Next time you see a mole digging in tree-root-filled soil in search of
supper, take a moment to ponder the mammal’s humerus bones. When seen
in the lab, they are nothing like the long upper arm bones of any other
mammal, says Samantha Hopkins, a paleontologist at the University of
Hopkins, a professor of geology in the UO’s Robert D.
Clark Honors College, studies the evolutionary history of burrowers, in
search of why and how they adapted a physique for digging in response
to environmental influences or other forced changes in habitats.
Society of America, Hopkins presented preliminary findings of one line
of her research. Moles and mole rats, she said, are examples of mammals
that have adapted to moving soil in rocky, root-packed soils, in
opposition to most other burrowing mammals that prefer softer, dryer
"It requires a lot of morphological adaptation, a
lot of tradeoffs, to be good at digging," she said. "That’s intuitive
to us as humans who have handled a shovel in the backyard. We know that
it’s really hard work to shift soil. Burrowing mammals acquire a
complex of features that lets them handle whole days moving soil. They
make for a great case for understanding convergent evolution because in
spite of how difficult it is to do this — in spite of all the costs of
doing this — it seems to be worthwhile enough that many mammals have
done it through time."
Convergent evolution is the development
of similar characteristics, necessary for survival, among unrelated
organisms in the same environmental conditions. Hopkins studies living
burrowers as well as the fossil records of such mammals, living and
extinct worldwide, to understand why some choose to live — and dig for
their food and to avoid predators — in harsher regions. Conventional
thinking, she said, is that mammals evolved into burrowers after being
driven into grassland habitats, where going into the soil is their only
option to eat or escape in the absence of trees.
however, some evidence that the morphology and behavior required to dig
in more compact or rocky soils is recognizably different from that used
in softer soils," she said. "This study (still ongoing) aims to
determine whether there is an influence of digging substrate on the
evolution of digging."
Fossils found in the field or in museum
collections allow Hopkins to examine and compare the structure of
burrowing mammals’ shoulders, skulls, legs, feet and claws to get clues
of what features developed and possibly at what points in time these
skeletal changes occurred.
In her talk, Hopkins briefly
described known methods of burrowing, including scratch, head-lift,
incisor and humeral rotation. Humans digging something out of sand with
their hands is an example of scratch digging. The other methods are
difficult or impossible for humans to even mimic, she said.
doctoral student at the University of California, Berkeley, Hopkins
studied the fossil record of the extinct burrowing mammal Ceratogaulus,
the only horned member of the digging group Mylagaulidae. The
gopher-like rodents used the head-lift technique, in which they use the
tips of their snouts, powered by enlarged neck muscles, to drive into
soil. As part of her dissertation, she showed in the Proceedings of the
Royal Society B in 2005 that the horns were used for defense against
predators — not to help with the digging as had been previously
Mylagaulids were once common in much of North America
some 20 million years ago. Existing head-lift diggers include marsupial
moles, blind mole rats and mole voles. More common land-dwelling
burrowers, which use a variety of techniques, are badgers, ground
squirrels, burrowing owls, aardvarks, nutria, kangaroo rats, shrews,
prairie dogs and armadillos.
University of Oregon. October 2007.