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.Electronic supplementary materialThe online version of this article (doi:10.1007/s10071-017-1131-5) contains supplementary material, which is available to authorized users.

The objectives in the field of comparative cognition are clear; efforts are devoted to revealing the selection pressures that shape the brains and cognitive abilities of different species and understanding cognitive processes in differently structured brains. However, our progress on reaching these objectives is slow, mostly because of several major practical challenges. In this review, we discuss 2 major shortcomings: (a) the poor systematics and low magnitude of the phylogenetic comparisons made, and (b) the weak comparability of the results caused by interfering species-specific confounding factors (perceptual, motivational, and morphological) alongside an insufficient level of standardisation of the methodologies. We propose a multiple-level comparative approach that emphasises the importance of achieving more direct comparisons within taxonomic groups at genus or family level as the first step before comparing between distantly related groups. We also encourage increasing interdisciplinary efforts to execute “team-science” approach in building a systematic and direct large-scale phylogenetic comparisons of bigger cognitive test batteries that produce reliable species-representative data. We finally revisit some existing suggestions that allow us to maximise standardisation while minimising species-specific confounding factors.

A comparative approach is required to investigate the evolutionary origins of cognitive abilities. In this paper, we compare the performance of four parrot species, spectacled parrotlets (Forpus conspicillatus), rainbow lorikeets (Trichoglossus haematodus), green-winged macaws (Ara chloroptera) and sulphur-crested cockatoos (Cacatua galerita triton) in standardized string-pulling and string-choice paradigms. We varied the spatial relationship between the strings, the presence of a reward and the physical contact between the string and the reward to test different cognitive skills requiring means-end comprehension. The species tested showed a high individual and inter-specific variation in their ability to solve the tasks. Spectacled parrotlets performed best among the four species and solved the most complex choice tasks, namely crossed-string task and broken-string task, spontaneously. In contrast, macaws and cockatoos failed to identify the correct string in these two tasks. The rainbow lorikeets were outperformed by the parrotlets, but outperformed in turn the macaws and the cockatoos. The findings can be best explained by the variation in social complexity among species, rather than in their ecology.

Economic decision-making involves weighing up differently beneficial alternatives to maximise payoff. This sometimes requires the ability to forego one’s desire for immediate satisfaction. This ability is considered cognitively challenging because it not only requires inhibiting impulses, but also evaluating expected outcomes in order to decide whether waiting is worthwhile. We tested four parrot species in a token exchange task. The subjects were first trained to exchange three types of tokens for a food item of low, medium, and high value and successfully learned to exchange these in an order according to their value. Subsequently, they were confronted with a choice between a food item and a token that could be exchanged for higher-quality food. In additional control conditions however, choosing a token led to an equal or lower payoff. Individuals of all species were capable of deciding economically, yet only large macaws outperformed the other species in one of the crucial controls. For some individuals, particularly African grey parrots, the token apparently had an intrinsic value, which prevented them from choosing economically in some control conditions and which should be considered as potentially confounding by future token exchange studies.