Volumetric and connectivity assessment of the caudate nucleus in California sea lions and coyotes.

In addition to a large (chimpanzee-sized) and heavily convoluted brain, one of the most striking neurobiological features in pinnipeds is the large size of the head of the caudate nucleus, which dwarfs the rest of the striatum. Although previous research has suggested carnivore striatum is small in comparison to that of primates, there are limited volumetric data on separate striatal structures in carnivores. Therefore, the apparent functional implication of a potentially hypertrophic caudate to carnivores has not been discussed. Here, for the first time, we obtained separate volumetric measurements of caudate and putamen in California sea lions and coyotes. Exemplars of both species had very large caudate nuclei, approximately 1/75th of total brain volume. In both the sea lion and coyote, the caudate dwarfed the putamen at a ratio of 13 to 1 or greater, a finding in strong contrast to measurements showing larger putamen than caudate in primates. In addition, using post-mortem diffusion tensor brain imaging, we mapped and compared white matter connections between the dorsal caudate and the motor, premotor and frontopolar, and orbitofrontal cortices in healthy adult sea lions and healthy adult coyotes. The sea lions showed some evidence of greater premotor and frontopolar connectivity. These findings bear on previously underexplored striatal characteristics of large carnivores, and we discuss potential interpretations related to cognitive flexibility and sensorimotor transformation.