STEINHAUSEN ET AL. (2016, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5043137/)
I have rearranged the species in Fig. 1 of this reference, in order of decreasing encephalisation quotient, within each order, as follows.
Asterisks* indicate domesticated spp./subspp.
(Please bear in mind that, by definition, an encephalisation quotient of 1.0 is average for mammals. Values >1 indicate brainy (encephalised) spp., whereas values <1 indicate non-brainy/decephalised spp.)
The greater the value for encephalisation quotient, the brainier the species.
PRIMATES
Homo sapiens 6.68 (https://www.inaturalist.org/posts/96127-data-on-braininess-in-mammals-part-5#activity_comment_6dc1762a-9bf4-422c-9361-ea7b2904354e)
Hylobates lar 2.86 https://www.inaturalist.org/taxa/43593-Hylobates-lar
Cebus capucinus 2.69 https://www.inaturalist.org/taxa/43432-Cebus-capucinus
Pongo pygmaeus 2.60 https://www.inaturalist.org/taxa/43582-Pongo-pygmaeus
Ateles geoffroyi 2.44 https://www.inaturalist.org/taxa/43411-Ateles-geoffroyi
Pan troglodytes 2.38 https://www.inaturalist.org/taxa/43577-Pan-troglodytes
Ateles fusciceps 2.26 https://www.inaturalist.org/taxa/43409-Ateles-fusciceps
Macaca mulatta 2.24 https://www.inaturalist.org/taxa/43460-Macaca-mulatta
Chlorocebus pygerythrus 2.13 https://www.inaturalist.org/taxa/68137-Chlorocebus-pygerythrus
Papio hamadryas 2.06 https://www.inaturalist.org/taxa/43535-Papio-hamadryas
Callithrix geoffroyi 2.01 https://www.inaturalist.org/taxa/43372-Callithrix-geoffroyi
Trachypithecus obscurus 1.77 https://www.inaturalist.org/taxa/43494-Trachypithecus-obscurus
Gorilla gorilla 1.23 https://www.inaturalist.org/taxa/43580-Gorilla-gorilla
Alouatta palliata 1.21 https://www.inaturalist.org/taxa/43436-Alouatta-palliata
Lemur catta 1.20 https://www.inaturalist.org/taxa/43617-Lemur-catta
Otolemur crassicaudatus 1.15 https://www.inaturalist.org/taxa/43567-Otolemur-crassicaudatus
Nycticebus coucang 1.01 https://www.inaturalist.org/taxa/43627-Nycticebus-coucang
My commentary:
SCANDENTIA
Tupaia glis 1.05 https://www.inaturalist.org/taxa/46283-Tupaia-glis
Tupaia everetti 1.02 https://www.inaturalist.org/taxa/565410-Tupaia-everetti
Tupaia minor 1.01 https://www.inaturalist.org/taxa/46286-Tupaia-minor
My commentary:
ARTIODACTYLA
Muntiacus muntjak 1.67 https://www.inaturalist.org/taxa/42186-Muntiacus-muntjak
Antilope cervicapra 1.47 https://www.inaturalist.org/taxa/42416-Antilope-cervicapra
Rusa unicolor 1.32 https://www.inaturalist.org/taxa/75053-Rusa-unicolor
Vicugna vicugna 1.31 https://www.inaturalist.org/taxa/42236-Vicugna-vicugna
Cephalophus dorsalis 1.45 https://www.inaturalist.org/taxa/42302-Cephalophus-dorsalis
Cervus elaphus 1.25 https://www.inaturalist.org/taxa/204113-Cervus-elaphus
Ammotragus lervia 1.07 https://www.inaturalist.org/taxa/42422-Ammotragus-lervia
Rangifer tarandus 1.06 https://www.inaturalist.org/taxa/42199-Rangifer-tarandus
Cervus canadensis 1.03 https://www.inaturalist.org/taxa/204114-Cervus-canadensis
Dama dama 0.99 https://www.inaturalist.org/taxa/42161-Dama-dama
*Capra hircus 0.93 https://www.inaturalist.org/taxa/123070-Capra-hircus
Axis axis 0.91 https://www.inaturalist.org/taxa/42166-Axis-axis
*Lama glama 0.90 https://www.inaturalist.org/taxa/121026-Lama-glama
*Ovis aries 0.78 https://www.inaturalist.org/taxa/121578-Ovis-aries
Choeropsis liberiensis 0.75 https://www.inaturalist.org/taxa/74192-Choeropsis-liberiensis
*Bos grunniens 0.68 https://www.inaturalist.org/taxa/889089-Bos-grunniens
*Bos taurus 0.56 https://www.inaturalist.org/taxa/74113-Bos-taurus
Taurotragus oryx 0.56 https://www.inaturalist.org/taxa/75192-Tragelaphus-oryx
Sus scrofa 0.54 https://www.inaturalist.org/taxa/42134-Sus-scrofa
*Sus scrofa domestica 0.44 https://www.inaturalist.org/observations?taxon_id=324492
Hippopotamus amphibius 0.37 https://www.inaturalist.org/taxa/42149-Hippopotamus-amphibius
My commentary:
PERISSODACTYLA
Equus quagga 'granti' 1.08 https://www.inaturalist.org/observations?taxon_id=447363
Diceros bicornis 1.08 (caveats: these authors give the genus as Rhinoceros', which casts doubt on the identification; and the body mass is misleadingly given as 655 kg, instead of about 1000 kg, which means that EQ has been exaggerated)
Equus caballus 0.92 https://www.inaturalist.org/taxa/209233-Equus-caballus
*Equus asinus 0.88 https://www.inaturalist.org/taxa/148030-Equus-asinus
Tapirus indicus 0.55 https://www.inaturalist.org/taxa/43356-Tapirus-indicus
My commentary:
PROBOSCIDEA
Elephas maximus 2.22 https://www.inaturalist.org/taxa/43697-Elephas-maximus
Loxodonta africana 1.75 https://www.inaturalist.org/taxa/43694-Loxodonta-africana
HYRACOIDEA
Dendrohyrax arboreus 1.08 https://www.inaturalist.org/taxa/43092-Dendrohyrax-arboreus
Procavia capensis 0.74 https://www.inaturalist.org/taxa/43086-Procavia-capensis
CARNIVORA
Potos flavus 1.78 https://www.inaturalist.org/taxa/41678-Potos-flavus
Pagophilus groenlandicus 1.61 https://www.inaturalist.org/taxa/41702-Pagophilus-groenlandicus
Canis latrans 1.59 https://www.inaturalist.org/taxa/42051-Canis-latrans
Ailurus fulgens 1.50 https://www.inaturalist.org/taxa/41653-Ailurus-fulgens
*Canis familiaris 1.46 https://www.inaturalist.org/taxa/47144-Canis-familiaris
Taxidea taxus 1.40 https://www.inaturalist.org/taxa/41789-Taxidea-taxus
Zalophus californianus 1.39 https://www.inaturalist.org/taxa/41740-Zalophus-californianus
Urocyon littoralis 1.36 https://www.inaturalist.org/taxa/42077-Urocyon-littoralis
Vulpes vulpes 1.27 https://www.inaturalist.org/taxa/42069-Vulpes-vulpes
Lynx canadensis 1.23 https://www.inaturalist.org/taxa/41974-Lynx-canadensis
Procyon lotor 1.20 https://www.inaturalist.org/taxa/41663-Procyon-lotor
Suricata suricatta 1.16 https://www.inaturalist.org/taxa/41943-Suricata-suricatta
Otocyon megalotis 1.16 https://www.inaturalist.org/taxa/42095-Otocyon-megalotis
Canis lupus 1.08 https://www.inaturalist.org/taxa/42048-Canis-lupus
Ursus maritimus 1.02 https://www.inaturalist.org/taxa/41644-Ursus-maritimus
Panthera pardus 1.02 https://www.inaturalist.org/taxa/41963-Panthera-pardus
Hyaena hyaena 0.92 https://www.inaturalist.org/taxa/41888-Hyaena-hyaena
Ursus arctos 0.82 https://www.inaturalist.org/taxa/41641-Ursus-arctos
Leptonychotes weddellii 0.81 https://www.inaturalist.org/taxa/41731-Leptonychotes-weddellii
Mungos mungo 0.81 https://www.inaturalist.org/taxa/41921-Mungos-mungo
Ailuropoda melanoleuca 0.80 https://www.inaturalist.org/taxa/41659-Ailuropoda-melanoleuca
Callorhinus ursinus 0.77 https://www.inaturalist.org/taxa/41757-Callorhinus-ursinus
Panthera tigris 0.69 https://www.inaturalist.org/taxa/41967-Panthera-tigris
Panthera leo 0.68 https://www.inaturalist.org/taxa/41964-Panthera-leo
My commentary:
RODENTIA
Glaucomys volans 1.31 https://www.inaturalist.org/taxa/46272-Glaucomys-volans
Sciurus vulgaris 1.22 https://www.inaturalist.org/taxa/46001-Sciurus-vulgaris
Dasyprocta 'agouti' 0.91 https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=43711&view=species
Chinchilla lanigera 0.85 https://www.inaturalist.org/taxa/43818-Chinchilla-lanigera
Peromyscus maniculatus 0.81 https://www.inaturalist.org/taxa/44396-Peromyscus-maniculatus
Peromyscus gossypinus 0.74 https://www.inaturalist.org/taxa/44416-Peromyscus-gossypinus
Peromyscus leucopus 0.71 https://www.inaturalist.org/taxa/44395-Peromyscus-leucopus
Myocastor coypus 0.69 https://www.inaturalist.org/taxa/43997-Myocastor-coypus
Hydrochoerus hydrochaeris 0.67 https://www.inaturalist.org/taxa/74442-Hydrochoerus-hydrochaeris
Hystrix cristata 0.63 https://www.inaturalist.org/taxa/44171-Hystrix-cristata
Gerbillurus pyramidum 0.62 https://www.inaturalist.org/taxa/44215-Gerbillus-pyramidum
Cricetulus barabensis 0.60 https://www.inaturalist.org/taxa/44215-Gerbillus-pyramidum
Mus musculus 0.53 https://www.inaturalist.org/taxa/44705-Mus-musculus
Mastomys natalensis 0.51 https://www.inaturalist.org/taxa/45326-Mastomys-natalensis
Castor canadensis 0.46 https://www.inaturalist.org/taxa/43794-Castor-canadensis
Castor fiber 0.45 https://www.inaturalist.org/taxa/43793-Castor-fiber
*Rattus norvegicus 0.45 https://www.inaturalist.org/taxa/44576-Rattus-norvegicus
Mesocricetus auratus 0.43 https://www.inaturalist.org/taxa/44919-Mesocricetus-auratus
*Cavia porcellus 0.40 https://www.inaturalist.org/taxa/119405-Cavia-porcellus
My commentary:
LAGOMORPHA
Lepus europaeus 0.59 https://www.inaturalist.org/taxa/43128-Lepus-europaeus
Oryctolagus cuniculus 0.51 https://www.inaturalist.org/taxa/43151-Oryctolagus-cuniculus
My commentary:
CHIROPTERA
Desmodus rotundus 0.87 https://www.inaturalist.org/taxa/41168-Desmodus-rotundus
Artibeus jamaicensis 0.84 https://www.inaturalist.org/taxa/41005-Artibeus-jamaicensis
Plecotus auritus 0.58 https://www.inaturalist.org/taxa/40416-Plecotus-auritus
Nyctalus noctula 0.41 https://www.inaturalist.org/taxa/40572-Nyctalus-noctula
'EDENTATA'
Choloepus hoffmanni 0.71 https://www.inaturalist.org/taxa/47101-Choloepus-hoffmanni
Bradypus griseus 0.63 https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=47065&view=species
Tamandua tetradactyla 0.52 https://www.inaturalist.org/taxa/47104-Tamandua-tetradactyla
Dasypus novemcinctus 0.44 https://www.inaturalist.org/taxa/47075-Dasypus-novemcinctus
EULIPOTYPHLA
Erinaceus europaeus 0.36 https://www.inaturalist.org/taxa/43042-Erinaceus-europaeus
My commentary:
MACROSCELIDEA
Elephantulus myurus 0.82 https://www.inaturalist.org/taxa/43213-Elephantulus-myurus
Elephantulus intufi 0.82 https://www.inaturalist.org/taxa/43212-Elephantulus-intufi
My commentary:
CETACEA
Tursiops truncatus 4.52 https://www.inaturalist.org/taxa/41482-Tursiops-truncatus
Stenella attenuata 3.56 https://www.inaturalist.org/taxa/41500-Stenella-attenuata
Phocoena phocoena 3.07 https://www.inaturalist.org/taxa/41440-Phocoena-phocoena
Pseudorca crassidens 2.85 https://www.inaturalist.org/taxa/41539-Pseudorca-crassidens
Orcinus orca 2.69 https://www.inaturalist.org/taxa/41521-Orcinus-orca
Physeter macrocephalus 0.54 https://www.inaturalist.org/taxa/74890-Physeter-macrocephalus
Megaptera novaeangliae 0.40 https://www.inaturalist.org/taxa/41566-Megaptera-novaeangliae
Eschrichtius robustus 0.33 https://www.inaturalist.org/taxa/41478-Eschrichtius-robustus
Balaenoptera musculus 0.19 https://www.inaturalist.org/taxa/41553-Balaenoptera-musculus
My overall commentary:
Species with encephalisation quotients <0.5, with the adaptive correlates, are (in no particular order):
Of the above, the most puzzling case is beavers, which have remarkably small brains for 'ecosystem engineers' capable of construction/architecture/vegetation management.
Species with encephalisation quotients slightly exceeding 0.5 (in no particular order): anteater, eland, rabbit, rat, mouse, wild boar.
Comments
THE BRAINIEST PRIMATES
The following is in decreasing order of braininess, based on another reference. Note that the 'great apes' (Pan and Pongo) are not brainy enough to qualify for this list.
Homo sapiens
Sapajus apella https://www.inaturalist.org/taxa/496700-Sapajus-apella
Miopithecus talapoin https://www.inaturalist.org/taxa/43543-Miopithecus-talapoin
Cercocebus galeritus https://www.inaturalist.org/taxa/43468-Cercocebus-galeritus
Saimiri sciureus https://www.inaturalist.org/taxa/43428-Saimiri-sciureus
Cebus albifrons https://www.inaturalist.org/taxa/43433-Cebus-albifrons
Lagothrix lagothricha https://www.inaturalist.org/taxa/1369209-Lagothrix-lagothricha
Hylobates pileatus https://www.inaturalist.org/taxa/43596-Hylobates-pileatus
Cebus capucinus https://www.inaturalist.org/taxa/43432-Cebus-capucinus
Saimiri oerstedii https://www.inaturalist.org/taxa/43425-Saimiri-oerstedii
Cercocebus torquatus https://www.inaturalist.org/taxa/43467-Cercocebus-torquatus
Hylobates lar https://www.inaturalist.org/taxa/43593-Hylobates-lar
https://www.researchgate.net/figure/Log-log-plot-of-brain-and-body-masses-among-extant-nonavian-reptiles-closed-circles_fig2_232681928
BRAIN SIZE IN HOMO SAPIENS
Source: Holloway (1980, https://onlinelibrary.wiley.com/doi/abs/10.1002/ajpa.1330530115)
Adult females:
Mean body mass 62.1 kg (height 164 cm), mean brain mass 1318 g
Adult males:
Mean body mass 72.1 kg (height 174 cm), mean brain mass 1457 g
Also see https://www.perplexity.ai/search/what-is-average-adult-brain-ma-4YQ64GdoQ6aZBySEtmXJ1Q
Stankowich and Romero (2017, https://royalsocietypublishing.org/doi/full/10.1098/rspb.2016.1857) state that the encephalisation quotient of Homo sapiens is 7.4-7.8
(compared to 0.54 for pangolins, and about 0.63 for armadillos)
FOR DATA ON BRAIN SIZE IN MARSUPIALS AND MONOTREMES
https://karger.com/bbe/article-abstract/71/3/181/46946/Encephalization-of-Australian-and-New-Guinean
https://www.sciencedirect.com/science/article/abs/pii/S0944200614000622
https://books.google.com.au/books?hl=en&lr=&id=NSydTn-XELwC&oi=fnd&pg=PR1&dq=Ashwell+2008+encephalisation&ots=TYT_THwXxU&sig=lOJ8BnaZD1OCmU2plr6I1BCNmzY&redir_esc=y#v=onepage&q=Ashwell%202008%20encephalisation&f=false
https://bioone.org/journals/zoological-science/volume-31/issue-9/zs140022/Reassessing-the-Relationship-Between-Brain-Size-Life-History-and-Metabolism/10.2108/zs140022.short
https://karger.com/bbe/article/85/2/125/42344/The-Evolution-of-Relative-Brain-Size-in-Marsupials
My commentary:
Tachyglossus aculeatus (body mass about 4 kg, brain mass about 25 g) can be compared with the canid, Otocyon megalotis (body mass 3.2 kg, brain mass about 28.5 g, encephalisation quotient 1.16).
BRAIN SIZE IN HYAENIDAE
https://karger.com/bbe/article-abstract/77/2/91/326183/Brain-Size-and-Social-Complexity-A-Computed?redirectedFrom=fulltext
https://link.springer.com/article/10.1007/s10914-023-09653-9
BRAIN SIZE IN THE LARGEST-BODIED MARSUPIAL: DIPROTODON
https://link.springer.com/article/10.1007/s10914-023-09653-9
Body mass 1048 kg, brain mass no more than 477 g
My commentary:
Diprotodon can be compared with the like-size (about 1 tonne) perissodactyl, Diceros bicornis, which has a small brain for its clade.
The brain of D. bicornis weighs 531 g (n=1), which is less than the value for the far smaller-bodied Equus caballus (brain mass mean 599 kg, at body mass mean 514 kg). Encephalisation quotients are 0.47 for D. bicornis and 0.8 for E. caballus.
The above data show that Diprotodon has a smaller brain than that of Diceros, which itself is small-brained for a mammal.
Examples of surprisingly encephalised dinosaurs: coelurosaurs and Stenonychosaurus
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