Brain/body relations among myomorph rodents

The observed increase in brain size (E) with body size (P) 'from mouse to elephant' may be described by a power relation E = kPb, where b is near 2/3 or 3/4. That this reflects a single, strong interaction between brain and body evolution is challenged by two observations: (1) different species with the same body size may have markedly different brain sizes, and (2) the value of b at the species level is usually nearer 1/3 than 2/3. Furthermore, the idea that a bigger brain means greater versatility on the part of its owner makes a strong statement about such animals. We examined these notions by measuring cranial volumes of 1,480 skulls from 62 subspecies of cricetid and murid rodents. Values of k and b were obtained by computing a reduced major axis on E and P across all specimens (b = 0.693), and when specimens were partitioned by genus (b = 0.456) and subspecies level (b = 0.338). Thus, the overall slope of 7/10 is not a simple extension of the developmental rules at the subspecies level (b near 1/3) nor even at the genus level (b near 4/9). Rather, it may reflect the most likely path for an interbreeding population subjected to varying selective pressures on one or more correlated traits. Furthermore, among the rodents studied, folivorous subspecies averaged about 2/3 as much brain as granivorous, insectivorous or generalist subspecies of the same body weight. Also, Old World rats, which may be more versatile than wood rats, gained their competitive advantage despite having relatively smaller brains.