سفر ذهنی در زمان در حیوانات؟

Are humans alone in their ability to reminisce about the past and imagine the future? Recent evidence suggests that food-storing birds (scrub jays) have access to information about what they have stored where and when. This has raised the possibility of mental time travel (MTT) in animals and sparked similar research with other species. Here we caution that such data do not provide convincing evidence for MTT. Examination of characteristics of human MTT (e.g. non-verbal declaration, generativity, developmental prerequisites) points to other avenues as to how a case for animal MTT could be made. In light of the current lack of evidence, however, we maintain that MTT is a uniquely human characteristic. Do animals reminisce about the good old days and ponder what the future might hold for them? Humans frequently engage in such mental time travel (MTT), reliving past events and entertaining possible future scenarios 1 and 2 (Box 1). It has been argued that MTT is unique to humans 1, 3 and 4, and that its emergence was a prime mover in hominid evolution [1]. Recently, a series of innovative studies on food-storing scrub jays has raised doubt about this claim. In recovering stored food, these birds appear to act in ways that depend on what they stored where and when in the past 5, 6, 7 and 8, and on what they might expect to happen in the future [9]. This has sparked interest in similar capacities in other species 10 and 11. Although we applaud these efforts, we argue here that current evidence does not yet warrant crediting other species with MTT. By examining other characteristics of MTT we point to different ways in which evidence could be obtained if the competence were to exist in animals.

The current evidence suggests that scrub jays have ‘www-memory’ – they can encode, store and use information about what they cached where and when. This need not imply that they travel mentally back to the original caching event or forward to the recovery. The hypothesis that MTT is uniquely human seems only worth upholding if it can – potentially – be refuted (hence Option 1b above). We identified avenues through which a case could be made. Given at least some indication of competence at prerequisites, our closest relatives, the great apes, might be the most likely candidates. But it is certainly worth finding out more about scrub jays' competence, and about their limits. For example, Clayton and colleagues recently provided some evidence for flexible memory use [39]. However, can jays use their skill outside of the domain of caching and recovery? Can they, for instance, learn different decay functions (e.g. use differently coloured soil with preservatives or mould to change decay time) and use them as predictors of significant non-food related events? One could present jays with choice paradigms contrasting one caching tray now versus two trays later, with differentially perishable foods to investigate delay of gratification and contrast present with future needs. The paradigm could also be used to investigate MTT prerequisites. Emery and Clayton found that jays that had pilfered others' caches would re-cache food in new sites if their own original caching had been observed by another jay [9]. One could investigate jays': (i) understanding of seeing (do they cache differentially in an area not visible to a potential pilferer); (ii) self-recognition (does their own mirror image result in similar re-caching behaviour – if not, does a mirror image of another observer do so?); and (iii) theory of mind (do they reduce re-caching when the observer holds a false-belief, for example when the observer is not privy to a change in the hiding constellation)? When evaluating new evidence for MTT, however, we remind scholars of Tinbergen's [40] four levels of explanation: function, causation, development and evolution. In terms of function, for example, MTT is clearly not unique in providing means for acting to enhance future survival and reproduction. All animals that have memory and a capacity to learn possess a mechanism that is future-oriented. Even the function implied by the Bischof-Köhler hypothesis [41] – to act before the adaptive problem or need is encountered – is clearly not unique to humans. Building a nest, preparing for hibernation and food caching make sense only in the light of the future needs. If we consider