The ground squirrels are members of the squirrel family of rodents which generally live on
or in the ground, rather than trees. The term is most often used for the medium-sized ground
squirrels, as the larger ones are more commonly known as marmots or prairie dogs, while the
smaller and less bushy-tailed ground squirrels tend to be known as chipmunks. Together, they
make up the "marmot tribe" of squirrels, the Marmotini, and the large and mainly ground
squirrel subfamily Xerinae, and containing six living genera. Well-known members of this
largely Holarctic group are the marmots, including the American groundhog, the chipmunks, the
susliks, and the prairie dogs. They are highly variable in size and habitus, but most are
remarkably able to rise up on their hind legs and stand fully erect comfortably for prolonged
periods. They also tend to be far more gregarious than other squirrels, and many live in colonies
with complex social structures. Most Marmotini are rather short-tailed and large squirrels,
and the Alpine marmot is the largest living member of the Sciuridae, at 53–73 cm in
length and weighing 5–8 kg. The chipmunks of the genus Tamias frequently
spend time in trees. Also closer to typical squirrels in other aspects, they are occasionally
considered a tribe of their own. The ground squirrel is especially renowned
for its tendency to rise up on its hind legs. It does this whenever it senses nearby danger,
or when it must see over tall grasses. The squirrel then curls its paws flat against
its chest and sends a screeching call to warn other family members about the presence of
predators. Ground squirrels may carry fleas that transmit
diseases to humans, and have been destructive in tunneling underneath human habitation
Evolution and systematics
Palaeosciurus from Europe is the oldest known ground squirrel species, and it does not seem
to be particularly close to any of the two to three living lineages of Marmotini. The
oldest fossils are from the Early Oligocene, more than 30 million years ago, but the genus
probably persisted at least until the mid-Miocene, some 15 mya.
It is not clear where the Marmotini originated. The subtribes probably diverged in the early
to mid-Oligocene, as primitive marmots and chipmunks are known from the Late Oligocene
of North America. Unfortunately, the fossil record of the "true" ground squirrels is less
well known, beginning only in the mid-Miocene, when modern susliks and prairie dogs are known
to have inhabited their present-day range already.
Whether the Marmotini dispersed between North America and Eurasia via "island-hopping" across
the Bering Straits or the Greenland region—which both at that time were temperate habitat—and
from which continent they dispersed to which, or if both continents brought forth distinct
subtribes which then spread to the other, is not known and would probably require more
fossil material to be resolved. In any case, that the fairly comprehensive fossil record
of Europe—at the relevant time separated from Asia by the Turgai Sea—lacks ancient
Marmotini except the indeterminate Palaeosciurus might be taken to indicate an East Asian or
western North American origin with trans-Beringia dispersal is the slightly more satisfying
hypothesis of it. This is also supported by the enigmatic Chinese genus Sciurotamias,
which may be the most ancient living lineage of this group, or—if the chipmunks are not
included here—close to the common ancestor of the Tamiini and the Marmotini sensu stricto.
In any case, expansion of the Marmotini to Africa was probably prevented by competitive
exclusion by their close relatives the Protoxerini and Xerini—the native terrestrial and palm
squirrels of that continent—which must have evolved at the same time as the Marmotini
did. Subtribes and genera
Basal and incertae sedis genera Palaeosciurus
Sciurotamias Subtribe Tamiina: chipmunks
Nototamias Tamias
Subtribe Marmotina: marmots Arctomyoides
Miospermophilus Paenemarmota
Palaearctomys Protospermophilus
Marmota Ictidomys: Thirteen-lined ground squirrel
and related species Subtribe Spermophilina: true ground squirrels
Spermophilinus Ammospermophilus
Cynomys Spermophilus
See also Squirrel
Tree squirrel Richardson's ground squirrel
Footnotes
References Helgen, Kristofer M.; Cole, F. Russel; Helgen,
Lauren E.; and Wilson, Don E. "Generic Revision in the Holarctic Ground Squirrel Genus Spermophilus".
Journal of Mammalogy 90: 270–305. doi:10.1644/07-MAMM-A-309.1. Steppan, Scott J.; Storz, B.L. & Hoffmann,
R.S.: "Nuclear DNA phylogeny of the squirrels and the evolution of arboreality from c-myc
and RAG1". Mol. Phyl. Evol. 30(3): 703–719. doi:10.1016/S1055-7903(03)00204-5 PDF fulltext
Thorington, R.W. & Hoffmann, R.S.: Family Sciuridae. In: Mammal Species of the World—A
Taxonomic and Geographic Reference: 754–818. Johns Hopkins University Press, Baltimore.
External links About California Ground Squirrels