Evolution of Brain Size

2006-04-10T19:25:00.000-05:00

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

2006-04-02T17:27:00.000-05:00

Bird-song - Evolution of HVC and repertoire size

2006-03-27T02:20:00.000-05:00

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

2006-03-15T15:33:00.000-05:00

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

2006-03-11T13:11:00.000-05:00

"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

2006-03-06T11:49:00.000-05:00

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

2006-02-27T01:46:00.000-05:00

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?

Linked Regularities in the development and evolution of mammalian brains

2006-02-22T11:15:00.000-05:00

Finlay and Darlington 1995.

This study deal mainly with brain development and evolution among different mammalian taxa. They test two hypothesis: 1) “Developmental constraint hypothesis” which predict that the size of any brain structure can be predicted, 2) “adaptation hypothesis” predict the opposite, were brain region are variable among different taxa.
The first part explore the relation between brain size and ten brain division, they found that the neocortex expand quickly in comparison with other brain part as brain size increase. Moreover, 96.29% of the brains size variation can be explain by increase in neocortex. If olfactory bulb is also taken into account 99.19% of the brain size variation can be explain by these two regions. The second part of the paper explore the importance of embryo developing time over brain structure size. The general idea is that the longer neurogenesis is delay, the more precursor cells can be formed and the larger the structure that result. The number of precursor is correlated with neuron number. They found that indeed larger brain size increase with late birth dates, increasing the number of precursor pool.
Why the neocortex increase faster than other brain areas? May be because increasing the capacity in complex functions such as sensory perception and spatial reasoning are more important than for expample vision abilities?
If the number of precursor is higly influence by the developing time, one my expect that animals like wales and ungulates may have high number of neurons. This data is not not available in this paper, but do we think that this is the case when compare to primates with shorter gestation time?

Good reference website

2006-02-20T22:32:00.000-05:00

I'm finding this website useful and thought I'd pass it along.

Brain size and sociality

2006-02-12T19:10:00.000-05:00

Evolution of Brain Size

Evolution of Gene Expression

2006-02-05T21:39:00.000-05:00

Compared to brains of other animals, the human brain is characterized by a number of “higher functions” including language and abstraction. Like our genetic similarity, the internal structures of our brains have few differences when compared to chimpanzees. Caceres et al. propose that the differences in size and function between our brains and the brains of chimps and other primates can be attributed to differences in the regulation of genes. Using a variety of methods I vaguely understand, they found that asymmetries in gene expression between humans and other primates are greater in brains than in other tissues such as the liver and heart. They propose that changes in gene regulation may be an adaptation to maintain neuronal functions over a greater period of time. Some questions: Does this explanation make sense when accounting for the “natural” life-span that characterized human evolution? Eg, Humans live longer in the present than they did hundreds of years ago and neuro-degenerative diseases, which generally affect older individuals, may not have had fitness consequences. Caceres et al. also propose that genes may be up-regulating metabolic processes in the human brain. Is this up-regulation characterized by a greater number of mitochondria? If so, how is the amount of mitochondria in different tissues regulated?

Uddin et al. conducted a similar study, also involving methods I don’t understand. They also found an up-regulation of genes that are involved in metabolism and neural functions. In contrast to Cacerres et al., Uddin et al. found that chimpanzees have gene regulation profiles more similar to humans than to gorillas. This demonstrates that chimpanzees are the sister group to humans. The similarity of gene regulation between chimps and humans could help elucidate the molecular mechanisms that associated with “higher” brain functions.

Some thoughts on last week's reading

2006-01-29T20:51:00.000-05:00

While all of you are reading (and posting) on Aiello and Wheeler, and on Armstrong, I thought I would pose a couple of questions that emerged from the reading of Striedter's Chapter 4.

As I mentioned in class, I thought that the explanation the text gave for the source of allometries was really very interesting. If you recall, the explanation had to do with the times at which brain growth and body growth cease. My first question is whether you think this is an adequate explanation. It seems to me that this is really an explanation for *how* such an allometry could emerge, but is there any reason a particular taxon couldn't alter the relative timing of these events? Could the taxonomic differences in these allometries reflect differences in, say, optimal metabolism/brain/body scaling from an ultimate standpoint?

Next, I would challenge you to think about what absolute brain size really means. It isn't at all clear, but it is obviously an interesting problem. This is a point Striedter makes throughout the book, and I think it will resurface repeatedly throughout the course as well.

Tissues are expensive

2006-01-29T15:45:00.000-05:00

Aiello and Wheeler propose that in order to compensate for increased encephalization primates and hominids shortened their gut. This allowed for the evolution of larger metabolically expensive brain tissue without a corresponding increase in BMR. Henneberg criticizes this hypothesis by pointing out that the concurrent change in these two structures does not necessarily indicate interdependence. Aiello and Wheeler argue that the adoption of a high-quality diet allows for a small gut and liberates more energy to be devoted to the development of a large brain. If high-quality food requires greater skill/intelligence to locate, I don’t see how the proposed coevolution between gut and brain size could have been initiated. It makes sense that a relaxation of metabolic constraints could allow for the development of larger brains, but what caused the change in diet? Aiello and Wheeler tried to address these complicated interactions by avoiding telling a “just so” story. Unfortunately, in the end, I think they are still storytelling. Their story just happens to involve a physiological mechanism.

Finally, Aiello and Wheeler refer to overall brain size in this paper. However, they fail to address changes in individual sections of the brain. Do all brain sections increase isometrically with an increase in size? Is it possible that there may be differential selection for some brain regions but not others depending on foraging strategy and dietary niche of an organism? For example, an increase in the size of the hippocampus could have allowed earlier hominids to adopt a more complex foraging strategy by increasing the ability for spatial memory. In this example, an increase in the hippocampus may not drastically increase overall brain size, but it might change foraging strategy and, therefore, the size of the gut.

2006-01-23T02:45:00.000-05:00

Is increased neuronal interconnectedness and absolute brain size associated with greater intelligence? Intuitively, it seems that the greater the level of communication between neurons throughout the brain, the higher the cognitive capability of an organism. However, we see that this is not always the case, as there are numerous tradeoffs associated with neural network interconnectedness and an ideal brain size and efficiency (metabolic, morphic, etc.) Can we imagine ideal brain architecture? In Fig. 4.16 (p. 128) Neural Network Allometry; we see two examples of connectivity: proportional and absolute. Proportional connectivity is prohibitive because of spatial and metabolic limitations associated with the number of axons necessary for such interconnectedness (very low efficiency), conversely, the absolute model allows for a decent level of connectivity while greatly decreasing the excess wiring associated with a network in which each neuron can communicate with any other neuron.

Striedter reveals that as absolute brain size increases, brain connectivity decreases. One plausible reason being that maintaining increased connectivity in very large brains may lead to incoherence. Streidter writes, "In an evolutionary sense, there is nothing wrong with being 'stupid' as long as you can get away with it." Examining the cognitive abilities of closely related species and measuring this against the design of each brain may prove very interesting. What are the best tradeoffs, and which anatomical specializations are emphasized in the "smarter" species?

2006-01-22T16:07:00.000-05:00

In general brain size tend to increase allometrically with body size, but when the brain:body ration is taken into account the tendency is that small animal have proportionately large brains. However, different taxonomical groups exhibit different scaling exponents, also the encephalization quotient may vary depending on which taxonomic group use to calculated its expected brain weight. Another relevant is comparing brain size among taxonomic groups is by measuring individual brain areas and neural circuits, especially when they are linked to specific behavior or physiological function. From the evolutionary perspective the brain size increase at slow rate from ray-finned fishes to reptiles, but with the origin of birds and mammal the brain increase disproportional compared to body size. Most cartilaginous fishes have large relative brain size that the primitive bony fishes at the same body size. Relative brain size increased once with the origin of jawed vertebrates and again with cartilaginous fish or that it decrease on the primitive bony fishes. Brain size didn’t change significantly when lobe-finned vertebrates first evolved, but it change when tetrapods appear. The reptiles brain are similar to those of terrestrial frogs, toads, and primitive lobe-finned vertebrates in terms of relative size. This suggest that the origin of reptiles didn’t bring major changes on relative brain size. With the origin of birds significant changes occurred in the brain size compare to reptiles of the same body size. Brain size is also larger in mammals when compared with reptiles of the same body size. Although this are the general pattern many variation occurred within and between taxonomic groups. Another factor that can influence brain size is the life history of the species, one can argue that species with complex foraging techniques and/or social structures should have bigger brains. This is the case in many species but there are also many exceptions. Another way to explain the difference on brains size among groups is to compare the ratio of brain:body size formation during the fetal stage. In general the brain of species with large brains relative to body size, continue to develop after the body weight is constant, but in many species the brain and body grow at similar rate (steep) and then the brain stop growing while the body continues to increase (flat).

2006-01-22T15:02:00.000-05:00

Reading for January 24th; Chapter 4 comments:

In Chapter 4 Striedter reviews changes in overall brain size, especially focusing on:

1) scaling relationships (e.g., brain size-body size scaling follows a power law in a similar fashion as the body size-basal metabolic rate relationship, though there is apparently no direct or easy relationship between brain size and basal rate of metabolism - Fig 4.12B)

2) how brain size varies across vertebrate groups and within vertebrate groups (of interest to me is what explains the residual variation within taxa - maybe we can discuss one or two particular groups)

3) in which lineages we find evolutionary increases and decreases in relative brain size (and how these changes are associated with concomitant changes in body size--dwarfism, gigantism, or changes in brain size independent of body size)

and 4) what behavioral correlates (e.g., foraging techniques, sociality) reflect variation in brain size within taxonomic groups.

So, the behavioral correlations with brain size that the author mentioned (the clever foraging and social intelligence hypothesis) were intriguing, but I wonder if these correlations can be more directly pinpointed to increased development of certain areas of the brain. For example, which regions of the brain are necessary for greater spatial memory (for food caching) or for color differentiation (in determining ripeness of fruits)? Another example, maybe songbirds have larger brains compared to other birds of the same size (Fig 4.8B) due to their capacity to learn their songs and incorporate local dialects--is this related to growth and development of particular brain regions?

Also, can we discuss the sub-section on ontogeny and brain size in chapter 4? So, Striedter suggests, that brain allometry relationships, levels of encephalization, etc, are set early on in development, and a major research goal is to figure out what limits or regulates brain development. From this perspective, any "selection pressures" for larger or smaller brains suggested by the patterns of brain size in groups of adult vertebrates must be expressed or realized during development. So, how does this help us understand evolution of brain size? Are we trying to view the evolution of brain size as resulting from selection pressures acting to favor larger/smaller brains within taxonomic groups, but while also considering the flexibility of or the constraints on an organism's development plan? ??

Also, I'm just curious about the timing of brain development...Do all the regions of the brain stop developing at the same time, more or less? Do different regions of the vertebrate brain develop at different rates or at different times during development?

First reading assignments

2006-01-12T11:19:00.000-05:00

Hi, all. The first reading assigned is from Georg Striedter's book "Principles of Brain Evolution." There will be no class on January 17th, but there is some assigned background reading -- Chapters 2 and 3 of Striedter. On January 24th we will discuss Chapter 4 (pp 93-125).

This book is not required -- I don't plan to go over additional chapters. It is, however, very interesting, and it would be a good book to have if you are interested in the topic. If you are not going to buy the book, let me know so I can make a few copies of the assignment.

On a more mundane note, the departmental server is down, which means I can't post pdfs of future reading assignments or the self-assessment worksheet. Feel free to e-mail or call with any questions. I'll be out of town January 14-21.