Parallel Evolution of Low-Frequency Sensitivity in Old World and New World Desert Rodents

Abstract

"It is therefore evident that these two geographically separated groups have each evolved highly derived large middle ears, large typanic membranes, and freely mobile ossicular systems with large lever ratios; but that they have done so in quite different ways. This is as we would expect in independently evolving groups which experience similar selective pressures but which would not be expected to have the same mutations in their gene pools." "On a theoretical level, one could point to the scarcity of natural cover in their arid environment; their need to travel considerable distances to forage for food, and their nocturnal behavior. In such an environment there would be significant selective value to sensory systems that allow an animal to detect the presence of predators in the dark. There ar also direct data to support this. Experiments have shown that at least some gerbils and kangaroo rats can avoid predation by owls and snakes except when their middle ear volumes are reduced -- and, in kangaroo rats, when very low light levels deprive them visually as well (Webster and Webster 1971; Lay 1974). Being able to hear the low frequency sounds produced by both snakes and owls just prior to striking (Webster 1962) - and thus to avoid their predation - would seem in these two cases to be the primary selective pressure that favored low-frequency sensitivity."