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. 2026 Jul 1;55(4):e70150. doi: 10.1111/ahe.70150

Creation of an MRI‐Based Three‐Dimensional Common Wombat (Vombatus ursinus) Brain Atlas and Linking Brain Morphometry to Wombat Behaviour

Natasha B Anthoniel 1, Kristy R Q Goh 1, Timothy J Stait‐Gardner 2, Nyoman D Kurniawan 3, Marianne D Keller 1,✉
PMCID: PMC13322708  PMID: 42385146

ABSTRACT

Marsupials lack a corpus callosum; in its place Diprodontia have a fasciculus aberrans connecting the hemispheres. However, despite this uniqueness, the realm of marsupial brain anatomy remains largely unexplored, especially through the lens of modern imaging techniques. Currently, there is a glaring absence of publicly available reference materials to explore normal brain morphometry to aid in defining and investigating marsupial neurodegenerative brain diseases. This study segmented magnetic resonance images of a wombat brain with an overall size of 41,745.46 mm3 to create a three‐dimensional atlas. The volume and ratio to the overall brain size of each segment were quantified. The common wombat has a moderate hippocampal formation (2464.34 mm3; 5.90%), thick hippocampal commissure (535.19 mm3; 1.28%), well‐developed tectum (481.42 mm3; 1.15% and a large epithalamus; 53.96 mm3; 0.13%). The common wombat's moderately sized cerebellum (5181.99 mm3; 12.41%) with prominent folia and fissures but less folding may suggest the low need for complex motor movement when burrowing. The common wombat also has a large diencephalon (3104.70 mm3; 7.44%), large thalamus (2206.81 mm3; 5.29%), a moderately sized hypothalamus (530.71 mm3; 1.27%), moderately sized basal ganglia (caudoputamen) (1345.26 mm3; 3.22%) and moderately sized pons and medulla (2475.26 mm3; 5.93%).

Keywords: brain mapping, CNS, Diprotodontia, fasciculus aberrans, marsupial, volumetry

1. Introduction

Marsupials are the closest known living relative of eutherian mammals, having diverged from each other 160 million years ago (May‐Collado et al. 2015). In the course of their evolutionary history, marsupials have developed various unique and specialised behavioural and functional adaptations to suit a diverse range of habitats.

MRI‐based brain atlases have been developed for humans and various companion and research animals, providing valuable insights into brain morphometry, shape and volumes of brain structures. These atlases can elucidate neurodegenerative diseases and neural development. However, Australia's unique native fauna remains significantly underrepresented in current brain research. We propose to expand existing brain atlases within the marsupial order of Diprotodontia, which currently only include the koala and the quokka (Kariyappa et al. 2022; Taylor et al. 2006).

Studying the brain anatomy of marsupials offers unique opportunities for neuroscience research. It opens the door to defining and investigating marsupial neurodegenerative brain diseases, like toxoplasmosis (Donahoe et al. 2015). Currently, there is a glaring absence of publicly available reference materials for veterinarians and scientists to explore normal brain morphometry.

The last published mapping of the wombat brain was done almost 30 years ago, using histological slides of half of the wombat brain, resulting in partial 2‐dimensional maps (Sanderson and Nelson 1998). The paucity of studies may be due to ethical difficulties in sourcing and experimenting on wild wombats (Gurovich et al. 2019). Previously, it has been shown that brain atlases do not need to use multiple specimens (Nowinsky 2021). In this study, we developed the wombat brain atlas using a similar approach due to limited sample availability.

A recent study has shown the success in using magnetic resonance imaging (MRI) for detailed neuroanatomical mapping in preserved brains (Gurovich et al. 2019). MRI also allows the creation of a 3‐dimensional atlas, revealing more accurate and detailed information and measurements regarding the wombat brain compared to the current atlas by Sanderson and Nelson (1998). The resulting atlas can be used as the basis of future neurological and evolutionary studies and possibly for clinical use (Ashwell 2010).

2. Materials and Methods

The brain of a deceased female 8‐month‐old common wombat ( Vombatus ursinus ) was obtained under Environmental Protection Agency NSW Scientific Licence SL102840 following a post‐mortem at the University Veterinary Teaching Hospital Sydney. No animal ethics application was required due to death from unrelated causes. The brain was fixed in 10% neutral buffered formaldehyde (pH 7.0) for 2 months. The fixed brain had a weight of 53.5 g.

Prior to imaging, the brain was washed in phosphate buffer saline with 0.2% v/v Magnevist (Bayer) at 4°C over 7 days to remove the formaldehyde and increase the image contrast. Both anatomical and diffusion tensor images were acquired using a 9.4 Tesla, Bruker Biospec MRI with a 300 mm Ultrashield 30 cm bore magnet, interfaced to a Bruker Avance III spectrometer running Paravison 7.0 for acquisition and reconstruction (Bruker Biospin, Ettlingen, Germany) using a 72 mm quadrature transmit and receive coil.

MRI scans consisted of a multi gradient echo (MGE) scan (TR = 70 ms; TE = 4 ms with 8 echoes; field of view = 30 mm; thickness = 40 mm), which was acquired in 1 acquisition with no repetitions, with an imaging time of 1 h and 14 min. A 3D diffusion weighted imaging (DWI) dataset was also acquired using a Stajeskal‐Tanner (DWI spin‐echo) sequence with the following parameters: TR/TE = 150/20 ms, δ/Δ = 4/10 ms, 3 b = 0 and 30 diffusion‐encoding directions with b = 3000 s/mm2 and acquisition time of 15 h and 28 min.

The advanced neuroimaging software AMIRA was available through Western Sydney University (WSU) and was used for brain segmentation and tractography.

Segmentation was done in the ‘Segmentation Editor’ workroom in AMIRA (Thermo Fisher Scientific, Massachusetts, USA). The hippocampal formation, hippocampal commissure, fimbria, anterior commissure, fasciculus aberrans, basal ganglia, thalamus, prethalamus, hypothalamus, epithalamus, superior and inferior colliculus, cerebellum and ventricles were mapped. The ‘grow tool’ was used to roughly segment structures based on signal intensity, and the ‘paintbrush’ tool was used to dissect, when necessary. A smoothing factor of 3 was applied to all segments. Any inaccurate smoothened edges were corrected manually. The volume and relative sizes of the brain and each segment were measured using the ‘Measure and Analyse/Material Statistics’ module in the ‘Project’ workroom in AMIRA.

For the slices used for with anatomical annotations, a brain mask of the brain was created by thresholding the higher b‐value DTI images of the DTI scans, where the background signal was suppressed and contrast between the brain and gel was significantly different. This mask could then be applied to any of the DTI b‐value images to highlight the brain structures and remove the background compared to surrounding signals.

3. Results

The brain atlases of other diprotodonts, specifically the tammar wallaby (Notamacropus eugenii) (Ashwell 2010) and quokka ( Setonix brachyurus ) (Kariyappa et al. 2022), were used as a guide to identify and locate the different structures of the common wombat brain. Due to their close evolutionary relationship, their brains are thought to be composed of similar structures located in similar locations. The boundaries of some of these structures were then matched to the available histological atlas of the wombat brain (Sanderson and Nelson 1998).

The MGE scans provided great contrast to delineate the anterior commissure and fasciculus aberrans, hippocampus proper, hippocampal commissure, fimbria of the hippocampus, diencephalon (thalamus, prethalamus, hypothalamus and epithalamus), basal ganglia, tectum, pons, medulla and cerebellum (Figure 1). In contrast, the DTI scans enabled volume rendering and calculation of the whole brain. The common wombat brain has well‐developed gyri and sulci with an overall volume of 41,745.46 mm3. The brain measured 56.66 mm in rostrocaudal length, extending from the most rostral point of the cerebrum to the caudal margin of the cerebellum. The maximum width was 52.19 mm, and the maximum height was 29.07 mm.

FIGURE 1.

FIGURE 1

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Composite image of sections through the brain of the common wombat ( Vombatus ursinus ) using MGE scans. First row: Hippocampal formation and commissure (demarcated and shaded in purple) and fimbria of the hippocampus (lavender); second row: Anterior commissure, including fasciculus aberrans fibres (green); third row: Basal ganglia (blue); fourth row: Epithalamus (pink), thalamus proper (brown), prethalamus (yellow) and hypothalamus (orange); fifth row: Tectum (aqua); sixth row: Pons and medulla (mint green); seventh row: Cerebellum (crimson red). The levels of the sections are indicated by lines drawn through 3D models of the brain made using DTI scans. Scale bar = 10 mm. Columns from left to right: First column: Frontal section of the brain. The left, top, right and bottom sides of the image are the left, dorsal, right and ventral parts of the brain, respectively. Second column: Sagittal sections of the brain. The left, top, right and bottom sides of the image are the caudal, dorsal, rostral and ventral parts of the brain, respectively. Third column: Axial sections of the brain. The left, top, right and bottom sides of the image are the left, rostral, right and caudal parts of the brain, respectively. Fourth column: 3D view of the relevant brain structures through a frontal section of the brain. The arrow points dorsally. A green arrow indicates a rostral view, whereas a red arrow indicates a caudal view. Fifth column: Top: 2D slice of the brain showing the level of the frontal (vertical line) and axial (horizontal line) sections. Bottom: 2D slice of the brain showing the level of the sagittal section (vertical line).

3.1. Hippocampal Formation, Hippocampal Commissure and Fimbria of the Hippocampus

The hippocampal formation, a C‐shaped structure (Figure 1), can be distinguished from the surrounding white matter due to its lighter signal intensity (Figures 2, 3, 4). In the rostral half of the brain, where it is difficult to distinguish its boundaries from the connecting grey matter, the medial boundary of the hippocampal formation was made vertically at the level of the medial end of the white matter. It has a volume of 2464.34 mm3, occupying 5.90% of the brain volume.

FIGURE 2.

FIGURE 2

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Annotated frontal slices (left) at the level indicated by the line on the 3D volume rendering using T1w scan (right). Slices progress from rostral (top) to caudal (bottom). The top of the image corresponds to the dorsal aspect of the brain.

FIGURE 3.

FIGURE 3

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On the left: Axial section through the brain of the wombat at the level indicated by the line on the 3D volume rendering (right). Sections from the cerebrum at the top to the beginning of the spinal cord segment at the bottom are shown.

FIGURE 4.

FIGURE 4

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Sagittal paramedian midline sections through the brain of the wombat (left) at the level indicated by the line on the 3D rendering using a T1w scan (right). Sections from the rostral cerebral cortex on the left to the brainstem on the right are shown.

The hippocampal commissure connects the two hippocampal formations (Figures 1 and 5) and can be easily distinguished from the surrounding grey matter due to its darker signal intensity (Figures 2, 3, 4). The thin fimbria of the hippocampus runs laterally from either side of the hippocampal commissure to the hippocampal formation (Figure 5). It runs dorsal to the lateral ventricle and ventral to the hippocampus formation. The hippocampal formation, at its tallest part, has a height of about four times the height of the fimbria of the hippocampus, forming an abrupt junction between the hippocampal formation and the fimbria of the hippocampus. This segment has a volume of 535.19 mm3, occupying 1.28% of the total brain volume (Table 1).

FIGURE 5.

FIGURE 5

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3D model in frontal view (left), isometric view (right). Cerebellum (crimson red) hippocampal formation (orange), fimbria of the hippocampus (light blue), anterior commissure and fasciculus aberrans (dark blue).

TABLE 1.

Absolute volume of the various brain structures of the common wombat ( Vombatus ursinus ) and their percentage relative to the entire brain.

Structure Volume (mm3) Percentage relative to the entire brain (%)
Whole brain 41,745.46 100
Hippocampal formation 2464.34 5.90
Hippocampal commissure and fimbria 535.19 1.28
Anterior commissure and fasciculus aberrans 295.68 0.71
Basal ganglia (caudoputamen) 1345.26 3.22
Diencephalon 3104.7 7.44
Thalamus 2206.81 5.29
Prethalamus 313.22 0.75
Hypothalamus 530.71 1.27
Epithalamus 53.96 0.13
Tectum 481.42 1.15
Pons and medulla 2475.26 5.93
Cerebellum 5181.99 12.41
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3.2. Anterior Commissure and Fasciculus Aberrans

Similar to other diprotodonts, the brain of the common wombat has an anterior commissure, which ventrally connects the left and right external capsule, and a fasciculus aberrans, which dorsally connects the left and right internal capsule. The anterior commissure and fasciculus aberrans have the same signal intensity, making it difficult to distinguish them from each other (Figures 2 and 4). However, both structures combined have a lighter signal intensity compared to the internal and external capsules (Figure 3), enabling the segmentation of the anterior commissure and fasciculus aberrans as one structure. On the sagittal view, the darker signal intensity anterior commissure and fasciculus aberrans are surrounded by the lighter intensity grey matter, forming an obvious distinction (Figure 4). The resulting cylindrical structure has a volume of 295.68 mm3, occupying 0.71% of the brain (Table 1).

3.3. Basal Ganglia (Caudoputamen)

The majority of the basal ganglia, especially the striatum, were distinguishable by the signal intensity. This structure was lighter than the surrounding white matter but darker than the adjacent grey matter and lateral ventricles (Figure 2). Although specific parts of the basal ganglia can be identified based on location, an accurate demarcation of borders is difficult due to all parts of this structure having the same signal intensity and texture.

The basal ganglia have a volume of 1345.26 mm3 which equates to 3.22% of the brain (Table 1). The caudate nucleus is mostly surrounded by the lateral ventricles dorsally and the white and grey matter medially (Figure 2). It is located dorsal and more rostral to the putamen, which is mostly surrounded by white matter ventrally and grey matter dorsally and laterally. Both the caudate nucleus and putamen are joined rostrally by the nucleus accumbens and are separated caudally, forming an upturned U‐shaped structure with two prongs going in the craniodorsal direction on both the left and right side (Figure 1). Visually, the caudate and putamen seem to each make up almost 50% of the structure.

3.4. Diencephalon (Thalamus, Prethalamus, Hypothalamus and Epithalamus)

The diencephalon was segmented dorsal to ventral into the epithalamus, thalamus proper (dorsal thalamus), prethalamus (ventral thalamus), hypothalamus and epithalamus (Figure 2).

The epithalamus, which forms the lateral and dorsal walls of the third ventricle, is distinguishable in all views. The darker and denser rounded walls of the epithalamus sit dorsal to the lighter and more porous thalamus proper, forming a visible boundary. The slightly darker, more porous thalamus proper and the slightly lighter, more fibrous prethalamus are demarcated by an obvious dark line that is formed by both signal intensity and fibre direction differences between the two structures. The slightly lighter hypothalamus, which occupies the ventral area of the diencephalon and surrounds the third ventricle, can be differentiated from the prethalamus through the signal intensity differences and the slightly visible medial forebrain bundle, mammillothalamic tract and hypothalamus fibre tracts, which run in a dorsomedial to ventrolateral direction. Although the hypothalamus and the thalamus proper have very similar signal intensities, they are demarcated from each other by the same dark line that differentiates the thalamus proper from the prethalamus.

The heart‐shaped diencephalon has a total volume of 3104.70 mm3, occupying 7.44% of the brain (Table 1). The epithalamus, thalamus proper, prethalamus and hypothalamus have volumes of 53.96, 2206.81, 313.22 and 530.71 mm3, making up 1.74%, 71.08%, 10.09% and 17.09% of the diencephalon, respectively (Figures 1 and 6). The small epithalamus, which is the most dorsal structure of the diencephalon, starts off as two structures rostrally and is joined caudally, forming a curved Y‐shaped structure (Figures 1 and 6). The thalamus proper makes up the dorsal two lobes of the diencephalon and can be seen as a round structure on the sagittal view (Figures 1, 4 and 6). The hypothalamus, which is the most ventral structure of the diencephalon, forms a horizontal rounded cylindric structure (Figures 1 and 6). The prethalamus, located between the thalamus proper and the hypothalamus, forms the lateral sides of the diencephalon (Figures 1, 2 and 6).

FIGURE 6.

FIGURE 6

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3D model isometric view (middle) and frontal view. Cerebellum (crimson red) thalamus (yellow) epithalamus (green) hypothalamus (blue) and prethalamus (on the right image only: Orange).

3.5. Tectum

The tectum consists of the superior and inferior colliculus. In the rostral part of the tectum, the superior colliculus is darker than the more medial periaqueductal grey and lighter than the more ventral tegmentum, making its borders easy to distinguish (Figures 1, 3 and 4). In the middle and caudal part of the tectum, the presence of the inferior colliculus and the similar signal intensity of the tectum to the tegmentum make its borders more difficult to distinguish. In the middle part, the surface of the brainstem has three lobes on each side, the first representing the superior colliculus, the second representing the inferior colliculus and the third representing the tegmentum.

Similarly, in the caudal part, the brainstem has two lobes on each side, the first representing the inferior colliculus and the second representing the tegmentum. Therefore, in these parts of the brainstem, the ventral boundary of the colliculus was made from the end of the lobe representing the inferior colliculus, following the visible fibre directions to the central periaqueductal grey. The resulting segment appears as two lobes at the caudal part of the brain (Figure 1). It has a total volume of 481.42 mm3 and occupies 1.15% of the brain (Table 1).

3.6. Pons and Medulla

Since the pons and medulla in marsupials are difficult to distinguish from each other and are usually regarded as one structure (Ashwell 2010), they were segmented together in this study. Rostrally, the boundaries of this segment are easily visible due to the different signal intensities caused by the lighter more dorsal tegmentum, forming a darker butterfly‐shaped structure in the frontal sections (Figure 4). The segment flattens and elongates caudally, forming a flattened heart‐shaped structure (Figures 1, 4 and 7). The caudal part of the middle cerebellar peduncle, which appears as a darker outpouching tear‐drop‐shaped structure from the pons, was excluded from this segment. However, in the rostral part of this segment, the middle cerebellar peduncle appears as a layer surrounding the pons and has similar signal intensity to the pons. Hence, the rostral part of the middle cerebellar peduncle was included into this segment due to the difficulty of differentiating it from the pons. The resulting segment has a volume of 2475.26 mm3 and makes up 5.93% of the brain.

FIGURE 7.

FIGURE 7

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3D model from left to right: Dorsal view, lateral view, frontal view, caudal view. Cerebellum (crimson red), cerebrum (yellow) pons and medulla (dark blue).

3.7. Cerebellum

The boundaries of the cerebellum were easily distinguishable on all views due to its unique cauliflower‐like shape (Figures 1, 2, 3, 4, 5, 6, 7). The cerebellum is mostly separated from the rest of the brain by the meninges and a small amount of space (Figures 3 and 4). The cerebellum is attached to the brainstem by the cerebellar peduncle, which is significantly darker than the cerebellum (Figure 2). In this study, the cerebellar peduncle and the meninges were not included in the cerebellum segmentation and volumetric measurement.

The cerebellum has a volume of 5181.99 mm3, occupying 12.41% of the brain (Table 1). Its fissures and subdivisions are prominent (Figures 2, 3, 4). The differentiation between the cerebellar grey matter and white matter is obvious. The cerebellar grey matter forms a tree‐like pattern with thin branches.

4. Discussion

Since no detailed anatomical atlases of the common wombat exist, drawings and histological information of the wombat were used to map the cerebral cortex, cerebral white matter, thalamus, hypothalamus, pineal body, hippocampus, midbrain, medulla oblongata, cerebellum and ventricles (Sanderson and Nelson 1998). In addition, information from other marsupials' brain anatomy was used to inform three‐dimensional (3D) segmentation utilizing the works of Ashwell (2010); Gurovich et al. (2019); (Haight and Nelson 1987); Kariyappa et al. (2022); Suárez et al. (2018) and Taylor et al. (2006).

In the presence of small susceptibility artefacts in the MGE scans, the boundaries for segmentation were estimated from all views and the 3D structure, taking the volumetric information of the DTI scan into account. The artefacts affecting segmentation were minor and did not compromise volumetric accuracy. These primarily consisted of sporadic small discontinuities (≤ 1 mm) and localized voxelation effects, which occurred infrequently and were likely attributable to post‐mortem handling and storage of the brain (Figure 1). The olfactory bulb of the brain was missing and therefore not included when calculating whole brain volume or relative volumes.

The whole brain volume of the wombat in this study (41,745.46 mm3) is smaller than the volume of the common wombat brain measured in a previous study (64,963 mm3) (Sharp 2016). This might be due to the presence of shrinkage (Stephan et al. 1981), the size of the animal and the absence of the olfactory bulb.

4.1. Hippocampal Formation, Hippocampal Commissure and Fimbria of the Hippocampus

It was difficult to distinguish the boundary of the hippocampal formation from the connecting grey matter in the rostral half of the brain due to their similar signal intensity. The brain atlas of the quokka brain clearly identified the cornu ammonis and dentate gyrus, but not the subiculum, which could be due to the subiculum's similar signal intensity to the grey matter (Kariyappa et al. 2022). In contrast, the histological atlas of the tammar wallaby brain segmented the subiculum with its medial boundary ending at about the level where the white matter ends (Ashwell 2010). As this is the only diprotodont brain atlas that includes the boundaries of the subiculum, this rule is followed in this study.

The hippocampus, which has a role in memory formation, has a C‐shaped structure consistent with that observed in other marsupials and mammals (Ashwell 2010).

Some studies have suggested that hippocampal size relates to home range size (Jacobs and Spencer 1994) and social behaviour (Todorov et al. 2019). When compared to mammals across a wide range of niches, the relative volume of the common wombat's hippocampus to the entire brain (5.90%) lies within the mid‐range reported for mammals (0.60%–11.01%) (Reep et al. 2007). This value is less than expected, considering the spatial demands for wombats to navigate their extensive burrowing systems and alternate between multiple (typically three to four) burrows every few days (Carver et al. 2024). However, the less than expected hippocampal size can be linked to the solitary nature of the wombat. This correlation to both large home range size and solitary behaviour is supported by the significantly smaller hippocampus of the koala (2.38%), a closely related solitary animal that spends most of its day resting (Goh et al. 2026). Additionally, hippocampal organisation and size can be subjected to plasticity within the species, based on individual experience (Clayton and Krebs 1994; Maguire et al. 2000) and home range sizes (Perdue et al. 2011; Sherry 2006; Smulders et al. 2000). Further investigations into the specific hippocampal region of the wombat brain are required to clarify the neural basis of the wombat's spatial and social behaviour (Todorov et al. 2019).

4.2. Anterior Commissure and Fasciculus Aberrans

To allow the visual separation of anterior commissure and fasciculus aberrans respectively, histological dissections are needed to aid in their spatial separation in future studies.

4.3. Basal Ganglia (Caudoputamen)

The basal ganglia segmented in this study mainly include the entire striatum, with a small number of other structures, such as the globus pallidus and the entopeduncular nuclei. Certain parts of the pallidum, amygdala and septal nuclei in some areas were difficult to distinguish from the surrounding white and grey matter. Hence, the volume of basal ganglia segmented in this study can be seen as an underestimation of the volume of the basal ganglia or an overestimation of the volume of the striatum. Yet, a previous brain atlas of the common wombat has segmented a structure similar to the basal ganglia segmented in this study as the ‘caudoputamen’ (Sanderson and Nelson 1998). Investigating the specific structures of the basal ganglia using histology can be helpful in producing a more accurate volume.

As the measurements of the basal ganglia in this study are a slight overestimation of the striatum, the relative volume of the basal ganglia to the entire brain in this study (3.22%) is comparable to other studies of the mammalian striatum. This ratio in the common wombat is moderate compared to that of other mammals (0.5%–9.19%) (Reep et al. 2007; Yin et al. 2009). Since the striatum consists of both a behavioural and sensorimotor component, it is difficult to relate the size of the wombat's striatum to their behaviour without investigating more specific structures within it through histology.

4.4. Diencephalon (Thalamus, Prethalamus, Hypothalamus, Epithalamus)

The large heart‐shaped diencephalon of the common wombat is similar in shape and size to that of other diprotodonts such as the tammar wallaby (Ashwell 2010) and the quokka (Kariyappa et al. 2022), but different from other marsupials, such as the long‐nosed bandicoot ( Perameles nasuta ), Southern brown bandicoot ( Isoodon obesulus ) and greater bilby ( Macrotis lagotis ) (Gurovich et al. 2019). The relative volume of the diencephalon to the entire brain in the common wombat (7.44%) is on the larger end of the range of that in other mammals (2.81%–8.00%) (Reep et al. 2007).

Similarly, the thalamus proper in the common wombat, which constitutes a significant part of the diencephalon (71.08% of the diencephalon), is relatively larger than that of various species of primates (42.80%–64.80% of the diencephalon) (Baron 1979), but seems similar to that of the tammar wallaby (Ashwell 2010) and the quokka (Kariyappa et al. 2022). Since the thalamus proper is a relay station for sensory and motor signals, investigation into the specific thalamic nuclei using histology is required to correlate this enlargement with the behavioural and functional adaptation of the common wombat.

In contrast, the relative volume of the epithalamus to the diencephalon (1.74% of the diencephalon) is of a large size in relation to the various species of primates (0.40%–2.60% of the diencephalon) (Baron 1979). Further microscopic studies into the wombat's epithalamic nuclei and tracts are required to assess the extent of epithalamic development.

The relative volume of the common wombat's hypothalamus to the diencephalon (17.09% of the diencephalon) is on the lower to moderate end of the range of that in various species of primates (10.20%–40.10% of the diencephalon) (Baron 1979). The hypothalamus is a complex structure supporting various basic life functions by receiving, processing and integrating sensory inputs, driving behaviour (Fong et al. 2023). A study showed that hypothalamic volume scales similarly in insectivores and primates, based on the equation [log y = 1.34 + 0.81 log x—0.033 (log x)2], where y is volume of hypothalamus (mm3) and x is volume of brain (cm3), irrespective of evolutionary history (Hofman and Swaab 1992). The relative volume of hypothalamus to the brain in the wombat aligns with this equation. Hence, further microscopic and connectivity investigations into the complex organisation of the hypothalamus in the common wombat are required to explore its relation to the wombat's behaviour. It is also important to note that hypothalamic physiology can be influenced by maternal environmental factors and stressors (Brown et al. 2023; Fong et al. 2023; Todd et al. 2026).

4.5. Tectum

The tectum, which is mainly responsible for visual and auditory pathways, multisensory integration and predatory behaviour (Isa et al. 2021), seems to be grossly well‐developed in the wombat brain. This is despite evidence that wombats rely more on olfaction for communication, with visual and auditory signals being comparatively less dominant, though still important (Carver et al. 2024). In a study of cartilaginous fishes, large‐eyed species that rely heavily on vision have larger tecta (Yopak and Lisney 2012). They also suggested potential links between the optic tectum, swimming speed and cerebellar size. Hence, the wombat's well‐developed tectum can be correlated to the need for motor and spatial sensory integration. Yet, further studies comparing the whole tectum and the visual, auditory and motor parts of the tectum of the wombat to other animals are required for a better understanding of their anatomically well‐developed tectum.

4.6. Pons and Medulla

In this study, the rostral part of the segment includes the middle cerebellar peduncle surrounding the pons due to difficulty in differentiating them because of similar signal intensities and visual appearance (Figure 2). However, towards the middle part of this segment, the middle cerebellar peduncle appears as an outpouching tear‐drop‐shaped structure lateral to the pons and was not included in this segment. In areas where two outpouchings are present on each side of the pons and medulla, the lower outpouching structures were regarded as the motor or sensory roots of 5n or the trigeminal nerve and were included in this segment. The upper outpouching structures were regarded as the cerebellar peduncle and were excluded from this segment. Caudally, the darker dorsally located cerebellar peduncles were also excluded from this segment. The inclusion of some parts of the middle cerebellar peduncle into this segment suggests that the volume of the pons and medulla measured in this study is an overestimation of the actual volume. Yet, this should not significantly affect the results as only a thin layer of the middle cerebellar peduncle is included. When compared to another study measuring only the medulla of other mammals, the volume of the pons and medulla to the entire wombat brain (5.93%) lies in the middle of this range (3.39%–10.48%) (Reep et al. 2007). However, since this study also includes the pons, it can be expected that the actual relative volume of only the medulla of the wombat brain without the pons is on the smaller end of this range (Figures 1 and 7).

4.7. Cerebellum

The cerebellum, which plays a role in motor control, can be expected to be large and well‐developed in the burrowing common wombat, due to their reliance on motor function (Carver et al. 2024; Maseko et al. 2012). Yet the relative volume of the cerebellum to the entire brain in the common wombat (12.41%) is moderate in size compared to that of other mammals (8.32%–18.97%) (Reep et al. 2007). This is supported by its similarity to another study reporting the average mammalian cerebellum size (12.80%) (Huang and Ricklefs 2013). The lesser need for complex movements can be correlated to the common wombat's smaller‐than‐expected cerebellum size. Contrastingly, the elephant has the largest relative cerebellar size in mammals, which has been attributed to the requirement for complex movements of their trunks (Maseko et al. 2012). Similarly, animals that require elaborate forelimb or beak control have been shown to have larger cerebellum volumes (Sultan and Glickstein 2007).

Additionally, although the common wombat cerebellum has prominent folia and fissures, similar to that of other diprotodontids such as the quokka and the tammar wallaby (Ashwell 2010, 2020; Kariyappa et al. 2022), it has less folding compared to some other animals such as the mandrill ( Mandrillus sphinx ), California sea lion ( Zalophus californianus ), harbour seal ( Phoca vitulina ), chimpanzee ( Pan troglodytes ) and humans (Heuer et al. 2023). This aligns with the observed positive relationship between relative cerebellar volume and the degree of folding (Heuer et al. 2023). Investigating the specialised regions of the wombat's cerebellum, especially the pontocerebellum, which is expected to be large and well‐developed, can help further explore these interesting findings and the relationship between the brain and the wombat's specialised behaviour of burrowing.

4.8. Future Studies

In‐depth studies into the morphology and size of the wombat cortex and its various regions can reveal exciting information regarding the sensory systems of the wombat (Ikeda et al. 2023; Krubitzer 1998).

Although differences in regional brain size may indicate possible functional specializations, behavioural interpretations remain tentative, as size alone does not determine function. Future work examining neurochemical architecture, projection density and cellular composition will be essential to suggest the relationship between neuroanatomy and the observed behaviour in wombats (Hardman et al. 2002). Future studies will include DTI, an advanced MRI modality, which will be used to map the white matter tracts in the brain (Kerbler et al. 2013). Exploring this in the wombat brain for the first time could reveal neural pathways and connectivity between different parts of the wombat brain, especially the fasciculus aberrans. Additionally, histology could aid in investigating and demarcating more specific structures with similar contrast. These studies can support more meaningful comparative analysis between the common wombat brain and those of other marsupials and eutherian mammals.

Additionally, the development of wombat joeys outside the womb, together with the use of MRI modalities, allows the investigation of the development of the brain with reduced environmental impact. Hence, creating and comparing segmented structures among wombats of different ages will give us more insight into developmental neuroanatomy in marsupials, and also eutherians.

5. Conclusions

The development of a comprehensive brain atlas for the wombat represents a significant advancement in marsupial neuroscience and veterinary medicine. By providing a detailed anatomical and functional map of the wombat brain, this project lays the groundwork for improved diagnosis and treatment of neurological conditions affecting this iconic Australian species. It enhances our ability to detect and understand the impacts of diseases such as sarcoptic mange, trauma and congenital disorders on the central nervous system. Furthermore, the atlas serves as a critical educational and clinical resource for veterinarians, wildlife carers and researchers, supporting evidence‐based care and rehabilitation strategies. Beyond its immediate applications, the atlas contributes to the broader field of comparative neuroanatomy, offering insights into the evolutionary trajectories of mammalian brains. Ultimately, this work not only deepens our understanding of wombat biology but also strengthens the foundation for their conservation and welfare.

Author Contributions

N.B.A.: study design, analysis and interpretation of data, drafting the article, revising the article. K.R.Q.G.: analysis and interpretation of data, revising article. T.J.S.‐G.: analysis and interpretation of data, revising article. N.D.K.: acquisition of data, revising article. M.D.K.: conception and design of the study, analysis and interpretation of data, drafting the article, revising the article, supervision M., grant funding.

Funding

This work was supported by Sydney School of Veterinary Science, University of Sydney, start‐up grant. Publication support was received from the Vincent Fairfax Foundation Bequest.

Conflicts of Interest

The authors declare no conflicts of interest.

Acknowledgements

We acknowledge the Sydney School of Veterinary Science (University of Sydney) start‐up grant and the Vincent Fairfax Foundation Bequest. We acknowledge the support from the National Imaging Facility (a National Collaborative Research Infrastructure Strategy capability) for the operation of 9.4 T MRI at the Center of Advanced Imaging, The University of Queensland and Western Sydney University for postprocessing support. We also want to thank Neil Horadagoda and James Maxwell for their help with specimen collection and handling. Open access publishing facilitated by The University of Sydney, as part of the Wiley ‐ The University of Sydney agreement via the Council of Australasian University Librarians.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon request.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon request.

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