Beneath the Garden Root

While tenacious Cape dune mole-rats dig their way into the frustrations of every farmer, gardener, horse rider and road fixer in the region, researchers are adamant Africa’s largest subterranean mammal improves soil fertility and can be sustainably managed with a little patience and without poison.

WORDS Francini van Staden

“It is neither a mole nor a rat,” says zoology professor Nigel Bennett of the University of Pretoria, a leading expert on the species. “While they behave like moles and resemble rats, their closest relative is in fact the porcupine.”

Endemic to the coastal environments of the Garden Route, the Cape dune mole-rat (Bathyergus suillus) is the largest truly subterranean mammal with the longest and deepest burrow systems in Africa. Their holes and tunnels are potentially dangerous to all who tread there, while their penchant for plant roots and bulbs has frustrated many a keen farmer and gardener. It has even led to special guidelines for road engineers to incorporate below-the-ground barriers to reduce the risk of road collapse on national highways.

Considered problem animals by many, others are charmed by Cape dune mole-rats’ laid-back mojo, cute pet-like features and tenacity. “They can easily grow as big as a rabbit and, because they are claw diggers and therefore not limited by worn teeth, a burrow system is often over 40 metres – even 80 metres – depending on the size of the animal,” says Nigel. A burrow system will incorporate a place for food storage, nesting and even separate toilet rooms. Mothers have distinct burrows for raising young – and when the young become aggressive in the confined space, they are sent off to go make burrows of their own.

The animals feed on plant roots and carbohydrate storages of geophyte plants such as bulbs, and require extensive burrow distances to harvest enough food. While large mounds may be a nuisance to humans, coming up to the surface is an energy intensive, high-risk, calculated decision in exchange for food reward, social interaction and protection from underground predators.

Cape dune mole-rats’ prolificacy is enhanced by easy living conditions in moist, sandy soil on the coast – unlike their Kalahari cousins whose burrowing is constricted to the rare occasions when it rains, forcing them to share space and storing more food. While up to 30 mole-rats may be living together in the desert, the Garden Route cruisers often have entire systems to themselves as all-year rainfall ensures soft soil and fresh vegetation to harvest throughout the year.

Historic connections
Mole-rats were here long before humans, and it is our occupation of land and resources that is 
inadvertently causing them to become a ‘problem’.

Archaeological excavations in the Garden Route uncovered several collections of mole-rat bones in littoral zone caves, alluding to a historic link between humans and mole-rats. Archaeologist Christopher Hensilwood found the inhabitants of the Blombos cave in the Late Stone Age caught and cooked mole-rats, presumably an important protein source supplementary to fish. Mole-rat fur also seems to have been desirable.

In the days of railway transport, mole-rat catchers were tasked with controlling the problem of soil collapsing beneath the George and Knysna railway.

Nowadays, a shift in predation is part of the reason why mole-rats are increasingly found in urban areas. Urban expansion has reduced snake, mongoose and jackal predators, creating a more favourable environment for mole-rats to thrive, but man, dogs and snakes are also their greatest modern-day threats.

Undermining infrastructure
Among the most dangerous and costly impacts are linked to the mole-rat’s strength and capability to burrow underneath built infrastructure such as roads.

Municipalities and road-building agencies are regularly faced with recurring potholes where mole-rat burrows and cavities cause road surface collapse.

South African National Roads Agency (SANRAL) district manager for the Western Cape, Kobus van der Walt, says while the required ongoing road maintenance has 
obvious financial implications, the unexpected dips and holes on a national road also hold serious safety risks for road users.

Hardened road surfaces prevent direct water seepage, creating ideal food pantry space for mole-rats. “Ground penetration radar detection technology has measured mole-rat food chambers as big as a square metre below national roads.”

For SANRAL, mole-rat barriers, such as those implemented between Wilderness and 
Sedgefield, are alleviating the problem. “We’ve found that bitumen treated barriers up to a depth of 1.2m keep most mole-rats out of road sub-surfaces. But some – I call them the MacGyvers – dig up to two metres deep.”

Road reinforcing through design and construction is also ongoing. “We can hardly control where they dig or burrow, and it is not their fault – humans have disturbed their habitat and now we are dealing with the subsequent challenges,” says Kobus.

SANRAL opposes any harmful elimination, referring to the implementation of mole-proof fencing at Cape Town International Airport to prevent mole-rats from undermining runway surfaces, as another success story. “Our best solution is to continue implementing mole-proof barriers and reinforcing road construction layers to buffer the road from deep mole-rat burrows,” says Kobus.

Management
Nigel says the mole-rats’ roaming lifestyle hold valuable ecological roles humans do not necessarily consider. “Mole-rats improve soil fertility by mixing vegetation, soil and excretion. Through soil moving, they aerate soil and improve its drainage properties.

“Mole-rats are not digging to ruin lawns; they have purpose within our ecosystems of which society bears equal benefit. People widely want to kill mole-rats, I feel we should look after them.”

Unless an underground food source is removed or inaccessible, mole-rats are unlikely to be permanently removed from a particular area. If killed, others will eventually come in their place.

Dispelling the myths that offensive smells, including phosgene gas, will chase them away, Nigel says mole-rats will simply close off the offending tunnels and move to a different area of their burrows for a while. Foreign objects such as bottles will be pushed out the door. Anything edible, like garlic and onion, will be eaten. With burrow systems covering extensive distances, drowning mole-rats is impossible.

Successful preventative solutions such as gardening without bulbs and vegetables, or making bulbs and vegetables inaccessible with underground mesh barricades, are rather recommended.

“Mole-rats are adaptive. If caught and safely relocated to vacant land, they will easily adapt to new surrounds. But they are somewhat tricky to capture,” admits Nigel.