In Focus: Can golden moles hear the earth move with a Thor-like ‘hammer’?

The golden moles (Chrysochloridae) are a group of silky and rather endearing group of mammals resembling, but distinct from, the true moles (talpids). Indeed, they are more closely related to elephants and manatees than to moles, shrews, rodents or other seemingly similar animals. They share many features with moles, such as adaptations to burrowing, and inhabit the grasslands, forests and deserts of southern Africa. For small insectivores such as the golden mole, desert environments present considerable challenges to foraging in particular. Food sources (typically termite colonies) tend to be few and far-between, the sandy substrate doesn’t allow the construction of permanent burrows, and orientation is notoriously difficult. This last obstacle could pose a particular problem to the moles – they are poorly-sighted animals, unable to use the sun’s azimuth position to navigate as several other desert species do. So how do they find enough food to survive without familiar tunnels or visual cues?

016106-01
Golden mole tracks and grassy tufts, Namibia. Image by Frans Lanting.

In 1997, biologist Peter Narins reported a curious behavioural phenomenon in the Namib golden mole, Eremitalpa granti namibensis. Although they travel by ‘sand-swimming’ beneath the surface during the day, they emerge to walk across the cooler sand at night to find food. While foraging, the animals were able to walk almost directly from one grassy patch to the next, despite their poor sight, the darkness, and the scattered grasses. What Narins observed was intermittent head-dipping by the moles, plunging their heads into the sand for several seconds before continuing. He noticed that the closer the moles came to the next tuft of grass, the more frequently the dipping seemed to occur.

head dip
Head dipping in the Namib golden mole, Eremitalpa granti namibensis. From Narins et al. 1997.

One well-known attribute of the desert golden moles, including E. g. namibensis, is their unusual aural anatomy. Of the three bones of the middle ear; the incus (anvil), malleus (hammer) and stapes (stirrup), the rotational “hammer” head of the malleus is greatly enlarged (see below). As a result, the bone accounts for an enormous 0.1% of the mole’s total body mass – and if that sounds underwhelming, consider that in the similarly-sized mouse, that figure is just 0.001%, and it is 0.00008% in humans! The hypertrophy of the malleus means its centre of mass is shifted laterally away from its axis of rotation, making it more vulnerable to imbalance. Several studies had proposed that this could be an adaptation to hearing very low-amplitude, low-frequency sounds: the increased inertia of the large malleus would mean that the skull might move around it under low-frequency vibrations, with the malleus remaining almost stationary and therefore displacing the other ossicles of the ear and generating a signal the mole can detect. Indeed, calculations made later by Matthew Mason (2003) and Willi et al. (2006) showed that peak conduction by the ossicles of the middle ear would occur at around 200-300 Hz.

malleus
Comparison of the malleus and incus of Eremitalpa grantii namibensis (a) and Amblysomus hottentotus (b), showing centre of gravity and axis of rotation. Note the enlarged head of the malleus in E. g. namibensis and the subsequent shift in the centre of mass. From Mason and Narins 2001.

In light of Narins’s new observations of dipping behaviour, he suggested that the moles use low-frequency seismic vibrations to detect grasses blowing in the wind and emitting low-level vibrations (which, when measured, gave a peak frequency of 310 Hz). Head-dipping would couple the skull more closely with the vibrating substrate, and the increased rate of dips as the animals approach the source could represent them zeroing in on it and the prey within. It has even been proposed that, once close enough, golden moles may pick out signals from the termites themselves.

To test this theory, Edwin Lewis, along with Narins and colleagues, buried eight speakers at regular intervals around the edge of a circular experimental arena in the Namib sand.

mole
A mole ‘locating’ active speakers emitting low-frequency sound. From Lewis et al. 2006.

They played simulated seismic signals through groups of three adjacent speakers to create a directional signal, and tracked the movements of animals released into a grass hummock at the centre of the arena, 5 metres away. Their analyses found that the moles moved towards the active speakers significantly more than they should under random movement, indicating that they could indeed detect and orient towards the low-frequency sounds. Of course, it must be borne in mind that this was not found across all the animals, and that to date there is no physiological evidence confirming this incredible foraging strategy. It is still a possibility under investigation, but the remarkable middle ear of the golden mole may have pointed biologists towards an incredible sensory capability in these desert species. Stay tuned!

Glossary

Aural: Relating to the ear and hearing.

Hypertrophy: Increased size of an organ or tissue by the enlargement of the cells comprising it.

References

Lewis, E. R., Narins, P. M., Jarvis, J. U., Bronner, G., and Mason, M. J., Preliminary evidence for the use of microseismic cues for navigation by the Namib golden mole, Acoustical Society of America (2006), 119, 1260-1268

Mason, M. J., Bone conduction and seismic sensitivity in golden moles (Chrysochloridae), J. Zool., Lond. (2003), 260, 405–413

Narins, P. M., Lewis, E. R., Jarvis, J. U. M., and O’Riain, J., The Use of Seismic Signals by Fossorial Southern African Mammals: A Neuroethological Gold Mine, Brain Research Bulletin (1997), 44, 5, 641–646

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