How to find the way

Subterranean mammals construct, occupy, and maintain very long and extensive burrow systems. An average subterranean mammal (single, weighing 5.3 oz [150 g]) controls about 203 ft (62 m) of burrows. Of course, there are species-specific, habitat, and seasonal differences. This also implies that an average mole-rat living in a group consisting of 10 family members has to be familiar with at least 2,034 ft (620 m) of burrows. The longest burrow systems were found in Cryptomys mole-rats and coruros. Yet, the burrow system is a complicated, complex, three-dimensional network. Although there is evidence that subterranean mammals have an extraordinary spatial memory based on well-developed kinesthetic sense (controlled, in part, by sensitive vestibular organs), this fact does not explain how subterranean mammals can steer the course of their digging and what, in absence of visual landmarks, is the nature of external reference cues for the kines-thetic sense. In 1990, the first evidence that Zambian mole-rats (Cryptomys anselli) show directional orientation based upon the magnetic compass sense was published. In a laboratory experiment, mole-rats collected nest materials and built a nest in a circular arena. They showed a spontaneous tendency to position their nests consistently in the southeast sector of the arena. When magnetic north was shifted (by means of Helmholtz coils), the animals shifted the position of the nest accordingly. This laboratory experiment on molerats has become the first unambiguous evidence for magnetic compass orientation in a mammal and a paradigm for further tests of magnetic compass orientation in small mammals.

The black-tailed prairie dog (Cynomys ludovicianus) is well known for its large burrows. (Photo by Tim Davis/Photo Researchers, Inc. Reproduced by permission.)

Convergent spontaneous directional magnetic-based preference for location of nests in the laboratory was demonstrated also in taxonomically unrelated blind mole-rats from Israel. In 2001, Nemec and associates observed, for the first time in a mammal, structures in a brain (populations of neurons in colliculus superior), which are involved in magnetoreception in Cryptomys anselli.

The problem of orientation underground was addressed as recently as 2003, when it was reported that blind mole-rats could avoid obstacles by digging accurate and energy-conserving bypass tunnels. Apparently, the animals must possess both the means to evaluate the size of the obstacle as well as the ability to perceive its exact position relative to the original tunnel that it will rejoin. At present, information about potential sensory mechanisms can be only speculated.

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