How to get through

The most significant challenge is a mechanical one: soil is a dense, more or less hard and compact medium that cannot be penetrated easily. Movement through soil is energetically very costly. Vleck (1979) has estimated that a 5.3-oz (150-g) pocket gopher burrowing 3.3 ft (1 m) may expend 300-3,400 times more energy than moving the same distance on the surface. To keep the energy costs of burrowing at the minimum, the tunnel should have a diameter as small as possible. To achieve this, subterranean mammals have a cylindrical body with short limbs and no protruding appendages. Even testes of most underground dwellers are seasonally or permanently abdominal. Subterranean mammals are mostly small-sized animals weighing 3.5-7 oz (100-200 g), but ranging from 1 oz (30 g) (Namib golden mole, naked mole-rat, and mole-vole) to 8.8 lb (4 kg) (bamboo rat). In order to penetrate the mechanically resistant medium, subterranean mammals need efficient digging machinery. Subterranean rodents dig (loosen soil) primarily with their procumbent, ever-growing incisors, or use teeth and claws, whereas subterranean insectivores, armadillos, and the marsupial mole use only robust, heavily muscled and large-clawed forelimbs. In teeth-diggers, the whole skull is subservient to incisors and well-developed chewing muscles. Interestingly, subterranean rodents belonging to the suborder Hystricognathi (Bathyergidae, Octodon-tidae) transport loosened soil backwards by pushing or kicking the soil with hind limbs, whereas representatives of the suborder Sciurognatha (Muridae, Geomyidae) turn in the burrow and push out the loosened soil with the head. Desert golden moles as well as the marsupial mole do not dig permanent tunnels (except for their nest burrow), but "swim" through the sand. Although sand-swimming requires less than a tenth of the energy required by mammals that dig permanent tunnels through compact soil, it is still much more expensive than running on the surface. Sand-swimming at a mean velocity of 25-97 ft/h (7.6—29.6 m/h) (as recently estimated for the Australian marsupial mole [Notoryctes typhlops]

The eastern mole (Scalopus aquaticus) has no external ears and a thin layer of skin protects its eyes. (Photo by Kenneth H. Thomas/Photo Researchers, Inc. Reproduced by permission.)

and Namib desert golden mole [Eremitalpa granti], respectively) is also substantially slower than walking or running above ground (about 1,476 ft/h [450 m/h]). It would apparently be energetically impossible for these mammals to obtain enough food by foraging only underground. Indeed, in one study of free-living Namib desert golden moles, the mean daily track length was 0.87 mi (1.4 km), but only 52.5 ft (16 m) of it was below the surface.

Subterranean mammals can move backwards with the same ease as forwards. The skin is usually somewhat slack, and the fur tends to be short and upright, brushing in either direction. These all may be burrowing adaptations to match frictional resistance, to facilitate moving and turning in tunnels. The extremes such as the total alopecia (hairlessness) of the naked mole-rat or the long hairs and thick pelage of the silvery molerat (Heliophobius argenteocinereus) are exceptions to the rule and should not be considered burrowing adaptations per se. Reduction or even absence of auricles (pinnae) may be beneficial for digging and moving in tunnels because of the reduced friction. The popular assumption that auricles are reduced or missing because, otherwise, they would have to act as shovels collecting all the dirt cannot withstand critical comparative analysis. Many burrowing rodents have rather prominent auricles and are apparently not handicapped. Probably more im portant than whether auricles are an advantage or disadvantage for burrowing is whether they are required for sound localization. If not needed for hearing, only then would they be reduced. The tail tends to be shortened in subterranean and fossorial mammals, yet there is no clear explanation as to the adaptive value of this feature. For instance, African mole-rats of two related genera, Heterocephalus and Cryptomys, differ in this trait markedly. Similarly, fossorial-subterranean octodon-tids have medium-sized tails, whereas related surface dwelling cavies have reduced tails. Vibrissae in subterranean mammals are also shorter and less protruding than in many surface dwellers. In sand-swimming golden and marsupial moles, they are inconspicuous, sometimes even missing.

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