September 5, 2006

Binturongs and the Dexterity/Brain Size Correlates

It's nice when you stumble across some scientific literature that answers a question that's been bugging you. Well, in this case, maybe half of a question.

I've always wondered if there was some connection between an organism's intelligence and its ability to manipulate objects with hands or some analog, and if there would be a way to quantify either attribute effectively. In my mind, cephalopods (squid, octopus) and primates are prime examples of intelligent manipulators, though this connection breaks down as soon as you browse the cetaceans (whales, dolphins).

In my search for literature pertaining to the binturong (which is scant, the animal is a virtually an unknown species), I came across a paper performing just that sort of experiment in a more focused manner. The researchers studied 41 different fissiped (terrestrial) mammalian carnivores, including Arctictis binturong and the red panda, Ailurus fulgens, and did the following:

  • Equated animal intelligence with brain size and brain weight, using a calculation based on such numbers
  • Correlated that number with the "total dexterity" (proximal and distal)

These methods are backed by evidence that the complex use of hands/forelimbs for feeding or manipulating instead of pure movement has lead to the selection of greater intelligence mainly in predators:

[...] carnivorous species within Carnivora may have increased brain size because of a more complex forgaging strategy involving selection for rapid prey detection, pursuit, capture (especially forepaw manipulation) and consumption (Gittleman).

Here's the short version of the results: In this experiment, brain size/weight was shown not to be connected to manual dexterity.

But that does not mean that intelligence and manual dexterity are not connected. A ratio of brain size to body size (called the encephalization quotient, or EQ), while certainly important, does not necessarily accurately describe or denote intelligence. There are other factors, such as the nervous wiring of the brain. The raccoon has a fairly normal EQ, not telling of any significant intelligence or skill with its hands, but we know otherwise; the raccoon can "see" with its hands in the dark. This may be because of how its brain is wired instead of raw brain size to body ratio.

As more brain and neuroscience is done, researchers are finding more and more that the EQ is an insufficient measure of intelligence. Intelligence probably has more to do with the rate at which the brain develops in the womb (think babies with giant heads), than sheer physical mass.

From Carl Zimmer's wonderful book, At the Water's Edge:

Intelligence may not be determined by relative brain size per se but by the rate at which the brain grows in the embryo. [...] primates grow with unusual timing : their brains swell early and quickly, but their bodies grow slowly for mammals of their size.


How skilled are the skilled limb movements of the raccoon (Procyon lotor)? Iwaniuk AN, Whishaw IQ. Behav Brain Res. 1999 Feb 15;99(1):35-44.

Brain size is not correlated with forelimb dexterity in fissiped carnivores (Carnivora): A comparative test of the principle of proper mass. Iwaniuk AN, Pellis SM, Whishaw IQ. Brain Behav Evol. 1999 Sep;54(3):167-80.

Carnivore Brain Size, Behavioral Ecology, and Phylogeny. John L. Gittleman. Journal of Mammalogy, Vol. 67, No. 1, 23-36. Feb., 1986.

Zimmer, C. (1998). At the Water's Edge. New York, NY: Simon & Schuster.

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