The Hagerman mole is no mo'
Most people's reaction to moles is a negative one. They're pesky animals that burrow in your lawn and make ridges as they dig their tunnels. If you’ve had mole holes in your yard it was probably not while you were living in Idaho. In this state they are not common, and only a few specimens are in museum collections. The mole that is found in Idaho today is the Coast Mole, Scapanus orarius. As the name suggests it is more commonly found along the coast. Most of the records of this species place it in Oregon on the coastal side of the Cascade Mountains. The distribution of the animal does extend across the northern part of Oregon and then crosses the Snake River into Idaho. The Idaho records are as far east as this species gets.
Moles are highly adapted for digging and this can be seen in their skeleton. The arms stick out to the side of the body so that the animal almost swims through the soil by doing the breaststroke. This digging requires powerful chest and arm muscles. The upper arm bone or humerus, bones of the forearm, ulna and radius, as well as the hand, are stout and show prominent ridges and processes where muscles attach. The hand is broad with an extra bone for support and the claws are strong. The distinctive shape and structure of these bones makes them easy for a paleontologist to identify.
Although moles are master diggers and produce extensive tunnel systems, they are still limited to where they can live. Since they live most of their life underground, they can only live where the type of soil permits burrows to be dug. If the soil is too hard, soft or wet, then it may not be possible to dig a burrow or it may collapse or fill with water. Certainly many parts of southern Idaho with its thick layers of basalt resulting from numerous lava flows are inhospitable for moles and may be one of the reasons why they are not present in the south-central and southeastern part of the state. But thick extensive lavas are not as common in the southwest part of the state. Much of the geology of Owyhee County is composed of sands, silts and clays of the Glenns Ferry Formation, the same as found at Hagerman Fossil Beds, yet there are no moles in this part of the state today. Perhaps because this part of the state is a high desert with limited rainfall. While moles may not like soil that is too wet, they can't live in an area that is too dry either, as it may not support enough earthworms and insects to sustain a viable population of moles.
While modern moles are limited in their distribution in Idaho today, in the past they were more widespread. The earliest fossil mole in the state is from Power County. This record is late Miocene or about 8 million years old. This early mole is the same genus, Scapanus, as the living species. Bones of a fossil mole have also been found at Hagerman. It is also the genus Scapanus, but was described as a new species, S. hagermanensis.
One would expect that a burrowing animal like a mole should be common as a fossil since it would be already "buried" and have a better chance of being preserved. But, like its modern relative it is not common and fewer than a dozen bones have been found in the Monument. The youngest fossil mole in the state is also from the Glenns Ferry Formation at Castle Butte in Owyhee County. This specimen is about 2.5 million years old, about a million years younger than the Hagerman specimens. After that there are no records until the living species.
Based on features of the teeth and skeleton, the Hagerman mole is related to the Coast mole and to Townsend's mole, Scapanus townsendi. Like the Coast mole, Townsend's mole prefers moister habitats and today lives to the west of the Cascade Mountains in Oregon and north into Canada. While we can't say for sure, it is not unreasonable to assume that like its modern relatives, the Hagerman mole preferred a moister environment. Studies of the pollen preserved at Hagerman Fossil Beds National Monument show that 3.5 million years ago, the types of plants that were here needed more moisture than the area gets today.
It has been calculated that twice as much rainfall fell in the area then, making it a much wetter, moister environment. Many of the animals found as fossils at Hagerman, such as beaver, muskrat, otter, and waterfowl, are species commonly found in a wetland habitat. Today, the Cascade Mountains to the west trap much of the moisture from the Pacific creating a rain shadow inland.
One of the possible reasons for the wetter environment in Idaho 3.5 million years ago is that the Cascades were not as tall and were not as effective in stopping moisture from the Pacific Ocean from reaching inland to Idaho. As the Cascades pushed upward they became a more effective barrier, resulting in an increasingly more arid environment in southern Idaho. As the environment became drier the different species of animals had to either adapt to the changing conditions, migrate, or go extinct. If the Hagerman mole is the ancestor to either the Coast mole or Townsend's mole then what we may be seeing in the fossil record is simply a reduction in the animal's range in response to this increasing aridity. The moles in Washington County may merely be relicts of a group that at one time was more widely distributed in Idaho.
The fossil record provides paleontologists with many opportunities to study how species respond to environmental change. One response is to expand or contract their ranges depending on whether the changes are more or less favorable for the animal. It is entirely possible that a change in weather conditions could increase the amount of rainfall and moisture in southern Idaho. If that happens, perhaps the moles in Washington County will expand their range into Owyhee County and throughout the southern part of the state (south of the lava fields). Who knows, perhaps sometime in the future more residents of Idaho may have to deal with pesky moles digging in their lawns and gardens.
FURTHER READING Hutchinson, J. H. 1987. Late Pliocene (Blancan) Scapanus (Scapanus) (Talpidae: Mammalia) from the Glenns Ferry Formation of Idaho. PaleoBios 12(45): 7 pp. Tedrow, A.R. 1997. The earliest fossil mole (Insectivora, Mammalia) from Idaho. Tebiwa 26(2):233-239. Yensen, E., D.A. Stephens, and M. Post. 1986. An additional Idaho mole record. The Murrelet 67:96. This article originally appeared in The Fossil Record, Winter 2000
Did You Know?
Lake Bonneville overflowed its rim about 15,000 years ago in southeastern Idaho near the present day city of Pocatello. The Snake River cut down through the ancient flood plain sediments and deepened and widened the Hagerman valley.