Data on braininess in mammals, part 8, including confusion about the encephalisation quotient of Cavia porcellus

@tonyrebelo @jeremygilmore @ludwig_muller @christiaan_viljoen

...continued from https://www.inaturalist.org/journal/milewski/96318-data-on-braininess-in-mammals-part-7-ontogenetic-progression-of-brain-mass-in-ruminants-from-birth-to-adulthood#

In this Post, I collate and discuss valuable data on encephalisation quotients (EQ) in three articles in the literature, viz.

Shigeno et al. (2017),
Papini (2008), and
Herculano-Houzel (2007).

This information covers a wide range of mammals. However, most noteworthy are certain rodents, in which I question the validity of the data.

A reminder to readers:

EQ indicates braininess,
by definition, the average mammal has EQ=1.0, and
values >1.0 indicate braininess exceeding expectations for a mammal, whereas values <1.0 indicate the opposite.

Within each of the three sets of data, I list the various spp. in order of decreasing EQ.

SHIGENO ET AL. (2017, https://link.springer.com/chapter/10.1007/978-4-431-56469-0_19)

Listed in order of decreasing EQ:

My commentary:

The two monotremes (https://en.wikipedia.org/wiki/Monotreme) are 'primitive' among mammals. For example, Ornithorhynchus has body temperatures 5-34 degrees Celsius, usually about 31 degrees Celsius - compared to 37 degrees Celsius in Homo sapiens.

However, monotremes are surprisingly brainy. This seems correlated with their extreme electrosensitivity (https://www.researchgate.net/publication/13087040_Electroreception_in_Monotremes).

The monotreme Tachyglossus aculeatus has the largest prefrontal cortex, relative to body mass, of any mammal. This contributes perhaps up to 50% of the volume of the cerebral cortex, compared to about 29% in Homo sapiens (https://www.perplexity.ai/search/what-percentage-of-human-brain-mI1HODdqQMaI.m6jc_5oMw and https://pubmed.ncbi.nlm.nih.gov/3427409/).

The opossum genus Didelphis is decephalised even among marsupials. This correlates with extreme fecundity (in the sense of many newborns per litter) and short lifespan.

PAPINI (2008,
https://www.amazon.com.au/Comparative-Psychology-Evolution-Development-Behavior/dp/1841694592)

Listed in order of decreasing EQ:

My commentary:

I suspect that, in the case of Bison bison, the value

refers to mature males, and
would be considerably greater (>0.6?) in adult females.

The value for Atherurus africanus is surprisingly great for a non-sciurid rodent. In the case of the 'edentate' Choloepus hoffmanni, I find the magnitude of the value to be incredible (possibly erroneous?).

The converse applies somewhat to Sus scrofa, the value for which seems too small (https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0157378).

The contrast in encephalisation is noteworthy between two microchiropteran bats (https://en.wikipedia.org/wiki/Microbat), namely Desmodus rotundus and Myotis lucifugus.

HERCULANO-HOUZEL (2007, https://d1wqtxts1xzle7.cloudfront.net/48887613/pdf-libre.pdf?1474043818=&response-content-disposition=inline%3B+filename%3DEncephalization_Neuronal_Excess_and_Neur.pdf&Expires=1719801754&Signature=fpHt-S-W9Z5opc4EEnbjOxfR4CSYsB7Aab-X1A~71YFdVlgfNnGEWCrYoFGFki~SrS1UAjHBOuj0iJWi719jy9wNRi7iaQYtW27f864iDohiBoSvcv-mMXVmlUJYUiT58s6rraZWQUM8a~5cBRZHKYLaJZkYTbbbTgkxoiz~bctcjF7hG9CYmzVj3rVgIXno7guL2tB15NkSjXiVhmvmp2R~LgJU~cARUVOSmIfrLc6FZrVTxf8Y1cGCLz30wrkRiJ3p4rNsqfFssqdAOhRrx5E1dk6xL6mKnqjk41NoRJu6EoUNzoksLLN4O5rLgdwlotiGGo~RlgRtvsJwaWxPYQ__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA)

Listed in order of decreasing EQ:

Cavia porcellus (n=2) body mass mean 311.0 g, brain mass mean 3.759 g, encephalisation quotient 1.711
https://www.inaturalist.org/taxa/119405-Cavia-porcellus

Dasyprocta sp. (species not stated) (n=3) body mass mean 2843.3 g, brain mass mean 18.365 g, encephalisation quotient 1.519
https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=43711&view=species

Mus musculus (n=4) body mass mean 40.4 g, brain mass mean 0.416 g, encephalisation quotient 0.949
https://www.inaturalist.org/taxa/44705-Mus-musculus

Rattus norvegicus (n=4) body mass mean 315.1 g, brain mass mean 1.802 g, encephalisation quotient 0.828
https://www.inaturalist.org/taxa/44576-Rattus-norvegicus

Cricetus cricetus (presumably; scientific name not stated) (n=2) body mass mean 168.1 g, brain mass mean 1.020 g, encephalisation quotient 0.745
https://www.inaturalist.org/taxa/45836-Cricetus-cricetus

Hydrochoerus hydrochaeris (n=2) body mass mean 47500.0 g, brain mass mean 76.036 g, encephalisation quotient 0.71
https://www.inaturalist.org/taxa/74442-Hydrochoerus-hydrochaeris

My commentary:

The encephalisation quotients for Rattus norvegicus and Hydrochoerus hydrochaeris agree with those of Papini (2008), shown above within this Post.

However, the encephalisation quotients found here by Herculano-Houzel (2007) differ surprisingly from those elsewhere in the literature. In the case of Cavia porcellus, I find the value (EQ=1.7) to be incredible.

For comparison, Steinhausen et al. (2016) found the following encephalisation quotients for the same spp. (see https://www.inaturalist.org/journal/milewski/96127-data-on-braininess-in-mammals-part-5#) :

Cavia porcellus 0.40
Dasyprocta sp. 0.91
Mus musculus 0.53
Rattus norvegicus 0.45
Cricetus cricetus -
Hydrochoerus hydrochaeris 0.67

In all cases except the extremely large-bodied H. hydrochaeris, the encephalisation quotients of Herculano-Houzel (2007) greatly exceed those of Steinhausen et al. (2016).

This suggests that

different mathematical methods were used, allometrically, and
those of Herculano-Houzel (2007) make the more sense, in terms of a correlation between braininess and intelligent behaviour.

However, in the case of Cavia porcellus, the disparity is so extreme that it is Herculano-Houzel (2007) who seems to have made some sort of basic error of exaggeration.

The difference in encephalisation quotient for C. porcellus, between the two references, is more than fourfold. A value of 1.7 makes no sense, because

the behaviour of C. porcellus does not, overall, indicate braininess exceeding either that of R. rattus, or that of the average mammal (https://www.perplexity.ai/search/which-behaviours-of-cavia-porc-kkPqrcR9TV.FvD.oTBVhhA), and
according to Kruska (2014, https://link.springer.com/article/10.1016/j.mambio.2013.12.005), C. porcellus has been decephalised by domestication.

Perhaps indicating a basic error of exaggeration of brain size in C. porcellus is the following quote from page 1286 of Herculano-Houzel (2007): "the agouti, which has a similar EQ to the guinea pig, does have a much larger number of 'extra neurons'...than the guinea pig...despite the smaller neuronal density found in larger rodent brains."

In possible resolution if this anomaly, please see Pirlot and Bee De Speroni (2009, https://www.degruyter.com/document/doi/10.1515/mamm.1987.51.2.305/html).

Also see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8200626/.

Posted on June 30, 2024 10:23 PM by

milewski

Data on braininess in mammals, part 8, including confusion about the encephalisation quotient of Cavia porcellus

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