The brain of the long-beaked echidna (genus Zaglossus)

Family Tachyglossidae

The brain of long-beaked echidnas is very similar in external and internal morphology to that of the short-beaked echidna, with the same sulci visible on the cerebral cortex.  On this basis the putative functional areas have been indicated in the image of brain (below) by extrapolation from the identified regions in the brain of the short-beaked echidna.  The brains of all echidnas almost completely fill the skull, so the sulci on the cortical surface are matched by ridges on the interior of the skull that are clearly visible in CT scans (see Fig. of skull CT). Cortical white matter is small in volume in all monotreme species, suggesting that there is a limited population of association/commissural and/or projection axons compared to therian brains of a similar size.  

The olfactory system is as prominent in the long-beaked echidnas as in the short-beaked species.  The nasal cavity has a very large volume and is filled with vertically orientated turbinate sheets.   The cribriform plate, which overlies the nasal cavity and transmits the many fine fibres of the olfactory nerve also has a large area.  The cribriform plate area (mean +SD) for both sides of the skull of Zaglossus bartoni bartoni is 439 + 135 mm2 (3 specimens).  This compares with 300 +40.5 mm2 in the short-beaked echidna (Ashwell et al., 2014). Central laminated olfactory structures (anterior olfactory nucleus and piriform cortex) are also large in all the tachyglossid monotremes (Tachyglossus aculeatus and Zaglossus bruijni; Ashwell and Gurovich, 2019).

All monotremes have poorly foliated corpus cerebelli for their cerebellar cortical volume (Ashwell, 2020), and this is particularly true for the long-beaked echidna Zaglossus bruijnii.  Flocculonodular cortex volume is also low in the long-beaked echidna (Zaglossus bruijnii) for its cerebellar size.

References

Ashwell, KWS (2020) Quantitative analysis of cerebellar morphology in monotreme, metatherian and eutherian mammals. Zoology 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 134, 38-57.

Ashwell KW, Hardman CD, Musser A (2014) Brain and behaviour of living and extinct echidnas. Zoology 117, 349-361.

Griffiths M (1978) The Biology of the Monotremes. Academic, New York.

Kolmer W (1925) Zur Organologie and mikroskopischen Anatomie von Proechidna (Zaglossus) bruijnii 1. Mitteilung Zeistchrift für Wissenschaft Zoologie. 125, 448–482.