Moles have exploded on the central coast in the last two years, and wherever I wander, I see their familiar sign—the ground covered with rumpled-up surface runs and those classic molehills we are admonished not to make mountains out of. What is going on? Why are they proliferating in such an unprecedented pulse? The answers take us right into their biological niche and amazing morphological adaptations.
The primary stimulus for moles’ recent success is the heavy rainfall of the last three years. Our broad-footed mole’s diet mainly consists of earthworms, along with occasional insects and other underground invertebrates. Earthworms require damp soil and plenty of decomposing plant material, both of which have been abundant. The mole’s primary foraging niche is right in those first couple of inches beneath the surface, where earthworms are most abundant and where the mole’s remarkable adaptations work best.
Moles’ diet puts them in the category of insectivores, rather than the herbivorous rodents with which we are most familiar. As predators, their populations and birth rates are lower, and their territories larger, than those of mice, rats, voles and gophers. A single mole can occupy a territory of up to an acre, while you might find a dozen gophers in the same area. But with more damp habitat and the corresponding increase in earthworm populations, the land is supporting much higher reproductive success and population densities for moles.
The teeth of the mole reveal its diet: long rows of sharp, jagged teeth lining the upper and lower jaws. These teeth are designed to puncture and slice their soft-bodied prey, and their arrangement is similar to shrews’ teeth but entirely different than those of herbivorous rodents, which have large gnawing incisors at the front, a row of flat-surfaced grinding molars at the back, and a space in between.
The diet of insectivores apparently makes moles unpalatable to predators such as bobcats, coyotes and even owls. This is why you occasionally find dead moles and shrews on trails, where the predator has caught but rejected them. Owls will go so far as to catch a mole and strip the skin up over the head before rejecting it, using their bill to pull the skin and leaving the poor mole looking like someone who pulled their T-shirt up over their head.
Moles’ highly evolved front foot looks very much like a tiny human hand, which makes those skin-stripped carcasses particularly creepy. But this foot is one of its most amazing adaptations. With its five-toes, it is like a tiny garden spade. It combines with the whole front leg structure for powerful, almost swimming-like, scraping strokes underground, as it parts the loose earth and moves through it in hunching thrusts that push surface soils up into the fissured runs we see.
In very loose ground, moles will use a two-armed breast-stroke motion, almost literally swimming through the ground. In harder ground, they will use more of a crawl-stroke, alternating right and left strokes while the opposite arm braces against the side wall of its tunnel.
Its hand, arm and scapula are so adapted to this digging style that the mole can’t place its foot flat on the ground. As a result, when it does rarely cross open ground, it moves in an odd wiggling movement reminiscent of an elephant seal. Its tracks are little arcs from the powerful toe-tips of its sideways-oriented foot.
There are a few more interesting adaptations to note. Their fur—the famous moleskin—is incredibly soft, smooth and fine, shedding dirt so completely that they are always immaculately clean. In addition, the fur folds easily forward or back, allowing them ease in running backward in their tunnels. Their stubby tail is highly sensitive to touch, helping them feel their way when they reverse direction. Related tunneling features are their short ribcage and extremely flexible lower vertebrae, which allow them to make 180 turns in tight tunnels.
The mole is almost totally blind, able to discern only light and dark—critical to keeping its tunnels closed—but it makes up for its blindness with incredibly sensitive nose structures. The mole feels its way through the ground with its nose and locates its prey by a combination of touch and smell. All these features—the enigmatic underground life, the stout fuzzy coat, the stubby arms, the pointed nose and blindness—add up to its wonderful characterizations in children’s stories such as “The Wind in the Willows.”
In average soil conditions, a mole can dig about 15 feet an hour, and it tends to work in shifts of three or four hours. Over a period of days, that can amount to quite a bit of digging. But the extent of its recent population increase was revealed in events I’ve never witnessed in a lifetime of field observation. Twice, two days after early rains, a mole tunneled far out into the open sands of the upper Abbotts Lagoon sand basin, digging complex networks of twisting, branching surface tunnels full of cul de sacs and reversals. There were maybe 200 feet of tunnels in all, covering an area about 50 feet in diameter. In one case, the mole dug a perfect circle about 15 feet wide.
What were they hunting out in the sand, far from earthworm territory? The local skunks provided the answer. Both were harvesting sand wasp larvae, revealing a perfect example of John Muir’s observation that when you pull on one string in nature, you find it connected to the whole universe.
Sand wasps have also profited from damper sand, allowing them to lay more eggs and hatch more larvae in their 6-inch- to 8-inch-deep excavations. Skunks are aware when there is a major hatch and are out busily digging them up, locating them by smell. In the unusual case of the mole, their tunnels were often going to the exact locations where the skunks were digging. It must have been quite the party night!
The world of a mole, then, involves extensive surface burrows just two or three inches deep, in a zone that is loose and shallow enough for digging and which is rich in prey. But moles have another tunnel system, several inches deeper, which connects their sleeping and birthing dens, food caches and latrine chambers.
To excavate these tunnels and cavities, they push the soil straight up and out, using their broad digging feet like bulldozer blades, resulting in the little conical mounds we know. If you look carefully, you will see that these mounds are fissured on the top, and often have the last plug of soil pushed out and lying on top.
Meanwhile, the gopher, just as dedicated a digger, employs an entirely different style of digging, without a spade-shaped foot. The gopher has a more typical rodent foot, though its digging claws are long and hard, that allows for more extensive dispersal running across the ground. It digs with a frenzied scratching motion, compared to the powerful scraping strokes of the mole. Its network of feeding tunnels is a couple of inches deeper than the mole’s, in root zones rather than worm zones, and it clears this network by constantly and frenetically pushing armfuls of soil up sloping ramps and out into the classic fan-shaped gopher mounds we see.
There is an interesting gradient from gopher habitat to mole habitat out in the fields. In the dry, open grass and chapparal, gophers are ubiquitous, their mounds sometimes completely covering whole meadow surfaces. But as you move toward the shade and the bottomlands, the ground is damper and more populated by earthworms. There, the dirt mounds begin to change from the fan-shaped, pushed-out, dry-soil gopher mounds to the pushed-up, more fissured and circular mole mounds. Once you understand these fundamental differences, these signs are easy to tell apart.
Richard Vacha is a Point Reyes Station resident and the founder of the Point Reyes Tracking School and the Marin Tracking Club.