposed. These hypotheses are traditionally separated into two categories; ecological [the spatiotemporal mapping hypothesis, Clutton-Brock and Harvey (1980); the foraging hypothesis, Milton (1981), Gibson (1986); the technical intelligence hypothesis, Parker and Gibson (1977), Byrne (1997)] and social (the social intelligence hypothesis, Humphrey (1976), Byrne and Whiten (1989), Dunbar (1998); the relationship intelligence hypothesis, Emery et al. (2007) [but see Scheiber et al. (2008); the cultural intelligence hypothesis, van Schaik et al. (2012)], although the two are sometimes linked (Dunbar and Shultz, 2007). Sensory motor factors have also probably played a role in the evolution of cognitive capacities via, for example sensory control of skilled movements (Whishaw, 2003;Sultan and Glickstein, 2007) or sensory adaptations (Paulin, 1993;Barton, 1998Barton, , 2004. ...
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