Brain Size of Mammals

Biologists have keen interest in the characteristics that enable a species to withstand the selective mechanisms of evolution. In particular, they drive evolutionary conclusions from investigating which physiological characteristics are related to brain size. In an attempt to study these characteristics a group of evolution biologists collected data on average values of brain weight, body weight, gestation lengths (length of pregnancy), and litter size for 90 species of mammals. The data were obtained from zoo records and literature. The data are available in the paper by G.A. Sacher and E.F. Staffeldt, "Relation of Gestation Time to Brain Weight for Placental Mammals; Implications for the Theory of Vertebrate Growth," American Naturalist 108 (1974): 593-613.

In order to determine if mammals with large litter sizes have bigger brains than those with smaller litter sizes, a cut off point was chosen using the information collected on average litter size and the mammals were thus categorized into small or large litter size groups.

The following two variables are derived from the original data set: Column Variable Name Description of Variable 1 BRAINSIZ Relative brain weights (brain weight divided by body weight), 2 LITTERSI Litter size = SMALL or LARGE

1. a) Compare the means and standard deviations of the two distributions.