Blind mole rats finding their way around their underground tunnels appear to face a tough challenge. They can't use any of the sensory cues that surface-dwellers take for granted: they can't see, and sounds and smells don't carry far through soil. But blind mole rats are actually spectacular navigators and expertly burrow the most efficient detours around obstacles. Tali Kimchi and colleagues at Tel-Aviv University now suggest that mole rats accomplish these astonishing feats using a form of seismic `echolocation'(p. 647).
`Mole rats need to live at a certain depth' says Kimchi, `because the roots and tubers that they eat are mainly found 20-40 cm below the surface.'Emerging on the surface is suicide since these blind animals are helpless against predators, while digging too deep costs energy and brings the mole rat out of range of its food supply. So how do mole rats estimate their distance from the surface? Mole rats communicate with one another using seismic signals produced by drumming their heads against their tunnel roofs. Could they be using the reflected vibrations of their own head-drumming to estimate their distance from obstacles and from the surface?
If mole rats use seismic `echolocation', they should head-drum more often during a complex than a simple spatial task. To record mole rat head-drumming,Kimchi dug ditches across mole rat tunnels in a field and placed small seismic earthquake detectors called geophones around the tunnels. She recalls that it was tricky getting recordings; the shy creatures ran away when they felt her approaching their tunnels. As she had expected, mole rats digging detours around her ditches head-drummed more often than mole rats navigating through straight tunnels, so mole rats do drum more often during complex tasks.
But to demonstrate that head-drumming provides information about the mole rats' surroundings, Kimchi needed to show that seismic waves are reflected in different ways depending on the obstacle. She took her recordings to Moshe Reshef, a geophysicist who produced a computer model that generated pulses exactly like those in Kimchi's recordings. The model showed that the amplitude, polarity and diffraction of reflected seismic waves provide information about the distance, density and size of obstacles. So reflected waves could help mole rats assess obstacles and probably also their digging depth.
But how were the animals detecting these waves? Mole rats pick up head-drumming signals from others by pressing their jawbone against their tunnel wall. They couldn't possibly drum their head and then press their jaw against the tunnel fast enough to detect their own signal reflected from obstacles ahead, so Kimchi wondered if mole rats use their feet to pick up the signal. To test this, she designed a tunnel T-maze in which mole rats could only detect vibrations through their feet. Sure enough, mole rats sped towards a head-drumming mole rat located down one of the maze's arms, suggesting that they could detect vibrations through their feet. Kimchi then showed that mole rats can also detect reflected seismic signals from their own head-drumming;mole rats had no trouble finding a mechanical shaker sending out similar signals to her field recordings. As a final touch, Kimchi found unique mechanoreceptors in the skin of mole rats' paws, which might detect vibrations.
Kimchi admits that while all her results point in the same direction, she does not have concrete proof that mole rats use seismic `echolocation'. She concludes: `Further work is needed to prove the dual function of head-drumming. This is just the tip of the iceberg.'