BURROWING AND THE EVOLUTIONARY RATCHET AMONG RODENTS: APPLICATIONS OF PHYLOGENETIC COMPARATIVE METHODS TO UNDERSTANDING PALEOECOLOGICAL PROBLEMS

Burrowing behavior, or fossoriality, has evolved a number of times among mammals, generating a diversity of morphological and physiological adaptations among the varied lineages that acquire this life habit. Because many of the adaptations that allow efficient burrowing also are detrimental to predator avoidance and escape aboveground, the evolution of fossorial specializations would be expected to drive an evolutionary ratchet; the evolutionary landscape is more favorable to acquiring greater specialization for burrowing rather than for decreases in the degree of burrowing specialization. Such a ratchet would explain the repeated convergent evolution of fossorial and even fully subterranean life habits in mammals. However, phylogenetic comparative analysis of the macroevolutionary pattern of acquisition of fossorial habits among extant rodents finds this asymmetry of evolution only into and out of subterranean habits and not between general terrestrial habits and less extreme fossoriality. However, if the evolutionary ratchet is in effect, intermediate degrees of fossoriality might not be expected to persist over evolutionary time when subterranean species evolve within a lineage, leading to underestimation of the frequency of evolution of fossoriality. Examination of the evolutionary patterns in morphology indicative of fossoriality is consistent with this explanation of pattern, as the acquisition of morphological digging adaptations shows indications of asymmetry among lineages of burrowing rodents.