Differences in the size of the brain between a marsupial and a comparable rodent: numbat vs eastern gray squirrel

Many readers will be familiar with a rodent called the eastern gray squirrel (https://www.youtube.com/watch?v=v8B0qy0vJj0 and https://www.youtube.com/watch?v=fkUyLrGiAxA and https://www.youtube.com/watch?v=oh-iJ7htAf8).

Few may be familiar with a marsupial called the numbat (https://www.youtube.com/watch?v=1BMc6HHGKU0 and https://www.youtube.com/watch?v=lrxD27Yyl-c and http://www.numbat.org.au/inthewild and https://www.australianwildlife.org/new-numbat-arrivals-boost-nsw-population/ and https://www.australianwildlife.org/numbats-return-to-central-australia/ and https://perthzoo.wa.gov.au/saving-wildlife/breeding-conservation/numbat-breeding-program and https://www.australianwildlife.org/numbats-reintroduced-to-nsw-national-park/).

However, these two mammals make for an interesting comparison, in terms of braininess and life history strategy (https://en.wikipedia.org/wiki/Life_history_theory).

The numbat (https://www.inaturalist.org/taxa/40262-Myrmecobius-fasciatus) has slow metabolism and reproduction. On this basis, I would expect it to have a smaller brain than the average mammal of its body mass.

The eastern gray squirrel (https://www.inaturalist.org/taxa/46017-Sciurus-carolinensis) has extreme longevity of and obviously intelligent behaviour, including an ability to learn from experience. On this basis, I would expect it to have a larger brain than the average mammal of its body mass.
 
These predictions seem true.
 
The numbat has an endocranial volume of 4.25 millilitres, suggesting that its brain volume is about 4 ml (https://books.google.com.au/books?id=NSydTn-XELwC&pg=PA13&lpg=PA13&dq=brain+size+in+myrmecobius+fasciatus&source=bl&ots=TWSXNCCXuQ&sig=N1S7lG0ngpz99bTbmm3wa1gvFiU&hl=en&sa=X&ved=0ahUKEwjY3YXisbzWAhXGXrwKHSkHBUEQ6AEIUDAJ#v=onepage&q=brain%20size%20in%20myrmecobius%20fasciatus&f=false).

Given that brains are fatty, I guess this might correspond to a brain mass of less than 4 grams.
 
The average brain mass of the eastern gray squirrel is 7.76 grams (https://cals.arizona.edu/research/redsquirrel/res_pdf/Other%20Squirrel%20and%20Sky%20Island%20Publications/Mamm%20Spec%20Sciurus%20carolinensis%2094.pdf).

So, the brain mass or volume of the rodent seems to be nearly double that of the marsupial, despite their having similar body masses.
 
However, there are three points to note here, as follows.
 
Firstly, the numbat does not have a small brain relative to the predictions for its body size based on Dasyurus (https://www.inaturalist.org/observations?place_id=any&taxon_id=40165&view=species).

The encephalisation quotients (https://en.wikipedia.org/wiki/Encephalization_quotient) for Dasyurus spp. are about 0.5 to 0.75, whereas the encephalisation quotient for the numbat is 0.8. (The average value for mammals is, by definition, 1.0.)

I suspect that one of the reasons why the numbat is relatively encephalised for such a slow-breeding marsupial, with such slow metabolism, is its diurnal specialisation, an aspect in which it resembles squirrels including the eastern gray squirrel.

Also, we would expect Dasyrus to be particularly small-brained based on the reduced lifespan in this genus.
 
Secondly, there is a general correlation in mammals between lifespan and brain size relative to body mass.

Therefore, the large size of the brain of the eastern gray squirrel is understandable (see https://anatomypubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1002/ar.20598 and https://pubmed.ncbi.nlm.nih.gov/17847061/ and https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.20598).

The eastern gray squirrel has a large brain despite its rapid reproduction, and because it combines diurnal activity with a long life in which there are many opportunities to learn.
 
Thirdly, there is a correlation, among rodents, between braininess and arboreal adaptation (https://www.researchgate.net/figure/Phylogenetic-Encephalization-Quotient-and-brain-component-percentage-comparisons-among_fig2_350811078).

The bottom line seems to be that, although the metabolism of the numbat is extremely slow, its brain is not extremely small.

And although the reproduction of the eastern gray squirrel is rapid, its brain is larger than expected for the average rodent.

As I see it, both observations may partly be explained by the diurnal behaviour of both species, and in the case of the squirrel also partly by its longevity, which exceeds that expected in a rodent.

Posted on 07 de setembro de 2022, 10:46 PM by milewski