We argue that the two temporal cognition systems are conceptually too confined to be helpful in understanding the evolution of temporal cognition. In fact, we doubt there are two systems. In relation to this, we question that the authors did not describe the results of our planning study on ravens correctly, as this is of consequence to their theory.

Evolution involves developmental change. Species comparisons play an important role in comparative cognition because they can uncover common patterns and shared principles in cognitive evolution. Developmental studies reveal foundational elements of cognitive abilities and how they are constructed and integrated. Sensorimotor cognition is such a key element that forms the foundation for later-developing cognitive skills, yet little is known about its development in animals. This study uses 37 behaviors and tasks to investigate the development of Piagetian sensorimotor abilities in five young ravens (Corvus corax) from ages two to eleven weeks. Their developmental pattern largely mirrored that of twelve other bird and mammal species, albeit at a markedly accelerated rate. They reached the final sensorimotor stage, which to date has been shown only in great apes. The onset and sequence of sensorimotor development was identical for all species. Absolute number of neurons in the pallium and rest of brain was associated with achieving a higher stage across these species. This was not the case for absolute or relative brain mass, or number of neurons in the cerebellum or whole brain. We discuss the independent evolution of sensorimotor cognition and the importance of developmental pace and pattern therein. These findings show that the study of sensorimotor development is a useful tool for comparative cognition research.

This commentary relates Hoerl & McCormack's dual systems perspective to models of cognitive development emphasizing representational redescription and the role of culturally constructed tools, including language, in providing flexible formats for thinking. We describe developmental processes that enable children to construct a mental time line, situate themselves in time, and overcome the primacy of the here and now.

A central claim by Hoerl & McCormack is that the temporal reasoning system is uniquely human. But why exactly? This commentary evaluates two possible options to justify the thesis that temporal reasoning is uniquely human, one based on considerations regarding agency and the other based on language. The commentary raises problems for both of these options.

The Gospel of Luke proclaims that ravens do not plan for the future; instead, God provides for them. However, in a recent study (Box 1) we showed that ravens are capable of domain-general planning, also called flexible planning, which we define as making decisions about futures outside one’s current sensory scope in domains for which one is not predisposed [ 1 Kabadayi C. Osvath M. Ravens parallel great apes in flexible planning for tool-use and bartering. Science. 2017; 357: 202-204 Crossref PubMed Scopus (96) Google Scholar ]. We subjected ravens to two such domains – token exchange with humans and tool use – that have also been studied with great apes and monkeys (e.g. [ 2 Osvath M. Osvath H. Chimpanzee (Pan troglodytes) and orangutan (Pongo abelii) forethought: self-control and pre-experience in the face of future tool use. Anim. Cogn. 2008; 11: 661-674 Crossref PubMed Scopus (240) Google Scholar , 3 Osvath M. Persson T. Great apes can defer exchange: a replication with different results suggesting future oriented behavior. Front. Psychol. 2013; 4: 698 Crossref PubMed Scopus (18) Google Scholar ]). The results strongly suggested that ravens’ actions were directed towards future, rather than immediate, rewards. Importantly, we sought to learn more about the deep natural history of cognition by charting behavioural complexities in birds, hitherto reported only for great apes. We investigated functional aspects of raven planning in comparison to apes; that is, whether they use similar operationally defined cognitive functions, such as self-control and long-term memory. Box 1The Setup of the StudyThe study [ 1 Kabadayi C. Osvath M. Ravens parallel great apes in flexible planning for tool-use and bartering. Science. 2017; 357: 202-204 Crossref PubMed Scopus (96) Google Scholar ] comprised four experiments, each with two conditions that were chosen to preclude behavioural predispositions of ravens: token exchange with humans and tool use (ravens are non-tool users in the wild). Before the experiments, the ravens learned to use the tool not through associative learning but from a single observation, and repeated this successfully five times (the only times they had ever used tools). They learned to exchange the correct token in less than 30 trials. The training deviated from the testing, as they never selected the items among distractors and the apparatus or exchanging human was present. Importantly, they never experienced the problem they had to plan for until after training: a situation where an item was needed but they had none [ 7 Köhler W. Zur Psychologie des Schimpansen. Psychol. Forsch. 1921; 1 (in German): 2-46 Crossref Scopus (17) Google Scholar , 8 Tulving E. Episodic memory and autonoesis: uniquely human?. in: Terrace H. Metcalfe J. The Missing Link in Cognition: Evolution of Self-Knowing Consciousness. Oxford University Press, 2005: 3-56 Crossref Scopus (61) Google Scholar ]. The experiments replicated previous studies with primates, but also included new controls. Experiment 1 tested decision-making for 15 min into the future while Experiment 2 tested for 17 h. Experiment 3 tested self-control in a 15-min setting and Experiment 4 in a <1-min setting. The results of these experiments were compared to evaluate the value of the items. All individuals performed significantly above chance per planning criteria in all experiments and conditions. The study [ 1 Kabadayi C. Osvath M. Ravens parallel great apes in flexible planning for tool-use and bartering. Science. 2017; 357: 202-204 Crossref PubMed Scopus (96) Google Scholar ] comprised four experiments, each with two conditions that were chosen to preclude behavioural predispositions of ravens: token exchange with humans and tool use (ravens are non-tool users in the wild). Before the experiments, the ravens learned to use the tool not through associative learning but from a single observation, and repeated this successfully five times (the only times they had ever used tools). They learned to exchange the correct token in less than 30 trials. The training deviated from the testing, as they never selected the items among distractors and the apparatus or exchanging human was present. Importantly, they never experienced the problem they had to plan for until after training: a situation where an item was needed but they had none [ 7 Köhler W. Zur Psychologie des Schimpansen. Psychol. Forsch. 1921; 1 (in German): 2-46 Crossref Scopus (17) Google Scholar , 8 Tulving E. Episodic memory and autonoesis: uniquely human?. in: Terrace H. Metcalfe J. The Missing Link in Cognition: Evolution of Self-Knowing Consciousness. Oxford University Press, 2005: 3-56 Crossref Scopus (61) Google Scholar ]. The experiments replicated previous studies with primates, but also included new controls. Experiment 1 tested decision-making for 15 min into the future while Experiment 2 tested for 17 h. Experiment 3 tested self-control in a 15-min setting and Experiment 4 in a <1-min setting. The results of these experiments were compared to evaluate the value of the items. All individuals performed significantly above chance per planning criteria in all experiments and conditions.

The ability to flexibly plan for events outside of the current sensory scope is at the core of being human and is crucial to our everyday lives and society. Studies on apes have shaped a belief that this ability evolved within the hominid lineage. Corvids, however, have shown evidence of planning their food hoarding, although this has been suggested to reflect a specific caching adaptation rather than domain-general planning. Here, we show that ravens plan for events unrelated to caching-tool-use and bartering-with delays of up to 17 hours, exert self-control, and consider temporal distance to future events. Their performance parallels that seen in apes and suggests that planning evolved independently in corvids, which opens new avenues for the study of cognitive evolution.

Inhibitory control refers to the ability to stop impulses in favor of more appropriate behavior, and it constitutes one of the underlying cognitive functions associated with cognitive flexibility. Much attention has been given to cross-species comparisons of inhibitory control; however, less is known about how and when these abilities develop. Mapping the ontogeny of inhibitory control in different species may therefore reveal foundational elements behind cognitive processes and their evolution. In this study, we tested the development of motor self-regulation in raven chicks (Corvus corax), using two detour tasks that required inhibition of motor impulses to directly reach for a visible reward behind a barrier. One task included a mesh barrier, which partly occluded the reward, and the other task used a completely transparent barrier, the cylinder task. The results suggest that the more visible a reward is, the more difficult it is to inhibit motor impulses toward it, and further, that this inhibitory challenge gradually decreases during development. The mesh barrier is reliably detoured before the animals pass the task with the wholly transparent cylinder. As the majority of the birds begun testing as nestlings, and as we provided them with experiences they normally would not receive in a nest, it is likely that they showed the earliest possible onset of these skills. A control subject, tested at a later age, showed that the mesh detours required no particular training, but that tasks including complete transparency likely require more specific experiences. Adult ravens without explicit training are highly proficient in inhibitory detour tasks, and, together with chimpanzees, they are the best performers of all tested species in the cylinder task. Our results suggest that their skills develop early in life, around their third month. Their developmental pattern of inhibitory skills for detours resembles that of children and rhesus macaques, albeit the pace of development is markedly faster in ravens. Investigating the development of cognition is crucial to understanding its foundations within and across species.

The ability to inhibit unproductive motor responses triggered by salient stimuli is a fundamental inhibitory skill. Such motor self-regulation is thought to underlie more complex cognitive mechanisms, like self-control. Recently, a large-scale study, comparing 36 species, found that absolute brain size best predicted competence in motor inhibition, with great apes as the best performers. This was challenged when three Corvus species (corvids) were found to parallel great apes despite having much smaller absolute brain sizes. However, new analyses suggest that it is the number of pallial neurons, and not absolute brain size per se, that correlates with levels of motor inhibition. Both studies used the cylinder task, a detour-reaching test where food is presented behind a transparent barrier. We tested four species from the order Psittaciformes (parrots) on this task. Like corvids, many parrots have relatively large brains, high numbers of pallial neurons, and solve challenging cognitive tasks. Nonetheless, parrots performed markedly worse than the Corvus species in the cylinder task and exhibited strong learning effects in performance and response times. Our results suggest either that parrots are poor at controlling their motor impulses, and hence that pallial neuronal numbers do not always correlate with such skills, or that the widely used cylinder task may not be a good measure of motor inhibition.

Overriding motor impulses instigated by salient perceptual stimuli represent a fundamental inhibitory skill. Such motor self-regulation facilitates more rational behaviour, as it brings economy into the bodily interaction with the physical and social world. It also underlies certain complex cognitive processes including decision making. Recently, MacLean et al. (MacLean et al. 2014 Proc. Natl Acad. Sci. USA 111, 2140–2148. (doi:10.1073/pnas.1323533111)) conducted a large-scale study involving 36 species, comparing motor self-regulation across taxa. They concluded that absolute brain size predicts level of performance. The great apes were most successful. Only a few of the species tested were birds. Given birds' small brain size—in absolute terms—yet flexible behaviour, their motor self-regulation calls for closer study. Corvids exhibit some of the largest relative avian brain sizes—although small in absolute measure—as well as the most flexible cognition in the animal kingdom. We therefore tested ravens, New Caledonian crows and jackdaws in the so-called cylinder task. We found performance indistinguishable from that of great apes despite the much smaller brains. We found both absolute and relative brain volume to be a reliable predictor of performance within Aves. The complex cognition of corvids is often likened to that of great apes; our results show further that they share similar fundamental cognitive mechanisms.

Recent years have seen acknowledgment from a number of researchers that similarities appear to exist in complex cognitive skills of distantly related species – most notably in corvids, parrots, delphinids, and great apes. Discoveries on complex cognitive skills in common hold the promise of interesting and fruitful new perspectives on cognition. That said, some theoretical approaches seem largely to be lacking. We draw attention to the importance of pre-existing constraints on and freedoms of the evolving animal, which might prove as important as external selective pressures in understanding the evolution of cognition. To elucidate our point, we briefly describe one contemporary cognitive-science approach to cognition. Accounts on cognitive evolution both in behavioral ecology and animal cognition are often hampered by simplistic input-output-based views on cognition. Cognition – in particular complex cognition – may influence animal behaviors in ways that cannot be captured by a purely selectionist account. We discuss the evolutionary processes underlying independently evolved yet similar characters. We highlight the importance of the difference between parallel and convergent evolution in understanding whether complex cognition arises repeatedly only through similar selective pressures; or whether underlying, previously evolved structures are crucial for the occurrence of cognitive similarities. In conclusion we suggest that the developmental sequences leading to apparently similar cognitive skills require further investigation to reveal the evolutionary processes behind them. Our aim is not one of providing ultimate answers to the questions we raise; instead, we draw attention to their existence, the better that they may be addressed.