The behaviour of the humpback whale (Megaptera novaeangliae) is a well-studied area of research. The humpback – a member of the order Cetacea, along with dolphins and porpoises – is known to have remarkable communicative abilities, and has the most complex vocalizations of the 77 known cretacean species. But, although the ethology of the humpback is relatively well known, there have been few studies of the organization and composition of this organism’s brain.
Now, after carrying out anatomical studies, a team of American researchers has established that the humpback’s brain contains a type neuron that was previously believed to be unique to humans and great apes. Patrick Hof and Estel Van der Gucht of the Department of Neuroscience at Mount Sinai School of Medicine in New York have in the past, carried out comparative neuroanatomical studies in order to determine the phylogenetic relationships between species. In the current study, they compared the brain of the humpback with those of a number of other cetacean species. The study, published online today in The Anatomical Record, shows that the humpback’s brain contains spindle neurons and that, like the human neocortex, that of humpbacks has a modular organization with certain cell types cortical organised in cortical “islands”.
Spindle neurons (left) were first discovered in the anterior cingulate gyrus of humans and great apes, and were were believed to be unique to these species. (Surprisingly, according to the original paper in which the cells are characterised, spindle neurons are absent from the neocortices of cetacean species.) Spindle neurons are projection neurons (i.e.they connect one region of the brain to another) and were given their name because of their spindle-like appearance. They have large cell bodies and, in the human cortex, occur in clusters of between 3-6 neurons, which are found exclusively in layer Vb. By contrast, in the brain of the chimpanzee, spindle neurons are distributed in a similar way, but are far less abundant than in the human brain, occurring on their own or in groups of two or three.
The belief that spindle neurons are involved in cognition is based on the observation the cells are severely affected byAlzheimer’s disease, with an average of about 60% lost due to the neuronal degeneration that takes place associated with the condition. Spindle neurons are thought to have emerged in a common hominid ancestor some 15 million years ago, as they are not found in the lesser apes or other species of primates. They are believed to be involved in the adaptive changes and functional modifications that occurred during human evolution. There is also some evidence that spindle neurons are abnormally located in the brains of autistics.
Spindle neurons are thought to have evolved in the cetacean lineage some 30 million years, and were perhaps retained only in those cetaceans with larger brains. Over a period of 55-60 million years, cetaceans have evolved highly specialized capabilities, such as echolocation, a complex mode of communication, sperm whales exhibit other complex social behaviours such as co-operation, cultural transmission, coalition-formation, and the use of tools. Underlying these behaviours is a complex neocortex.
Considering the new evidence of the existence of their existence in the humpback brain, and their apparent involvement in autism, it is tempting to speculate that these cells have some role in social behaviour. There would also appear to be some kind of correlation between the abundance of spindle neurons and the complexity of social structures in a given species. Finally, it is interesting that the authors conclude that “cetacean and primate brains may be considered as evolutionary alternatives in neurobiological complexity.”