Abstract
It is widely accepted that parrots show remarkable cognitive abilities. In mammals, the evolution of complex cognitive abilities is associated with increases in the size of the telencephalon and cerebellum as well as the pontine nuclei, which connect these two regions. Parrots have relatively large telencephalons that rival those of primates, but whether there are also evolutionary changes in their telencephalon-cerebellar relay nuclei is unknown. Like mammals, birds have two brainstem pontine nuclei that project to the cerebellum and receive projections from the telencephalon. Unlike mammals, birds also have a pretectal nucleus that connects the telencephalon with the cerebellum: the medial spiriform nucleus (SpM). We found that SpM, but not the pontine nuclei, is greatly enlarged in parrots and its relative size significantly correlated with the relative size of the telencephalon across all birds. This suggests that the telencephalon-SpM-cerebellar pathway of birds may play an analogous role to cortico-ponto-cerebellar pathways of mammals in controlling fine motor skills and complex cognitive processes. We conclude that SpM is key to understanding the role of telencephalon-cerebellar pathways in the evolution of complex cognitive abilities in birds.
Highlights
It is widely accepted that parrots show complex cognitive abilities
This was confirmed with pANCOVAS that tested if there are grade shifts in the size of spiriform nucleus (SpM) in other orders that have positive SpM residuals (Anseriformes, Strigiformes, Passerifomes and Pelecaniformes)
We found that SpM, a nucleus that acts as a relay between the forebrain and the cerebellum, is large in parrots compared to other birds (Table S3 and Fig. 2), and that the relative size of SpM is strongly correlated with the relative size of the telencephalon across all birds (Table 1)
Summary
It is widely accepted that parrots show complex cognitive abilities. These include: tool manufacture, mirror self-recognition, object permanence, meta-cognition, theory of mind, vocal learning, mental time travel and complex social cognition[1,2,3]. The expansion of the cortex in primates has been accompanied by an increase in the size of the cerebellum and different components of cortico-cerebellar pathways[9,12], in particular, the circuit between the PFC and cerebellum that includes the pontine nuclei and parts of the thalamus (Fig. 1)[9,12,13,14,15]. If the evolution of complex cognitive abilities are dependent upon enlargements of a cortico-ponto-cerebellar pathway[9,12,13,14,15], parrots should have enlarged pontine and/or SpM nuclei, and these nuclei will covary with the size of their afferent and efferent targets. We test these hypotheses in a large, comparative dataset and show that SpM is enlarged in parrots, but that only the SpM nucleus covaries with telencephalon size in birds, adding further evidence to the convergent evolution of brain structure and cognition in parrots and primates
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