Comparison of Dolphins' Body and Brain Measurements with Four Other Groups of Cetaceans Reveals Great Diversity.

We compared mature dolphins with 4 other groupings of mature cetaceans. With a large data set, we found great brain diversity among 5 different taxonomic groupings. The dolphins in our data set ranged in body mass from about 40 to 6,750 kg and in brain mass from 0.4 to 9.3 kg. Dolphin body length ranged from 1.3 to 7.6 m. In our combined data set from the 4 other groups of cetaceans, body mass ranged from about 20 to 120,000 kg and brain mass from about 0.2 to 9.2 kg, while body length varied from 1.21 to 26.8 m. Not all cetaceans have large brains relative to their body size. A few dolphins near human body size have human-sized brains. On the other hand, the absolute brain mass of some other cetaceans is only one-sixth as large. We found that brain volume relative to body mass decreases from Delphinidae to a group of Phocoenidae and Monodontidae, to a group of other odontocetes, to Balaenopteroidea, and finally to Balaenidae. We also found the same general trend when we compared brain volume relative to body length, except that the Delphinidae and Phocoenidae-Monodontidae groups do not differ significantly. The Balaenidae have the smallest relative brain mass and the lowest cerebral cortex surface area. Brain parts also vary. Relative to body mass and to body length, dolphins also have the largest cerebellums. Cortex surface area is isometric with brain size when we exclude the Balaenidae. Our data show that the brains of Balaenidae are less convoluted than those of the other cetaceans measured. Large vascular networks inside the cranial vault may help to maintain brain temperature, and these nonbrain tissues increase in volume with body mass and with body length ranging from 8 to 65% of the endocranial volume. Because endocranial vascular networks and other adnexa, such as the tentorium cerebelli, vary so much in different species, brain size measures from endocasts of some extinct cetaceans may be overestimates. Our regression of body length on endocranial adnexa might be used for better estimates of brain volume from endocasts or from endocranial volume of living species or extinct cetaceans.

Sperm whales and killer whales with the largest brains of all toothed whales show extreme differences in cerebellum.

Among cetaceans, killer whales and sperm whales have the widest distribution in the world's oceans. Both species use echolocation, are long-lived, and have the longest periods of gestation among whales. Sperm whales dive much deeper and much longer than killer whales. It has long been thought that sperm whales have the largest brains of all living things, but our brain mass evidence, from published sources and our own specimens, shows that big males of these two species share this distinction. Despite this, we also find that cerebellum size is very different between killer whales and sperm whales. The sperm whale cerebellum is only about 7% of the total brain mass, while the killer whale cerebellum is almost 14%. These results are significant because they contradict claims that the cerebellum scales proportionally with the rest of the brain in all mammals. They also correct the generalization that all cetaceans have enlarged cerebella. We suggest possible reasons for the existence of such a large cerebellar size difference between these two species. Cerebellar function is not fully understood, and comparing the abilities of animals with differently sized cerebella can help uncover functional roles of the cerebellum in humans and animals. Here we show that the large cerebellar difference likely relates to evolutionary history, diving, sensory capability, and ecology.

Serotonin dysregulation in Fragile X Syndrome: implications for treatment.

Fragile X Syndrome (FXS) is a trinucleotide repeat disorder that results in the silencing of the Fragile X Mental Retardation 1 gene (FMR1), leading to a lack of the FMR1 protein (FMRP). FMRP is an mRNA-binding protein that regulates the translation of hundreds of mRNAs important for synaptic plasticity. Several of these pathways have been identified and have guided the development of targeted treatments for FXS. Here we present evidence that serotonin is dysregulated in FXS and treatment with the selective serotonin reuptake inhibitor (SSRI) sertraline may be beneficial for individuals with FXS, particularly in early childhood.