Bird brains might not be quite as useless as their puny size suggests, and may even contain structures that enable them to perform complex cognitive tasks normally associated with mammalian brains. This being the case, the authors of a new study in the journalTrends in Cognitive Science argue that certain high-level mental capabilities may not depend on brain size or structure, but on types of connectivity.
In mammalian brains, an area known as the prefrontal cortex (PFC) controls executive functions such as working memory, reasoning, and the ability to empathize with others. Covering most of the forebrain, the PFC is arranged in six layers – known as laminae – each of which contains a different type of neuronal cell and a particular pattern of connectivity.
The presence of this particular structure has therefore been identified as an essential precursor to the advanced cognitive abilities possessed only by mammals.
However, numerous studies over the past few decades have indicated that some birds may in fact possess mental aptitudes on par with certain species of primate. In one such study, an African grey parrot named Alex learned to label items and used communication skills to solve a range of cognitive tasks. Other experiments have revealed that ravens employ cunning tactics to try and deceive others when competing for food, while magpies and jackdraws appear to recognize themselves in a mirror, indicating a level of self-awareness.
In light of such evidence, the authors of the new study suggest that bird brains must contain an equivalent to the PFC, which coordinates executive function yet occupies a much smaller volume than the mammalian cortex. Reviewing the literature on avian neuroanatomy compiled over several decades, they attempt to explain how “birds with small, non-cortical brains of 5 to 20 grams can show identical capabilities to apes with large cortices and brain weights of between 275 and 500 grams.”
African grey parrots are famous for their communication skills. FrameAngel/Shutterstock
They begin with the work of Harvey Karten, who showed in the 1960s that, even though avian and mammalian brains differ in structure, they actually contain many of the same types of neurons and patterns of connectivity. Future studies then showed that the neurons of an avian brain region called the nidopallium caudolaterale (NCL) connect with one another in a remarkably similar way to the neurons of the mammalian PFC.
For instance, the NCL has been shown to be packed with dopamine-releasing fibers, which delay the transmission of signals in order to store information during working memory tasks, in exactly the same way as the neurons in the PFCs of monkeys do.
Further studies have indicated that injuries to the NCL severely disrupt birds’ “decision making, rule tracking, encoding of subjective values, and the association of outcomes to actions.”
As such, the researchers claim that the NCL is the avian equivalent of the PFC. Remarkably, however, these two structures are entirely dissimilar in both their location within the brain and in form. For instance, the NCL does not contain laminae, seemingly discrediting the notion that the layered structure of the PFC is necessary for the development of advanced cognitive capacities. Instead, the type of connectivity between neurons may be a more reliable source of mental aptitude.
Furthermore, because the evolutionary lines of birds and mammals diverged some 300 million years ago, the researchers suggest that they probably did not both inherit this connectivity from a common ancestor, but evolved to possess it independently of one another.
As such, they claim that “the NCL and PFC possibly represent a spectacular case of evolutionary convergence,” which refers to the process by which unrelated species develop the same evolutionary traits.