Evolution of Brain Size

The metabolic cost of neural information

Laughlin et al. 1998.

This paper show that transmitting neural information is costly. Moreover, the energy can change with the type of information is been transmitted, and also with the adaptations of the organs involve in collecting information from the environment (e, g., vision). I like the way they integrate different fields, physics, chemistry and anatomy, to explore the cost of information transmitted. I have few questions, How can we put this in to evolutionary context? and where selection is acting? During the developing stage or as an adult?. I just have a few ideas about this. If storing, gathering and analyzing information is very expensive one should expect that organism that are not energetically constraint will have higher brain capacity. But this is hard to accomplish in the long term as resources has been changing throughout time which means that energy availability also had change. So been able to manipulated the constancy of this resources has been the key to maintained the energy expenditure in human brain?. On the other hand if selection is acting during the embryonic develop one may expect that organism that live in constant and energy rich environments have been able to 1) put more resource into embryo formation or 2) delay the embryonic developing to allow the formation of more complex brain networks, these are not mutually exclusive mechanism.

Vertebrate Brain Development

Evolution of Brain Size

Bird-song - Evolution of HVC and repertoire size

Devoogd et. al. (1993) discusses how levels of song repertoire and number of syllables in a song relate to the volume of brain nuclei. They find that there is a correlation between the high vocal center and the number of song types. Do the same species always have the same repertoire? In any case, the researchers note that the total number of syllables in the complete repertoire is the best index of learning. Apparently, brevity is no sign of intelligence among avian species. There are two neural paths associated with song systems: the caudal projections [song production] and rostral projections [song acquisition]. The data showed that the differences in the numbers of songs species sing are associated with the relative size of the HVC.

Airey et. al. (2000) sought to determine if the volume of song-control nuclei was heritable. They started with three premises: male behavior determines female preference, behavior is intimately linked to brain anatomy, and the differences in behavior must be heritable. Material and Methods: (p. 2100 – “Birds were supplied with … hard-boiled chicken egg.”) Evolvability, or the proportional response for measured traits, was greater for the high vocal center in the species studied. Most importantly, the researchers find that there is “a moderate heritability for HVC size.” It is also correlated (in size) to the RA. Song production shows more evolvability for nuclei than song acquisition.

Spencer et. al. (2005) show that canaries, which have been exposed to malaria, have decreased reproductive fitness. This is due to a decline in repertoire. Previous studies linked song complexity to parasite loads. The study also sought whether coritcosterone (a stress hormone) had increased as a response to the introduction of the parasite. But, levels of corticosterone were not found to be higher in the parasitized birds. The study gives strong evidence to the developmental stress hypothesis, that is, physiological stressors will reduce reproductive fitness. Especially if they act against the HVC, in certain avian species.

Half of the world's human population is infected with Toxoplasma: Mind control by parasites

EO Wilson and James Watson on Charlie Rose

Cortical plasticity mini-symposium

Hi, everyone -- just want to alert you to talks by two luminaries in the field of cortical plasticity, a topic that seems to be cropping up repeatedly in the course. Both talks are on April 4 in the McKnight center down the hill. Both are members of the National Academy and have done some seminal work in neuroscience. I suggest we knock off a little early on the 4th to go to these talks. The detailed announcement follows.

The Mini-Symposium, entitled "Lifelong Brain Plasticity and Learning" will be held on Tuesday, April 4, 2006, from 11:00 am to 1:00 pm in the DeWeese Auditorium, LG-101A. Dr. Jon Kass will speak on "The Reorganization of Sensory and Motor Cortex after Nerve and Spinal Cord Injury: When a Little Means a Lot", and the title of Dr. Michael Merzenich's talk is "Brain Plasticity: Acquisition, Loss, Rejuvenation".

Old-World Primates Evolved Color Vision to Better See Each Other Blush, Study Reveals

"For a hundred years, we've thought that color vision was for finding the right fruit to eat when it was ripe," says Mark Changizi, a theoretical neurobiologist and postdoctoral researcher at Caltech. "But if you look at the variety of diets of all the primates having trichromat vision, the evidence is not overwhelming."

Link to the article

Auditory Specializations

Kubke et al. 2004

Differential increases in brain sections can be mediated either by increases in overall brain size (passive) or selective increases in specific cell groups (active). Because there are constraints on increases in overall brain size (metabolic, developmental, etc), sensory specialization may be accompanied by an increase in cell groups associated with the specific sensory modality. Kubke et al (2004) used barn owls to investigate the regulation of auditory brain structures in birds. They found that barn owls had a greater increase in auditory structures than would be accomplished by increases in global brain size, suggesting an increase in size of specific cell groups. Changes in size of specific brain structures can be mediated by changes in cell birth, rates of mitosis, and apoptosis.

Kubke et al. 2004 discuss interspecific variation in the size and development of brain structures, but what about intraspecific variation? Specifically, I wonder how plastic is specialization in different brain structures? Can differential investment in specific brain regions be modulated during development in response to maternal and/or environmental cues?

Kubke et al. briefly discuss the importance of tropic factors in regulating the extent of cell death. What do they mean by tropic factors (ecological influences?)? Also, why are increases in brain structure size important for increases in sensory modalities? What about synaptic connections or morphological characters (ears)?

Convey (2005)

Bats also have specialized brain centers in response to echolocation and environmental/feeding guild factors. That’s about all I take from these 13 pages.

Visual and Olfactory Brain Systems

Barton et al. 1995 examine the correlation of size of sensory structures in the brain with each other and with ecological factors of activity schedules and diet within primates, bats and insectivores. Controlling for brain size and phylogenetic relatedness, they show several correlations of residual contrasts between visual and olfactory brain structures that are in several ways consistent with ecological lifestyles of the species in question. For example, in primates there is an overall negative correlation between olfactory and visual structures, where diurnal species have larger visual cortices and nocturnal species larger olfactory structures. Additionally, diurnal frugivore-omnivores have larger visual cortices than diurnal foliovores. In bats, there is a positive relationship between olfactory and visual structure size, though it is suggested that these two characteristics are likely involved in a trade-off with echolocation abilities. Insectivore analyses typically had low sample sizes, though trends suggest a trade off between fossorial abilities and visual structure size. This study is well done, especially considering that it was published ten years ago. It sets the stage for genetic-based and functionally-based studies of these “trade offs” within particular groups (i.e. trichromatic vision in primates).

Gilad et al. provide a comparative study of the proportions of olfactory receptor pseudogenes across 19 primate species, finding that routine trichromatic primates have a higher proportion of pseudogenes than dichromatic (or allelic trichromatic) primates, suggesting decreased reliance on the olfactory sense relative to vision in trichromatic primates. This high proportion of pseudogenes association was also consistent for the howler monkey, the only new-world monkey to acquire routine trichromatic vision, an independent event from the evolution of trichromacy in old-world primates.

So, we see a decrease in utility of OR genes coincident with the evolution of trichromatic vision in primates, although Gilad et al. cannot show that this is due to a direct relationship between olfaction and vision. Here’s an idea—maybe can we test the relationship between olfactory and visual systems in other ways, looking at utility via the signature of neural connectivity across individuals in a population. Many (most? all?) new world monkeys have “allelic trichromacy” where the optic gene on the X-chromosome is polymorphic for sensitivity to medium and long wavelengths; as a result, female heterozygous individuals are trichromatic and female homozygous individuals and males are not. Assuming there is some advantage to trichromacy, and if the neural sensitivity in sensory systems are labile within individuals, maybe we could expect to see increased neural sensitivities in the visual brain regions with a corresponding decrease in olfactory neural sensitivity/connectivity for female heterozygous individuals compared to other dichromatic individuals in a population. ???

Another offhand comment--one concept I’m not completely clear on is “constraint”. It is easy to understand that brain size is constrained by the size of the skull cavity, or energetic expenses for development and maintenance of particular organs are constrained by the proportion of energy allocated to others, but I feel that the term is sometimes used loosely, or at least the constraining factors are less obvious. In this paper, the author suggests near the end of the discussion that while some OR genes are accumulating coding region disruptions, others are evolving under evolutionary constraint. Constrained by what exactly? And how could this constraint be tested?