Posts Tagged ‘striped skunk dispersal history’

Striped Skunk (Mephitis mephitis) Dispersal During the Pleistocene

May 30, 2017

Most people don’t even think about where and when the various species of wildlife inhabiting their neighborhood originated. No matter how common a particular species may seem, it has not always been there.  The striped skunk is a generalist species that occurs all across the United States, and it is quite common in many regions, especially rural farm country. It is found in forests, fields, wilderness and suburbs.  They are an adaptable species, thanks to their omnivorous diet and unique defense strategy.  Yet, striped skunks have not always existed over their present day range.

The ancestors of all American skunk species came to this continent by crossing the Bering Land Bridge over 5 million years ago.  Paleontologists assign fossils of this ancestral species to the extinct  Martinogale genus.  About 2 million years ago striped skunks in the Mephitis genus diverged from spotted skunks in the Spilogale genus.  There is fossil evidence of early species of Mephitis skunks from the early and mid-Pleistocene in Nebraska, Colorado, and Florida.  However, this early species must have gone extinct over much of its range.  By 300,000 years ago, Mephitis skunks were restricted to what today is northern Mexico and southern Texas.  All present day striped skunks descend from this ancestral population, according to a study of striped skunk genetics.  Scientists studied genetic information from 314 specimens chosen from 20 states and determined striped skunks spread east and west from this population.  Early striped skunks, like their closest living relative–the hooded skunk (M. macroura), were probably well adapted to desert environments but evolved characteristics that helped them survive in woodlands and grasslands.  Over 250,000 years ago, striped skunks crossed the Mississippi River and colonized the entire southeast.  This probably occurred during a glacial stage when the river ran low and numerous sandbars facilitated the crossing.  The lower Mississippi River has served as a barrier, isolating populations of striped skunks ever since.

Geographic distribution striped skunk phylogroups based on 601 base pairs of cytochrome-b gene in mitochondrial DNA. Pie charts indicate the proportional representation of groups in each state. The hypothesized Pleistocene and Holocene dispersal patterns for striped skunk phylogroups are indicated by unique dash marks.

Dispersal of striped skunk population during the Pleistocene based on genetic evidence from 314 specimens taken from 20 states. Map from the below referenced study.

Image result for mephitis macroura

The hooded skunk (Mephitis macroura) is the closest living relative of the striped skunk.  Its range is Mexico and the extreme southwestern U.S.  Genetic evidence suggests this is also the geographic range where striped skunks originated.

Image result for striped skunk

Nice photo showing coat variation within the striped skunk population.  Striped skunks colonized southeastern North America about 300,000 years ago.  A primitive closely related species occupied this region before that.  It’s unclear when this predecessor became extinct.

~200,000 years ago striped skunks advanced up the Rocky Mountains from their southwestern refugium.  This population split into 2 clades on either side of the Great Basin 130,000 years ago.  This western population expanded east and colonized the Midwest.  Following the end of the last Ice Age, southeastern skunks colonized New England and expanded west, coming into contact with western populations in the Midwest.  This has resulted in an admixture of once genetically distinct populations.  The history of this dispersal explains why skunk physical characteristics vary so much. The upper Mississippi River is smaller than the lower part and is not an insurmountable barrier.  Admixtures occur along the upper part of the river.  Genetic studies of raccoons, deer mice, northern short-tailed shrews, 5-lined skinks, and leopard frogs show similar dispersal histories with the Mississippi River acting as a barrier isolating populations from each other.

Reference:

Barton, Heather; and Samantha Wisely

“Phylogeography of Striped Skunk (Mephitis mephitis) in North America: Pleistocene Dispersal and Contemporary Population Structure”

Journal of Mammalogy 93 (1) 2012