Squeeze it like toothpaste: the flexible brain of marsupial mammals

Being stretchy and squeezable may be the key to finding space for the brain in mammals, including humans.

An international study, co-led by Vera Weisbecker of Flinders University in Australia, has revealed that marsupial mammals like possums, kangaroos and wombats appear to have a lot of flexibility when it comes to accommodating their brains into their skulls. The paper, to be published in the journal Evolution, included contributions by Ted Macrini, Ph.D., Chair and Professor of Biological Sciences at St. Mary’s University.

“The brain is one of the heaviest parts of the head, particularly in smaller mammals. But it needs to be placed in a way that doesn’t interfere with the many vital functions of the head, such as seeing, hearing, smelling and of course feeding,” Weisbecker said.

“This study examines the evolution of brain shape in marsupial mammals and provides some new insight as to how the brain co-evolves with the skull of mammals,” Macrini said. “The most interesting finding is the great degree of malleability of the marsupial brain, allowing it to fit into a variety of skull shapes and still do its job.”

Mammal brain stowing

“Stowing” a large brain into the head is a general challenge for mammals, which have much larger brains relative to their body size compared to their reptile-like ancestors. It is also a particularly intriguing issue in humans and their primate relatives, which tend to have extremely large relative brain sizes compared to other mammals,” Weisbecker said.

“We wanted to use a diverse group of mammals to assess if there is a general pattern of brain shape variation to explain the ability of mammals to fit their brains into the great diversity of head shapes without interfering with head function. Marsupial mammals fit the bill perfectly because they are a well-understood group of mammals with diverse head sizes and functions,” Weisbecker said.

The team used CT scanning and 3D visualization to extract the shape of the brain cavity in the diverse group of Australian marsupial mammals.

“These so-called ‘endocasts’ give us a good idea of what the brain itself looks like,” Weisbecker said.

“I then placed landmarks — which work a bit like a coordinate system for shape — all over the endocasts of just under 60 marsupial species, including some really cool fossils like the marsupial lion.”

Extreme stretching patterns

When co-author Emma Sherratt from the University of Adelaide in Australia analyzed the data, the team was in for a surprise:

“The biggest difference between the brains is basically whether they are more cylindrical or more globular. We were struck by how extreme this stretch-compress pattern was – we saw brains that look like marbles, and others that nearly look like tubes!” Sherratt said.

“Intriguingly, this matches the characteristically rounded shape of the human brain, which is sometimes thought to arise from the need to “stack” all the brain tissue into the skull efficiently,” Weisbecker said.

“It is, therefore, possible that human brain shape arises from an ancient general tendency of the brain to assume a range of shapes from rounded to elongate.”

Outlandish brain shapes

Another interesting find was the many unusual brain shapes the team saw.

“Within the general pattern of spherical versus stretched-out, we saw some outlandish endocast shapes. For example, some species had totally flat brains, while others seemed to have parts of the brain “squished aside” by the bone around the middle ear.”

The researchers say that the Tasmanian tiger has enormous olfactory bulbs, which are responsible for smell, and some unusually compact and rounded cerebral hemispheres.

“We believe that round-to-cylindrical brain shapes are probably the most easily achieved evolutionary pathway of fitting the brain into the skull. However, within this overall pattern, the brain seems incredibly flexible, nearly as if it was toothpaste than can be squeezed into any mould,” Weisbecker said.

“This might also explain why we saw substantially different brain shapes in individuals of the same species.”

The teams find matches well with evidence that the brain of some mammals can change size and shape during an animals’ lifetime.

“We suspect that a flexible brain is the key to success in other animals as well. For example, some crocodiles and ancient coelacanth fishes have extremely long brains; and birds have their eyes imprinted on their brain shape. It appears that the brain is capable of functioning regardless of where it goes in the skull.”

“The most interesting finding is the great degree of malleability of the marsupial brain, allowing it to fit into a variety of skull shapes and still do its job.”

Ted Macrini, Chair and Professor of Biological Sciences

The research shows brain function might not be easy to determine from brain shape.

“We found no correspondence of brain shape with movement patterns, for example, if a species climbs trees, glides, hops or walks on all fours,” Weisbecker said.

“We suspect that the overall shape of the mammalian brain is strongly determined by the requirements of the skull. Understanding specific adaptations of the brain probably require investigation of finer detail than overall brain shape.”