Do bandicoots show the naturally-selected antithesis of human braininess among mammals?

(writing in progress)

The braininess of various mammals can be quantified by EQ (encephalisation quotient).
 
Bandicoots (https://en.wikipedia.org/wiki/Bandicoot) are in a sense the mammals most specialised for non-braininess. I have not seen this pointed out as such in the literature.
 
What have been pointed out previously in the literature (e.g. Tyndale-Biscoe 2005, pp. 167-181) are the following.

• Bandicoots grow exceptionally rapidly for marsupials, without having metabolism any more rapid than that of the typical marsupial. In general, marsupials tend to grow more slowly than comparable eutherians (a good ref. for the latter point is Tyndale-Biscoe 2005, page 174), but bandicoots break this rule.
• Bandicoots are unusual among marsupials in possessing an allantoic placenta – which is consistent with the first point because rapid growth just after conception tends to be facilitated by the placenta.

Some useful context on braininess in land mammals:
 
Although certain primates are extremely aberrant in braininess, most land mammals deviate little from typical mammalian braininess. Another way of saying this is that most mammals have encephalisation quotients of 0.5-1.5, where 1.0 is average for mammals.

Humans represent an extreme outlier on the positive side, but there are no corresponding outliers on the negative side, i.e. no mammal is as decephalised as humans are encephalised. Going below an EQ of 1.0 as a flip-side of the human extreme would bring us into the domain of reptiles.

For  example, as far as I can calculate, the komodo dragon (https://www.inaturalist.org/taxa/39449-Varanus-komodoensis) is about as small-brained, relative to the average mammal, as Homo sapiens is large-brained. But I know of no actual mammals which are that small-brained. The brain of the komodo dragon is only about as big as the two human eyeballs together. All mammals have brains larger than this, and seem to score at least EQ 0.3 on the mammalian scale.
 
It is the least encephalised land mammals, with EQ values of 0.5 or less, that concern me here.

Some of these small-brained mammals may be actually decephalised, in the sense that they have proportionately smaller brains than the mammalian ancestor. Please bear in mind that primates are actually an ancient group of mammals, and it seems fair to assume that the first primate, which arose during, not after, the time of the dinosaurs https://www.sciencedaily.com/releases/2010/01/100119154710.htm , had EQ of at least 1.0. So, the smallest-brained of living mammals consist of some primitively unencephalised forms and some actually decephalised forms.

I suspect that Peramelemorphia (https://en.wikipedia.org/wiki/Peramelemorphia) fall into the latter category, i.e. that are actually specialised for small-brainedness, as it were.
 
The eutherians with the proportionately smallest brains tend to be those with the slowest metabolism https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2941275/ . This metabolic correlation does not apply to marsupials or monotremes (Weisbecker and Goswami 2011).
 
Bandicoots are placental mammals despite being marsupials; i.e. bandicoots are ‘placental marsupials’. This is not oxymoronic because:

• being a marsupial is not merely a matter of lacking a placenta, and
• all eutherians have placentae but not all placentals are eutherians.

Here, the facts are known and my only originality is the directness of my expression: most authors stop short of stating that ‘bandicoots are placental marsupials’ although that is a defensible claim.
 
Now something rather paradoxical which I have not seen pointed out as such.
 
In general, marsupials tend to be less brainy than comparable eutherians. Another way of saying this is that marsupials tend not to be as encephalised as comparable eutherians.

The relatively small brains of marsupials seem consistent with their relatively slow metabolism and relatively slow growth, adding up to a syndrome adaptive to landmasses which are limited in productivity and relatively free from predators. Where the habitat is rather poor but there are consequently few predators, it makes sense to have a limited pace of life and limited anti-predator adaptations.
 
What is noteworthy about bandicoots is that their aberrance in terms of growth rates is opposite to their aberrance in terms of braininess. Thus, bandicoots are not merely the most eutherian-like of marsupials, they are unique in combining apparently incongruent specialisations. Bandicoots are specialised, among marsupials, for rapid growth; but at the same time they are specialised, among marsupials, for ‘slow thinking’.
 
From a human perspective it may be difficult to imagine the adaptive advantages of limited encephalisation. However, brains are energetically expensive, with certain requirements also in terms of nutrients. What seems to emerge is that bandicoots are ‘specialised for small brains’.
 
I realise that it is one thing to claim that bandicoots have small brains, and another to claim that they are ‘decephalised’. This is because decephalisation implies that, during the evolution of the lineage, there was an actual process of loss of brain size. This is not necessarily true for bandicoots, which are an ancient group that may always, from the early Eocene onwards, have been small-brained. However, in the sense that bandicoots fail to follow the trends seen in comparable mammals including other marsupials, I am suggesting that they are effectively = adaptively decephalised.
 
The following shows how small-brained bandicoots are. The far-left category is Peramelemorphia, the order including bandicoots and bilbies. Note that most of the other orders also include species with EQ < 1.0.
 
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2941275/ (the basic bar-chart comparing EQ in marsupial orders and eutherian superorders)
 
For example, in Diprotodontia some kangaroos have EQ < 0.5. In Dasyuromorphia at least one species has a similar score for EQ, despite the fact that vatrious species reach scores of > 1.5. Ameridelphia consists of American opossums (https://en.wikipedia.org/wiki/Opossum), and here again the least encephalised forms, such as Didelphis marsupialis and possibly Monodelphus domestica, have EQ < 0.5. In order Xenarthra, some armadillos have EQ < 0.5, such as Dasypus novemcinctus with a value of 0.37. In Afrotheria the decephalised forms are golden moles and tenrecs. In Laurasiatheria the decephalised forms are pangolins, colugos, hedgehogs, solenodons, certain shrews, certain artiodactyls, a few Carnivora, etc.
 
Most of the decephalised land mammals are either specialised herbivores (e.g. colugos, folivorous possums, koalas, etc. https://www.researchgate.net/profile/Maciej_Henneberg/publication/233726395_1998_koala_brain/links/09e4150acb6041e673000000/1998-koala-brain.pdf ), relying on extremely bulky, fibrous diets, or being extremely defended in relying on armour (e.g. armadillos, hedgehogs, pangolins) or chemical defences (e.g. skunks).

One exception is pigs (Suidae), which like the partly aquatic hippos (Hippopotamidae) are puzzlingly decephalised although more omnivorous than herbivorous. It is debatable whether pigs can be considered as ‘extremely defended’ by their canine teeth. Although ‘basal insectivores’, which tend to have limited EQ, sound generalised, many of them are specialised defensively in significant ways; e.g. solenodons are among the few venomous mammals, and shrews are so distasteful that their carcases are not eaten by many Carnivora.
 
In the case of ‘extreme herbivores’ some, but not all, of the effect is attributable to the ‘inflation’ of the body by bulky guts and gut contents. This effect accounts in part for why even those primates specialised for a diet of leaves, e.g. Gorilla (https://en.wikipedia.org/wiki/Gorilla) and Nasalis (https://en.wikipedia.org/wiki/Proboscis_monkey), are decephalised relative to omnivorous primates (although not to the point of having EQ values <0.5, except possibly in the most extremely herbivorous of lemurs https://en.wikipedia.org/wiki/Lemur).
 
The only mammals which have extremely limited EQ, without being either extremely herbivorous or extremely defended, seem to be certain American opossums. Here some explanation may be found in the extreme rates of reproduction, which exceed those of bandicoots. Opossums possibly rival bandicoots in combining omnivory with a combination of fecundity and decephalisation; a difference is that bandicoots are terrestrial whereas opossums climb.
 
Bandicoots are trophically comparable with some armadillos (https://en.wikipedia.org/wiki/Armadillo), but the main difference is that all armadillos are extremely defended by armour, to a degree not seen in any Australian mammal including the short-beaked echidna (https://en.wikipedia.org/wiki/Short-beaked_echidna). Armadillos also seem more specialised than bandicoots in their claws, perhaps helping to explain why bandicoots are more dexterous than armadillos in holding food items above ground.
 
It is an odd aspect of this study that both bandicoots and pigs turn out to be decephalised. This is because, in some ways, bandicoots can be thought of as extremely small, marsupial pigs (see my article in Wildlife Australia magazine).
 
So where does broader survey of decephalisation in mammals leave our interpretation of the decephalisation of bandicoots?
 
Well, bandicoots are the only terrestrial land mammals for which it can be said that they have extreme decephalisation without also having extreme herbivory, extreme defensiveness, or extremely rapid reproduction.

I realise that bandicoots can be said to reproduce rapidly by the standards of Australian marsupials, but that is a two-sided phenomenon.

• On one hand, bandicoots do tend to be fecund compared with Diprotodontia (https://en.wikipedia.org/wiki/Diprotodontia) and Dasyuromorphia (https://en.wikipedia.org/wiki/Dasyuromorphia), and this helps to explain why some bandicoots are among the few marsupials in the vulnerable range of body mass which have evaded extinction and even, in the case of a few species, continue to live in suburbia.
• On the other hand, the fecundity of bandicoots is not comparable to that seen in various eutherians such as rodents, which have large litters, give birth frequently, and grow rapidly. Or, as far as I can ascertain, with certain opossums. (Please bear in mind that although all Peramelemorphia possess eight teats the litters never consist of eight neonates.)

So far, the bottom line seems to be this:
 
Bandicoots are generalised mammals in most ways, the main exceptions being that they

• follow the marsupial mode of reproduction by bearing small, almost embryonic neonates and housing these in a pouch, and
• have smaller brains than expected for terrestrial omnivores lacking extreme defences against predation.

Once one appreciates just how unremarkable bandicoots are in most ways, relative to other land mammals, their decephalisation emerges as one of the most interesting aspects of them. And I can ultimately offer no better explanation for this decephalisation than the fact that bandicoots are entirely restricted to Australasia, a landmass with less predation pressure than is found on larger landmasses with more reliable climates and richer soils.

(Not only do bandicoots not need the armour evolved by armadillos, but they do not need the normal intelligence of mammals?)

(marsupials are so consistently committed to slow reproduction and growth that the only way they make an exception to this is by giving up some of their braininess, which was below-par to start with)

(writing in progress)

Posted on June 18, 2022 11:11 PM by milewski