How to find food

Subterranean mammals are animals that live and forage underground. However, the underground ecotope is low in productivity, burrowing is energetically demanding, and, in addition to these costs, foraging seems to be inefficient. It is widely assumed that subterranean rodents must forage blindly without using sensory cues available to and employed by surface dwellers. Indeed, vision is ineffective underground, there are no air currents to transmit airborne odorants over longer distances, high frequency sounds are damped by the soil, and low frequencies cannot be localized easily; touch and taste are only useful on contact. Carnivorous and/or insectivorous subterranean mammals such as moles can dig a stable foraging tunnel system into which prey may be trapped. Moles run-

The thumb of the alpine marmot (Marmota marmota) has a nail instead of a claw to aid in digging. (Photo by St. Meyers/Okapia/Photo Researchers, Inc. Reproduced by permission.)

ning along existing burrows can locate prey by hearing their movement in the tunnel system. Prey animals may also leave scent trails that the insectivorous predator can follow. In most cases, the food is detected at encounter or in the immediate vicinity through touch. The most spectacular example for this type of foraging is the star-nosed mole (Condylura), with its Eimer-organ-invested rostral tentacles. Mason and Narins have shown that the Namib golden mole may use low-frequency vibrations produced by isolated hummocks of dune grass and orient its movement toward the hummocks and the invertebrate prey occurring there. However, in contrast to the prey of moles, tubers, bulbs, and roots are stationary and silent.

It has been demonstrated that subterranean rodents are able to dig in relatively straight lines until they encounter a food-rich area and then make branches to their tunnels to harvest as much as possible from this area. Tunneling in relatively straight burrows conserves energy because the animals do not search in the same area twice. Because geophytes are generally distributed in clumps and patches, extensive burrowing around one geophyte upon encountering it increases the chances of encountering another. Although this dual strategy has been described and its functional meaning recognized in different species of subterranean rodents, sensory mecha nisms that may underlie the switch from linear to reticulate digging have not been addressed until a recent study. It has been shown that subterranean rodents can smell odorous substances leaking from growing plants and diffusing around the plant through the soil. Thus, herbivorous mammals are able to identify the presence of the plants and possibly even to identify particular types of plants specifically. They may be able to orient their digging toward areas that are more likely to provide food sources.

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