One of the great questions in life. Clearly, this had been plaguing Alana Sharp, a postdoctoral researcher from Australia, so much that she had to go out and research it for herself!
In part of a memoir series published by the Victoria Museum, Alana carefully analysed the skulls of fossil and living wombats to see how their sinuses and brains evolved through time.
Sinuses are part of the skull that form as cavities due to resorption and deposition of bone. This happens due to the different stresses that impose upon the skull as it develops during growth.
Some marsupial species are known to have pretty large sinuses. Even extinct animals, such as Diprotodon, the largest marsupial ever discovered, had massive sinuses that extended all the way through the skull and over the brain towards the back. The evolutionary context for this has remained a mystery though, until now.
Using CT scanning, a 3D digital imaging technique originally designed for medical purposes, Alana was able to investigate the interior sinus anatomy of a range of wombats and their close relatives. Wombats form a group with their ancestors known as vombatiforms, which kinda sounds a bit like a rubbish German EDM band. All vombatiforms are browsing herbivores that arose during the Cenozoic period (66 million years ago to now), and includes the modern koala and wombat.
Alana wanted to know what the relationship between body size, brain size, and sinus size was in vombatiforms. For example, as they got bigger through evolution, did their brains decrease in size while their sinuses increased? So, better sense of smell, less brain power to figure out what it is they were smelling. A classic example of an evolutionary trade-off.
What she found was that in the larger, extinct vombatiforms, the sinuses seemed to almost surround the inner brain, but in the smaller, living species, the sinuses were much smaller. As well as this, the size of the sinuses positively scales with the volume of the whole cranium. Because of this, the size of the brain negatively scales with increasing cranial size, meaning that larger-headed species have bigger sinuses but smaller brains in general.
The reasons for larger marsupials, including these vombatiforms, having smaller brains could be due to any number of reasons, including the brain being an expensive machine to run metabolically, or environmental conditions relating to seasonality. It might be then that smaller brains are necessary in order to compensate for higher energy demands due to having a large body, something known as the Expensive Brain Hypothesis.
Cranking up the size of the sinuses also increases the area available for attachment of the muscles responsible for chewing, which compensates for the decrease in external surface area of the braincase due to brain shrinkage.
Alana thinks there is much more room for growth in this area though. For example, how do individuals within a species vary in their sinuses? What about other marsupials, such as kangaroos and other macropods – do they follow similar patterns? Do we see variations between the sexes at all?
Hopefully all will be revealed in time as we learn more and more about the weird and wonderful wombats!
Sharp, A.C. 2016. A quantitative comparative analysis of the size of the frontoparietal sinuses and brain in vombatiform marsupials. Memoirs of Museum Victoria 74: 331–342 (link)