A Caltech Library Service

Evolution and Scaling in Mammalian Brains


Bush, Eliot Christen (2004) Evolution and Scaling in Mammalian Brains. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/P9FG-7K79.


Here I look at three stages in the evolutionary development of mammalian brains. Chapter one addresses how connectivity in neocortex scales with brain size. This is of evolutionary interest because it helps define the basic mammalian condition. Neocortical white matter increases disproportionately in large brains. This might reflect increases in the number of connections per neuron. It might also reflect scaling in axon diameter. I compare these hypotheses by examining white matter-gray matter scaling in cerebellum. Because the white matter of cerebellum lacks cortico-cortical connections, the connectivity theory predicts that cerebellar white matter should not hyperscale relative to gray matter. I have measured white matter and gray matter volume in a large sample of mammals and I find that cerebellar white matter does not hyperscale. This supports the proposition that neocortical hyperscaling reflects an increase in the number of connections per neuron in large brains.

In chapter two I use independent contrasts analysis to examine the scaling of frontal cortex in a large sample of mammals. I find significant differences in scaling between primates and carnivores. Primate frontal cortex hyperscales relative to the rest of neocortex and the rest of the brain, and the primate slope is significantly greater than that for carnivores. This suggests that there are substantial differences in frontal cortex structure and development between the two groups. Combined with with anatomical differences, it suggests that primates have evolved a number of unique adaptations in frontal cortex.

Chapter three examines the evolution of brain size in anthropoid primates. Living anthropoids have larger brains than strepsirrhines. What about early anthropoid fossils? I measure brain size in the early anthropoid Parapithecus grangeri using computed tomography. I find that relative to the living anthropoids, Parapithecus had a small brain for its body size. Thus large brains did not develop at the same time as a number of other anthropoid adaptations.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:endocranial volume; frontal cortex; gray matter; Primates; white matter
Degree Grantor:California Institute of Technology
Major Option:Biology
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Allman, John Morgan
Thesis Committee:
  • Allman, John Morgan (chair)
  • Konishi, Masakazu
  • Perona, Pietro
  • Shimojo, Shinsuke
Defense Date:4 March 2004
Non-Caltech Author Email:Eliot_Bush (AT)
Record Number:CaltechETD:etd-03292004-144927
Persistent URL:
Bush, Eliot Christen0000-0001-5476-8005
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1191
Deposited By: Imported from ETD-db
Deposited On:02 Apr 2004
Last Modified:27 Jan 2021 22:09

Thesis Files

PDF (eliot-bush-thesis.pdf) - Final Version
See Usage Policy.


Repository Staff Only: item control page