Ontogeny of Cognition and Communication in Corvids

Abstract

Studies on the evolution of language and cognition focused traditionally on our closest living relatives – the great apes – following a divergent approach. However, the comparison of cognitive processes among more distantly related species, such as birds and mammals might shed light into the question why and how humans have evolved such unique cognitive skills. In the last decades, scholars started focusing on distantly related species, for example corvids. Corvids are known for their advanced cognitive abilities but also for their flexible communicative skills. Ravens and crows are renowned to possess social cognitive abilities that are on par with the skills of great apes. Ravens are known to tactically deceive conspecifics, reconcile with- and console valuable partners, remember former group mates and their relationship with them over years, understand third-party relations, and use such understanding to prevent others from forming too strong alliances. Furthermore, recently it has been shown, that ravens not only vocalise to communicate, but also use gestures as communicative means. However, little is known about the ontogeny of these communicative means and the underlying cognitive abilities. Development is a key mechanism of evolution. Using a comparative developmental approach can aid to shed light into the nature of cognitive skills and their origins.In my doctoral thesis I examined the ontogeny of communication and cognition of ravens (Corvus corax) and carrion crows (Corvus corone corone). To unravel developmental trajectories, I tested and observed crows and ravens during the first 1½ years of life. In the course of my PhD I combined observations of the birds’ behaviour and communication, manipulation of the social situation (inducing a tense feeding condition to enhance conflict and post-conflict behaviour), and testing the birds in a comprehensive test battery to assess possible developmental milestones and compare them directly with great apes. To ensure comparability across species, the thesis was based on methods developed by scholars investigating communicative and cognitive development of human and non-human primates. Moreover I got the opportunity to include corresponding data of the performance of great apes in the test battery, which enabled an even more detailed comparison.The major results of my thesis are summarised below:In Chapter II I revealed reconciliatory behaviour in carrion crows. Former studies on corvids’ post- conflict behaviour only found third party affiliation, but not reconciliation between former combatants (except studies on ravens). In my study I tested, if ravens are really outstanding regarding their post-conflict behaviour or if other factors influence the incidence of reconciliation. To do so, I confronted sub-adult carrion crows throughout their first 1½ years of life, with a tense feeding situation to enhance conflict and- post conflict behaviour. I revealed for the first time, that within the corvid species, not only ravens possess the ability for reconciliation but that the social relationships structure impacts on the occurrence of this ability. The strategy of reconciliation thus represented a crucial postconflict behaviour in crows during development where knowledge about individuals is gained and social relationships are negotiated.In Chapter III I compiled, for the first time the gestural repertoire of a bird species, in this case ravens, by applying the methods from non-human primate gestural research. Furthermore I conducted the study throughout the first 1½ years of life to reveal possible developmental milestones. The ravens used twelve different gestures in a flexible way and produced gestures more often intentionally than displays. One major finding of the study was that although repertoire size increased with age, distinct developmental milestones could not be assigned. A rapid development of gestures within the first 4 months of life might increase the fitness of the birds by enhancing cooperation between alliance partners, during e.g. search for food after aggregating into non-breeder flocks.In Chapter IV I conducted a comprehensive cognitive test battery based on a study on great apes and humans. I tested ravens in nine physical and six social tasks four times throughout the first 1½ years of life to reveal possible developmental milestones. Incorporating the data from the great apes of the original study, allowed me to reveal cognitive similarities and differences between great apes and the “feathered apes”. Ravens performed on par with the great apes in most of the social cognitive tasks and in some physical cognitive tasks. Furthermore I did not find differences between the physical and the social domain within the ravens, suggesting a possible general intelligence rather than domain specificity. Concerning development, I did not find an increase throughout the testing period, which suggests a rapid development of cognitive skills within the first 4 months of life (before testing started). To master and engage with this socially complex environment it thus may be adaptive to develop advanced cognitive and communicative skills already at a relatively young age. Furthermore physical cognitive skills need to be developed when the ravens leave their parents to successfully access food.Overall, my thesis provides hitherto undocumented evidence how and when development impacts on communicative and cognitive abilities and social behaviour in corvid species. Furthermore I present that a rapid development of specific skills is linked to a complex social environment and the life history of the birds, which leave their parents early to aggregate in non-breeder flocks. With my work I hope to have stimulated future studies to also adopt a more biocentric view of the evolution of cognition and communication. Moreover, future comparative research on cognitive and communicative abilities should comprise a developmental approach to test how ecology and social structure of a species shapes their skills.