Short-beaked echidna brain (Tachyglossus aculeatus)
Family Tachyglossidae
The brain of the short-beaked echidna is large (mean endocranial volume + SD = 24.7 + 3.3 ml) for its body size and the cerebral cortex is highly folded (gyrencephalic). Encephalisation (relative brain size for body weight) for the short-beaked echidna is comparable to the domestic cat and lies close to that for New World primates such as the squirrel monkey. Values of the gyrification index (GI) for short-beaked echidnas range between 1.316 and 1.426 (Hassiotis et al., 2003). This is comparable to that in many highly encephalised placental mammals; for example the GI in the domestic cat is approximately 1.530. The cerebral cortex is large (volume of 11,400 mm3 or 43% of brain volume; Pirlot and Nelson, 1978). Most of this is isocortex (i.e. 6 layered), but the allocortical regions devoted to olfaction are very large and form a ventrally projecting piriform lobe. Terminology for the (approximately) nine sulci of the echidna cerebral hemispheres is based around a system of Greek letters assigned in rough descending order of prominence of the sulci (Ziehen, 1897, 1901; Elliot Smith, 1899; Griffiths, 1978). Eight of these sulci are visible on the dorsal (alpha, beta, gamma, delta, epsilon, zeta, omicron) or ventral (mu, eta) surfaces, while sulcus psi is best seen on the medial surface of each hemisphere. Sulci alpha and beta define the borders between motor (M) and primary somatosensory (S1), and motor and frontal cortex (Fr1, Fr2), respectively.
More than 50% of the rostral cortex (i.e. areas Fr1, Fr2, Fr3) has no attributable primary motor or sensory function and this region has often been considered as an expanded prefrontal cortex (Welker and Lende, 1980; Divac et al., 1987a, b). If this interpretation is correct, then the proportion of isocortex occupied by the prefrontal area in the short-beaked echidna (around 50%) is larger than in humans (a mere 29%; Divac et al., 1987a, b). Krubitzer and colleagues reported the existence in this echidna of the same four somatosensory fields (S1, PV or C, R and M) that those authors found in the platypus (Krubitzer et al., 1995). PV is located posterior to the auditory cortex, while R is located in the depths of the alpha sulcus.
Despite the large isocortical volume and gyrification of the cerebral hemispheres, the underlying white matter of the brain of the short-beaked echidna is relatively small in volume. This suggests that the forebrain of the short-beaked echidna (like the platypus and long-beaked echidna of New Guinea) has fewer association, commissural and/or projection connections than those of similarly sized forebrains of therian mammals (Ashwell and Gurovich, 2019).
The cerebellar hemispheres of the short-beaked echidna brain are also remarkably small given the large size and gyrification of the cerebral cortex (Ashwell, 2020). This is in contrast to therians, among whom the cerebellar hemispheres expand in step with isocortical enlargement. The poor development of the cerebellar hemispheres suggests that the cerebro(ponto)cerebellar feedback loop is less developed in the brain of the short-beaked echidna compared to therians.
References
Ashwell KWS (2020) Quantitative analysis of cerebellar morphology in monotreme, metatherian and eutherian mammals. Zoology (Jena) 139, 125753. doi: 10.1016/j.zool.2020.125753.
Ashwell KWS, Gurovich Y (2019) Quantitative analysis of forebrain pallial morphology in monotremes and comparison with that in therians. Zoology (Jena) 134, 38-57.
Divac I, Holst M–C, Nelson J and McKenzie JS (1987a) Afferents of the frontal cortex in the echidna (Tachyglossus aculeatus). Indication of an outstandingly large prefrontal cortex. Brain Behavior and Evolution 30, 303–320.
Divac I, Pettigrew JD, Holst MC and McKenzie JS (1987b) Efferent connections of the prefrontal cortex of echidna (Tachyglossus aculeatus). Brain Behavior and Evolution 30, 321–327.
Elliot Smith G (1899) Further observations on the anatomy of the brain in the Monotremata. Journal of Anatomy and Physiology 33, 309–342.
Fig. 1. Superior view of brain. A view of the brain of the short-beaked echidna from above, showing the main sulci and functional areas. Nomenclature of cortical features is based on Krubitzer et al. (1995) and Hassiotis et al. (2003). Body part names within S1 (manus, trunk, pes, tail) indicate the part of S1 where that body region is represented.
Fig. 2. Inferior view of brain. A view of the short-beaked echidna brain from below showing the main sulci, cortical areas and cranial nerves. Nomenclature of cortical features is based on Hassiotis et al. (2003).
Fig. 3. Horizontal section through brain. A horizontal section through the brain of the short-beaked echidna. The inset shows the position of the section. Cortical regions are named according to Krubitzer et al. (1995) and Hassiotis et al. (2003).