A user-friendly guide to braininess in mammals

Fig. 1 in http://www.pnas.org/content/107/37/16216.full compares brain sizes, relative to body sizes, in various lineages of mammals. This constitutes a powerful summary that will be of interest to many readers.

However, most naturalists may find some of the categories so complicated as to be baffling. So, here is a user-friendly guide to the interpretation of this chart.
 
Braininess can be defined as brain size relative to body size, using a mathematical correction for scaling factors. Braininess can be quantified as Encephalisation Quotient (EQ, https://en.wikipedia.org/wiki/Encephalization_quotient), which by definition has a value of 1 for the average mammal.
 
Please note, immediately, this study omits the monotremes (https://en.wikipedia.org/wiki/Monotreme).

Marsupials (https://en.wikipedia.org/wiki/Marsupial) are divided into several clades. Indeed, in this classification, marsupials are not a coherent category as such. For example, American opossums (https://en.wikipedia.org/wiki/Opossum) are unrelated to Australian possums (https://en.wikipedia.org/wiki/Possum), being convergent rather than stemming from a common ancestor.

Equally surprising, in the opposite way, is that primates (https://en.wikipedia.org/wiki/Primate) share the same clade as rodents (https://en.wikipedia.org/wiki/Rodent). This means that we humans are more closely related to rats than possums are to opossums.

And then, as if to discourage any lay person trying to fathom this chart:
One clade, namely Laurasiatheria (https://en.wikipedia.org/wiki/Laurasiatheria), is so extremely diverse that it ranges from bats to whales and from skunks to zebras.
 
Not allowing ourselves to be deterred by these bewildering cladistic associations, let us move from left to right across the same chart (Fig. 1 in http://www.pnas.org/content/107/37/16216.full).
 
On far left we have Peramelemorphia (https://en.wikipedia.org/wiki/Peramelemorphia), the group of marsupials consisting of bandicoots and the bilby. These marsupials have particularly small brains. Look at the great difference in relative brain sizes between bandicoots and the human species, which is the solid dot in the column for Euarchontoglires (https://en.wikipedia.org/wiki/Euarchontoglires), and located right at the top of the chart.
 
The values second from left refer to the large and diverse group including kangaroos, wombats, possums, etc. As readers can see, Diprotodontia (https://en.wikipedia.org/wiki/Diprotodontia) tend to have relative brain size less than the average for mammals, but not as extremely so as in the bandicoots.
 
The next two columns, third and fourth from left, refer to the dasyurid marsupials (Dasyuromorphia, https://en.wikipedia.org/wiki/Dasyuromorphia) and the American opossums (Didelphimorphia, mentioned above). Both groups have relative brain sizes not much different from the mammalian average. This, by the way, is one of the reasons why I find it significant that both the marsupial lion (Thylacoleo) and the thylacine (Thylacinus) are inferior in EQ to Carnivora (see my other Post).
 
So far, what this means is that a quoll (Dasyurus, https://en.wikipedia.org/wiki/Quoll) is likely to be brainier than a dorcopsis (think rat-kangaroo, https://en.wikipedia.org/wiki/Dorcopsis) of similar body size, which in turn is likely to be brainier than a bandicoot of similar body size. And the Virginia opossum https://en.wikipedia.org/wiki/Virginia_opossum, despite its reputation as brainless, would be worth checking to see if in fact it is brainier than some bandicoots.
 
The fifth column from the left, Xenarthra (https://en.wikipedia.org/wiki/Xenarthra), refer to the ‘edentates’, an American group including armadillos (https://en.wikipedia.org/wiki/Armadillo), sloths (https://en.wikipedia.org/wiki/Sloth), and anteaters (https://en.wikipedia.org/wiki/Anteater). As readers can see, this clade has relatively small brains although not as much so as in the case of bandicoots.
 
The sixth column is diverse, apart from an African association common to most of them: elephants (https://en.wikipedia.org/wiki/Elephant), dugongs (https://en.wikipedia.org/wiki/Dugong), golden moles (https://en.wikipedia.org/wiki/Golden_mole), tenrecs (https://en.wikipedia.org/wiki/Tenrec), aardvarks (https://en.wikipedia.org/wiki/Aardvark), and elephant shrews (= sengis, https://en.wikipedia.org/wiki/Elephant_shrew).

These Afrotheria range from decephalised in e.g. tenrecs (which are both insular on Madagascar and extremely defended with e.g. spines) and dugongs, to encephalised in e.g. elephants. The most relevant aspect of Afrotheria in the current context is the fact that elephant shrews, although comparable with bandicoots, score higher than bandicoots in EQ.
 
The seventh column, Laurasiatheria, contains ungulates (https://en.wikipedia.org/wiki/Ungulate) and Carnivora (https://en.wikipedia.org/wiki/Carnivora) with fairly average brain size for mammals. However, it also contains other groups which include extreme values for EQ.

For example, pangolins (https://en.wikipedia.org/wiki/Pangolin) are included in Laurasiatheria, and they are decephalised in keeping with their extreme armour and their staple diet of ants and termites. At the other extreme we have dolphins (https://en.wikipedia.org/wiki/Dolphin), which surprisingly are in the same clade as pangolins and are so encephalised as to rival humans in EQ.
 
The eighth column shows Euarchontoglires, including primates, treeshrews (https://en.wikipedia.org/wiki/Treeshrew), rodents, lagomorphs (https://en.wikipedia.org/wiki/Lagomorpha), and colugos (https://en.wikipedia.org/wiki/Colugo). This clade shows the greatest range in EQ values of all the clades, going from colugos and hares at the bottom to the human species at the top.
 
Because primates – which specialise in braininess – greatly boost EQ in this clade, the authors have seen fit to present a ninth column for Euarchontoglires minus the primates. This shows that, without the primates, this clade would tend to have average EQ for mammals.
 
Many interesting patterns are revealed by this chart. However, one of them concerns the bandicoots.

What emerges is that the Peramelemorphia are unique, among mammals, in being a distinct clade with EQ always < 1.0. No other clade shows such consistency in EQ, and furthermore this consists of decephalisation without the usual associations of small brains such as armour, venom, extreme herbivory, etc.

When one thinks ‘primate’, it is fairly valid to think ‘brainy’ (although some of the extremely herbivorous lemurs may perhaps score < 1.0 in EQ, I need to check that). By the same token, when one thinks ‘bandicoot or bilby’ it is fairly valid to think ‘un-brainy’. I.e. if one had to pick the salient characteristic of bandicoots and bilbies, it would be that they are decephalised, rather than that they are marsupials.

There are many taxa of mammals, in six of the remaining seven major clades of mammals, that turn out to be even more decephalised than bandicoots, along the lines of rat-kangaroos (https://en.wikipedia.org/wiki/Rufous_rat-kangaroo and https://en.wikipedia.org/wiki/Musky_rat-kangaroo and https://en.wikipedia.org/wiki/Desert_rat-kangaroo), opossums, armadillos, tenrecs, manatees (https://en.wikipedia.org/wiki/Manatee), hippos (https://en.wikipedia.org/wiki/Hippopotamus), and colugos.

However, those decephalised mammals belong to clades that also extend to some degree of encephalisation (least so in Xenarthra).

In summary:
The range in values for EQ is great within any given clade of mammals, with a tendency to ‘outliers’ on both the positive and the negative sides of braininess. Whereas most naturalists are familiar with the braininess of certain mammals such as primates and dolphins, what deserves new appreciation is the 'unbraininess' of relatively unfamiliar groups, some of which are not just simply 'primitive'.

Putting off many other themes we could explore here, what emerges for now is a new view of bandicoots. These are not just a minor variant of marsupials. Instead, they are remarkable in their own right, in being the mammalian clade most specialised for limited EQ.

Publicado el 21 de junio de 2022 por milewski milewski

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The western barred bandicoot (Perameles bougainville, https://www.inaturalist.org/taxa/43308-Perameles-bougainville) and the checkered elephant-shrew (Rhynchocyon cirnei, https://www.inaturalist.org/taxa/1123576-Rhynchocyon-cirnei) are similar in body mass (about half a kilogram), and both have long noses and the kind of subtle camouflage patterning which tends to hide the animal in dappled shade.

Both eat mainly invertebrates. Both are, in a sense, ‘living fossils’, because giant elephant-shrews today are similar to their ancestors up to 30 million years ago despite living on the hectic continent of Africa. The two forms are fair examples of evolutionary convergence, despite being unrelated and having radically different reproductive modes.

The western barred bandicoot is native to Western and southern Australia, while the checkered elephant-shrew lives in Africa, where it is the closest thing to a bandicoot in overall appearance.
 
What is particularly interesting in this comparison is the difference in brain sizes. The western barred bandicoot has a brain mass of only about 3.5 g, whereas the checkered elephant-shrew has a brain mass of about 6 g.

Rhynchocyon cirnei, body mass 0.507 kg, brain mass 5.90 g:

https://animals.fandom.com/wiki/Rhynchocyon?file=0e9d363b2ac3e6b8d235c0f763476ec5.jpg

https://www.biolib.cz/en/image/id175494/

https://www.biolib.cz/en/image/id315565/

https://alchetron.com/Rhynchocyon
  
Perameles bougainville, body mass 0.457 kg, brain mass 3.33 g:

https://www.australianwildlife.org/wildlife/western-barred-bandicoot/
 
https://www.wanaturalists.org.au/reports/travel-across-the-world-for-bandicoot-treasures/

Publicado por milewski hace casi 2 años

The following graph shows that monotremes (pale blue inverted triangles) are far brainier than bandicoots (green). The horizontal axis is body mass.
  
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2941275/figure/fig02/

Publicado por milewski hace casi 2 años

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