Are parrots poor at motor self-regulation or is the cylinder task poor at measuring it?

Abstract

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.

Keywords: Brain size; Cylinder task; Detour-reaching task; Inhibition; Motor self-regulation; Psittacidae; Self-control.

Fig. 1

Cylinders used in the study (left: transparent cylinder, right: opaque cylinder)

Fig. 2

Individual learning curves of the 38 birds included in the study, presented separately for each species. Horizontal axes represent the trial numbers, and vertical axes represent the cumulative numbers of correct responses on the cylinder task. The black lines show the individual birds’ scores. The shaded gray areas indicate the upper and lower limits of 95% confidence intervals for the species averages

Fig. 3

Each species’ estimated change in the cylinder task score (proportion correct responses) over the 10 trials, predicted from the generalized linear mixed-effect regression analysis, with correct or incorrect as the binary outcome variable

Fig. 4

Each species’ estimated change in the proportion of ‘away’ failures (i.e., cases where the contact made with the cylinder was directed away from the food, and thus not food-related) over the 10 trials predicted from the generalized mixed model regression analysis, with error type (toward or away) as a binary outcome. The gap in the regression line for the great green macaw is due to the fact that there were no errors at the seventh and eighth trial in this species