Does Brain Weight Have Meaning?

Dr. Michel Thireau, French National Museum of Natural History

The vertebrates' main neural axes end with the brain, a very special organ housed in the skull of all vertebrates. The brain is especially complex in its organization, and its functioning reaches an extreme degree of sophistication within the human brain.

In the last two centuries, researchers have tried to reduce the brain complexity. Several methods have been pursued but one method brought credit to the hypothesis that the simplification of the brain complexity through quantitative estimation may be the correct procedure, thus showing that it is necessary to know the animal's brain and body weights when studies are conducted. The relationship between the brain and the body weights have been analyzed in numerous scientific papers using allometry (i.e. growth disharmonies). In this regard, it became possible to use an allometric correction (anisometric growth) to determine a series of encephalic indices. With some judicious comparisons, these indices bring quantified information on various adaptations and the general evolution of species.

However, a problem remains unsolved concerning the human brain: why genius may have either a small or large brain, even after allowing for allometric correction? It is difficult to correlate brain-weight with the level of functional capacities because the level of functional neural-networks by weight unity is not easily perceptible.  Moreover, in an enlightened brain such as the human organ, capacities are more or less fluctuant depending upon diverse circumstances which can hinder its thinking independently of the weight of the brain.  So would it be better to give up any interpretative essays on volumetric brains results?  Not at all, and we will see why.

The problem we shall try to "solve" is not the knowing of the functional capacities of the brain but determining what the quantitative pattern of brain organization actually is, and if accessible, then determining what it's meaning is. In recognizing the brain's internal neural population and their volumes, then the brain's organization can subsequently be approached by way of this general cytoarchitectonic method, with which we hope to reach a simplified but correct overview.  When the brain is being developed (ontogeny), morphogenetic fields are moving together to occupy the braincase, mainly under the species genetic program.  In advanced brains, a genetic expression of achieved volumetric anatomical organization is able to follow a light reshaping according to various environmental restraints. In this manner, the pattern of brain organization is, simultaneously, largely stabilized but not definitive, and advanced brains remain influenced by a light individual reorganization additional to genetic specificity.  So is it actually possible to determine a deep signature in brain pattern while various growth possibilities occur?  The answer seems to be positive if we respect some essential conditions.  What are some of them?

II. Essential Conditions:

Mammalian brains have long interested numerous neuroanatomists and seem to be the "Royal Road" to access the understanding of the uniqueness of the human brain.  But all mammals have brains with dozens of tiny recognizable areas, each of them with a typical neural population. A correct and objective identification of all these areas requires years to be adequately studied and analyzed so that their characteristics can be recognized.  Even if this research is successful, mammalian brains are adapted in various ways along numerous evolutive lines. Thus the possibility to discover a general hidden signature of evolution is limited. For that reason, such work on mammals should be associated with the study of vertebrates which apparently have a more simplified brain, such as newts and salamanders (i.e. Amphibians, Urodela).  The telencephalon of Urodela, (the rostral and most evolutive section on all vertebrate brains) appears to be very simple with only 16 distinct areas.  The study of the telencephalon section of the Urodela brain allows us to know the tri-dimensional position, form, limits and volume of each of these 16 areas (neural populations).

Among Urodela, two families are largely represented: Plethodontidae in the new world and Salamandridae in the old world.  Naturalists are able to separate these two families by various morphological anatomical and molecular observations, but had never been able to ascertain as much from the brain's volumetric pattern organization alone.  This became possible in 1997 with the application of the Correspondence Factorial Analysis (CFA) on data matrix-crossing species and neural populations volumes of the Urodela.

CFA can analyze species according to neural population volumes, as well as neural population volumes (according to species) inside a virtual space, which can then be described according to its factorial axes (i.e. all its dimensions).  Factorial maps allow direct comparisons between the position of species and the position of associated brain areas.  Using CFA, in 1995 we discovered that a natural separation -- neurotaxonomy -- exists between Plethodontidae and Salamandridae on the basis of the 16 intratelencephalic volumes areas.  Today, neurotaxonomy seems to be a general signature in all vertebrates.


This illustration presents a factorial map (2D) according to factorial axis (phi1-phi2) of a hyperspace (4D).  It is a plane projection of 5 animals (4 newt species and one salamander subspecies living in France and Europe) as well as the volumes of their intratelencephalic brain's sections, illustrating that all newts are different compared to the salamander.  For the first time (1997) in the history of quantitative neurobiology, it became possible to obtain a distinction between species solely based upon knowledge of the volumetric organization of the rostral brain section.  The name of that discovery, probably a "revolution" in neurosciences, is "neurotaxonomy".

III. For Further Consideration:

What are some of the probable future consequences associated with Neurotaxonomy?

1) Access to the process of evolution becomes possible by the way of volumetric brain structural organization with a special significance regarding the animal's taxonomic position.

2) Developmental biology (Evo-dev) can use the results of neurotaxonomy to separate genetic and epigenetic factors.

3) Human brain development compared to the brain development of large monkeys shall suggest new processes of human phylogenetic origins.

4) The neurosciences profit from a new framework of reference, which may be utilized for a general interpretation with more precise results.

Various teams of researchers throughout the world (including the U.S., Canada, Great Britain, Australia, et al) are currently studying volumetric variations in brains areas.  It is hoped that our common interests shall drive us toward a closer and more coordinated collaboration. This close collaboration is necessary when recognizing the complex difficulties involved with studying the brain, and is certainly most important in bringing a full success to the neurosciences during the 21st century.


Thireau, M. (1977) - Analyse volumetrique comparee de l'encephale, et en particulier du telencephale, des Amphibiens Urodeles.  Ph.D. (X+230).

Thireau, M., Dore, J.-C. & Viel, C. (1997). - Neurotaxonomie (N. Novum) et representation du genre Triturus au sein des Amphibiens Urodeles, a partir de l'analyse multivariee du volume des structures intratelencepahliques. Bull. Soc. Zool. de France, 122, 4, 393-411.

Thireau, M. & Dore, J.-C. (1998) - An introduction to neurotaxonomy: multidimensional analysis of the volumetric organization of the telencephalon in Amphibia Urodela.  Current studies in Herpetology, Societa Europaea Herpetologica, 425-433.

Thireau, M. & Dore, J.-C. (1999). - Neurotaxonomie. Pas si betes! Mille cerveaux  mille mondes. Museum National d' Histoire Naturelle et Nathan, 22.

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