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.
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)
|94||0||Cynocephalus||Philippine Flying Lemur|
|18||4||Colobus||Black and White Colobus|
|-62||1||Mirza||Giant Mouse Lemur|
|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|
|—||0||Oreonax||Yellow-tailed Woolly M.|
|—||0||Prolemur||Greater Bamboo Lemur|
The 95% confidence level for the above figures should be somewhere around ±75/√C, where C = the certainty
|C||Key to Certainty Level|
|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.
|37200*||285||Globicephala melas||pilot whale|
|15000*||226||Balaenoptera physalus||fin whale|
|5593||162||Loxodonta africana||African elephant|
|2875||119||Papio anubis||olive baboon|
|1917||92||Ara ararauna||b.&y. macaw|
|1710||85||Macaca mulatta||rhesus monkey|
|1660||83||Macaca radiata||bonnet monkey|
|1340||69||Saimiri sciureus||squirrel monkey|
|1140||59||Sapajus apella||tufted capucin|
|1135||58||Cacatua galerita||s.-cr. cockatoo|
|850||39||Psittacus erithacus||Af. grey parrot|
|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|
|529||9||Garrulus glandarius||Eurasian jay|
|53,8||-140||Cynomys sp.||prairie dog|
|22.3||-197||Microcebus murinus||mouse lemur|
*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 nonhuman 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.