Does Eating Fruit Lead Brain Evolution?

There has been a dearth of important academic papers directly pertaining to cetacean intelligence over these past few months. Today I post my much delayed analysis of a significant paper on primate intelligence. It was published in Nature Ecology & Evolution back in May, and I have waited in vain long enough for a suitably relevant paper on whales with which to balance it. I apologise in advance for temporarily skewing of the posts on this site away from its prime focus on cetaceans.

Primate Brain Size is Predicted by Diet but not by Sociability

The authors start by noting their surprise that they could find no evidence for primate social group size correlating with brain size or intelligence. This is not a one off result, but adds to the powerful body of evidence against the Machiavellian Intelligence Hypothesis, which I had already noted last year in the second half of THIS post. They look at several other correlates, but only the proportion fruit in a species diet stands out. Disappointingly, they make no attempt to follow-up Deaner et al 2007, where they find the second highest correlate of intelligence to be dietary breadth. Since fruit is seasonal it is highly likely that fruit eaters require a more varied diet to survive, and I predict that, when adjustments are made for dietary breadth, the effect they found will disappear.

From earlier findings on primate intelligence, and confirmed by a later study using self control, we can explain at least 80% of the variance in primate intelligence by two measures: brain size and dietary breadth. Furthermore, we have reason to believe that these metrics are similarly predictive for all vertebrates, if we just change the form to forebrain neuron number and dietary breadth. I have a fair bit of data for the former metrics, but none on the latter. If anyone knows where I can find objective data on dietary breadth, I would be grateful if you let me know, as I have currently been reduced to the use of qualitative findings to make crude approximate adjustments, or to forgo the latter predictive factor altogether.

Primate IQ

As the only group for which comprehensive data is available, primates must act as the basis of any objective scale of nonhuman intelligence, against which other animals must be compared. The point of difference of this website, is that objective measures of cognition are its focus.

I have replaced the earlier ad hoc values I obtained for brain weight with data from the above study. This I did by averaging the value given for the three species for which the most data was available in each genus. If less than three species were known, then I pooled all the data. The main exception was the night monkey, where one member of its genus had a much larger body and brain weight than the rest. There I discarded the larger value.

Two values for brain weight badly mismatched available body weight (Philippine flying lemur and the pig-tailed langur), and relied on single published figures. I have included them anyway for completeness sake. This, plus the addition of DUF 1220 data completes my current data for primate intelligence.

My Raw Figures for Primate intelligence as of 30 September (figures set such that human genetic intelligence = 250, and 1PIQ point = 1IQ point on our familiar human scale)

 Score  C  Genus  Common Name
 246  4  Homo  Humans
 201  9  Pan  Chimpanzee
 186  9  Pongo  Orangutan
 150  9  Gorilla  Gorilla
 149  0  Simias  Pig-tailed Langur
 105  9  Macaca  Macaque
 99  2  Symphalangus  Siamang
 94  0  Cynocephalus  Philippine Flying Lemur
 90  1  Nomascus  unnamed Gibbon
 90  1  Rhinopithecus  Snub-nosed Monkey
 88  9  Ateles  Spider Monkey
 88  1  Semnopithecus  Grey Langur
 87  3  Sapajus  Robust Capuchin
 86  1  Hoolock  Hoolock Gibbon
 85  9  Papio  Baboon
 82  1  Lophocebus  Crested Mangabey
 79  1  Pygathrix  Douc
 79  1  Nasalis  Proboscis Monkey
 68  9  Cercopithecus  Guenon
 63  4  Theropithecus  Gelda
 63  4  Erythrocebus  Patas Monkey
 59  1  Piliocolobus  Red Colobus
 59  1  Chlorocebus  Green Monkey
 57  1  Trachypithecus  Lutung
 56  1  Allenopithecus  Swamp Monkey
 51  9  Mandrillus  Mandrill
 51  4  Presbytis  Surili
 50  5  Lagothrix  Woolly Monkey
 50  1  Procolobus  Olive Colobus
 47  7  Cebus  Gracile Capuchin
 46  1  Chiropotes  Bearded Saki
 44  9  Cercocebus  White-eyed Mangabey
 37  9  Hylobates  Dwarf Gibbon
 31  1  Indri  Indri
 29  3  Alouatta  Howler Monkey
 25  2  Pithecia  Saki Monkey
 23  4  Saguinus  Tamarin
 18  4  Colobus  Black and White Colobus
 8  2  Callicebus  Titi
 4  3  Daubentonia  Aye-aye
 3  3  Varecia  Ruffed Lemur
 -11  5  Otolemur  Greater Galago
 -16  4  Eulemur  Brown Lemur
 -16  2  Callimico  Goeldi’s Marmoset
 -17  1  Hapalemur  Bamboo Lemur
 -20  3  Loris  Slender Loris
 -22  4  Propithecus  Sifaka
 -22  9  Saimiri  Squirrel Monkey
 -23  6  Nycticebus  Slow Loris
 -23  1  Pygathrix  Potto
 -27  6  Aotus  Night Monkey
 -33  6  Lemur  Ring-tailed Lemur
 -36  1  Avahi  Woolly Lemur
 -41  1  Lepilemur  Sportive Lemur
 -42  3  Cebuella  Pygmy Marmoset
 -43  2  Phaner  Fork-marked Lemur
 -46  1  Mico  unnamed Marmoset
 -49  4  Leontopithecus  Lion Tamarin
 -53  1  Arctocebus  Angwantibo
 -62  1  Mirza  Giant Mouse Lemur
 -63  9  Callithrix  Marmoset
 -64  1  Euoticus  Needle-clawed Bushbaby
 -67  8  Galago  Lesser Bushbaby
 -78  1  Cheirogaleus  Dwarf Lemur
 -88  1  Tarsius  Tarsius
 -89  1  Galagooides  Dwarf Galago
 -90  1  Carilito  Philippine Tarsier
 -93  1  Cephalopachus  Horsefield’s Tarsier
 -96  6  Miopithecus  Talopoin
 -115  4  Microcebus  Mouse Lemur
 24  3½  average 70/80  primate
 136  5½  average all 8  ape
 8  4½  average 15/19  n. w. monkey
 62  3½  average 21/22  o. w. monkey
 -90  1  average all 3  tarsier
 -47  3½  average 7/10  galago&lorisid
 -33  2½  average 13/15  lemur
 —  0  Rungwecebus
 —  0  Oreonax  Yellow-tailed Woolly M.
 —  0  Branchyteles  Muriquis
 —  0  Cacajao  Uakari
 —  0  Callibella
 —  0  Sciurocheirus
 —  0  Perodicticus
 —  0  Pseudopotto  False Potto
 —  0  Allocebus
 —  0  Prolemur  Greater Bamboo Lemur

The 95% confidence level for the above figures should be somewhere around ±75/√C, where C = the confidence

 C  Key to Confidence Score
7-9  score derived from multiple and varied tests
3-6  score derived from at least three tests of at least two different types
   2  score calculated from brain weight and one cognitive test
   1  score calculated from brain weight
   0  score calculated from suspect brain weight

PIQ as it Correlates to Forebrain Neuron Count for Primates and Non-primates

Now, let us forget about adjustments for dietary breadth and simply list all animals of known forebrain neuron number, that have been determined by isotropic fractionation and which derive PIQ >0. Furthermore, let me augment that list with the two best figures for cetaceans, but determined by the less accurate traditional method of stained count.

 count  PIQ  species  common name
 37200*  285  Globicephala melas  pilot whale
 16340  232  Homo sapiens  humans
 15000*  226  Balaenoptera physalus  fin whale
 9100  194  Gorilla gorilla  gorilla
 8900  192  Pongo sp.  orangutan
 7400  180  Pan troglodytes  chimpanzee
 5593  162  Loxodonta africana  African elephant
 2875  119  Papio anubis  olive baboon
 1917  92  Ara ararauna  b.&y. macaw
 1730  86  Giraffa camelopardalis  giraffe
 1710  85  Macaca mulatta  rhesus monkey
 1660  83  Macaca radiata  bonnet monkey
 1340  69  Saimiri sciureus  squirrel monkey
 1281  66  Nestor notabilis  kea
 1204  62  Corvus corax  raven
 1140  59  Sapajus apella  tufted capucin
 1135  58  Cacatua galerita  s.-cr. cockatoo
 850  39  Psittacus erithacus  Af. grey parrot
 820  37  Corvus frugilegus  rook
 801  36  Macaca fascicularis  Long-tail. mac.
 763  32  Tragelaphus strepsiceros  Greater kuru
 599  17  Cacatua goffiniana  Goffin’s cockatoo
 575  14  Psittacula eupatria  Alexandsrian parrot
 571  14  Damaliscus dorcas  bontebok
 529  9  Garrulus glandarius  Eurasian jay
 492  2  Corvus monedula  jackdaw
 <0  selected animal
 307  -27  Sus scrofa  pig
 258  -38  Nymphicus hollandicus  cockatiel
 149  -74  Melopsittacus undulatus  budgerigar
 77.3  -116  Sciurus carolinensis  Squirrel
 53,8  -140  Cynomys sp.  prairie dog
 31  -176  Rattus norvegicus  rat
 22.3  -197  Microcebus murinus  mouse lemur
 13.7  -229  Mus musculus  mouse

*figures determined by stained neuron count

Note that, while forebrain neuron count might prove the best predictor of intelligence, dietary breadth, for which I have no quantitative figures, is also significant. If I had such figures, those with the widest diet, which is topped by humans and rats, would have an expected PIQ around 50 points higher than the given values above. Likewise, those that consume only one or two plant species, such as the giraffe, would have their estimates knocked down a similar margin.

And one last footnote…

The Wide Cognitive Divide Among the Capuchin

Measuring primate intelligence might be easier than attempting to do so for cetaceans, but it’s not without its quarks. One complication is that capuchins have traditionally been grouped as a single genus, and much data is still published pooling results from both their robust and gracile genera. The more I study this group, the greater the intellectual divide between its two halves becomes apparent. One way of explaining this might be, that despite having brains of similar sizes, the published stained forebrain neuron count for graciles is only half the isotropic fraction count given for its robust cousin above.

Last year, a large cache of ancient stone tools was discovered by researchers from Oxford University. Despite their distinctive nature, these were not of human origin, but robust capuchin. To open the cashews they use two tools: large flat anvil stones, and hammer stones. The anvil stones are very heavy, and must have required great effort to drag into place by the cashew trees. This requires foresight, and possibly cooperation, on a level not so far unobserved among non-great-ape-primates. Sadly, no one has yet seen a new anvil being dragged into place. The hammer stones are more easily available, and display a more typical type of nonhuman tool use.

Robust capuchin might be habitual and extraordinary multiple tool users, but tool use in graciles has never been observed for most species, and in the few that do, it is sparing.