Cellular scaling rules and primate brains - revisited

Kelsey Faria blogs about a paper, earlier reviewed here, for her contribution to the Evolution class.

The order of Primates is known for a variety of species that are energetic, inquisitive, social, and intelligent. Whereas the order of Rodentia typically lack the range or number of skills that primates encompass. Theses differences seem to put these two orders in completely different categories, although species in each order have relatively similar brain sizes. So the question arises what could be different about their brains that it affects their behavior? The question raises the possibility that primate brains differ from rodent brains in their cellular composition (Herculano-Houzel et al 2006).

The authors examined the cellular scaling rules for primate brains and show that brain size increases isometrically as a function of cell numbers. This isometric function is in contrast to rodent brains. Rodent brains have the ability to increase faster in size than in numbers of neurons. As a result of the linear cellular scaling rules, primate brains have a larger number of neurons than rodent brains of similar size (Herculano-Houzel et al 2006). In all probability this would give primates an advantage over the rodents which may explain the richer behavioral repertories and better cognitive abilities (Herculano-Houzel et al 2006).

Brain size fluctuates across mammalian species, and several studies have focused on finding any shared regularities behind brain morphology and cellular composition across species with different brain sizes. With these regularities this leaves room to discover new hypothesis about the underlying development and evolution of the brain. Studies have proven that animals from different species differ in their behavioral repertoires, and one would assume differences in cellular composition of the brain

The authors conducted an analysis in which cellular scaling rules were applied to rodents and primate brains. They concluded that the average neuronal size is larger in larger brains, whereas the average non-neuronal cell size remains comparatively stable. They also discovered that the neuron ratio increases with increasing brain size (Herculano-Houzel et al 2006).

With the information collected through this analysis the authors were interested in applying the scaling rules to other mammalian orders. There main goal was to set rules that can be applied to all brains and possibly reflect characteristics from a common ancestor which would conclude why there is a phylogenetic variance across orders. They were particularly interested in cellular scaling differences that might have arisen in primates. If the same rules relating numbers of neurons to brain size in rodents also applied to primates, a brain comparable to ours, with approximately 100 billion neurons, would weigh approximately 45 kg and belong to a body of 109 tons, about the mass of the heaviest living mammal, the blue whale (Herculano-Houzel et al 2006).

Inevitably their study indicates that there must be scaling differences between rodent and primates due to their behavior relative to similar brain size. The authors used the isotropic fractionators, which is a non-stereological method, which estimates the total number of neuronal and non neuronal cells in the cerebral cortex, cerebellum, and remaining structures of the brain (Herculano-Houzel et al 2006). They examined across six species of the order Primata, from Callithrix to Macaca, and in the closely related tree shrew, which is in the order Scandentia.

From the results gathered the authors concluded that the cellular scaling rules for primate brains differ from those of rodents. There was a distinct difference between the primate brains and rodent brains, primate brains do not hyper scale as they gain neurons, as rodent brains do. Primate brains also increase in size according to their number of neuronal cells which means that the average neuronal cell remains constant. The rodent brains increase in size faster than they gain neurons, which results in a increasing in the average neuronal cell size. When looking at neuronal densities they remain stable in primates and they tend to decrease in rodent brains. Primate ratios of non-neuronal neuronal cells to Mbr (brain mass) do not correlate although rodent’s ratios do.

The authors also included the tree shrews included in the experiment did not alter the results. Therefore the tree shrews are in fact a close relative of the primates and they to conform to the primate scaling rules.

Some implications for humans according to the data are that larger brains do not have a larger relative number of neurons in the cerebral cortex. From their results both the cerebral cortex and the cerebellum represent fractions of brain mass but do not differ significantly with an increase in brain size. Although, relative cortical size is seen to increase significantly with increasing brain size when larger species, such as great apes and humans, are considered (Herculano-Houzel et al 2006). It will be interesting to see if these scaling rules will apply if and when the addition of apes and humans are included in the experiment. A primate brain containing 100 billion neurons would be expected to weigh about 1,450 g and belong to a body of 72.7 kg, values that match the average mass of a human’s brain and body. This would conclude that humans and their brains are in fact isometrically scaled up versions of a common primate plan (Herculano-Houzel et al 2006).

Reference:

Suzana Herculano-Houzel, C. E. C., Peiyan Wong, and Jon H. Kaas (2006). "Cellular scaling rules for primate brains." Proceedings of the National Academy of Sciences 104(9): 3562–3567. doi: 10.1073/pnas.0611396104.