Okay, I admit, last week's entry for Taxon of the Week was a little, well, weak. Basically I didn't feel like writing on monkeys at the time. This week's entry is much better - squirrels. Specifically, the mostly North American ground squirrels - Marmotini (see here for some pictures and an article).
[As an aside, I saw Bridge to Terabithia with Jack this Saturday just been, which presented me with a bit of a biogeographic quandary. The movie looked very much like it had been filmed in New Zealand - I recognised the vegetation. Though the intended setting was not specified, I'm guessing it was somewhere in the American Midwest. A couple of scenes featured squirrels, obviously filmed in America (New Zealand animal importation laws expressly forbid importing squirrels under any circumstances). However, in one scene where the main character traps and releases an animal that has been attacking his family's greenhouse, the animal in question is a brushtailed possum - not, to my knowledge, present in North America, but very much present in New Zealand. Sometimes, I have to admit, I wish I didn't realise these things so I could just ignore it and enjoy the movie.]
The Marmotini are one of the few clades of squirrels to make it to North America, along with the flying squirrel genus Glaucomys and the genera Tamiasciurus and Sciurus*. There are five genera - the more arboreal Tamias (chipmunks) and the terrestrial Ammospermophilus, Spermophilus, Marmota (marmots) and Cynomys (prairie dogs) - though some authors divide up some of these genera, notably Tamias and Spermophilus (which is probably paraphyletic). Tamias, Spermophilus and Marmota include Eurasian members, while Ammospermophilus and Cynomys are solely North American. I haven't been able to find what are the specific morphological characters uniting this clade (reading between the lines in Callahan & Davis (1982), I suspect they're genitalic characters), but it is also recovered with molecular phylogenies (Mercer & Roth, 2003). All Marmotini hibernate during winter to some extent (A. Watts).
*There is another genus present in North America, Microsciurus, but Mercer & Roth (2003) find it to be nested within Sciurus.
Perhaps the most interesting features I've come across in a quick scan of the Marmotini are their reproductive patterns and their sociality. Marmotini produce large litters of relatively small cubs or pups or whatever a baby squirrel is called, and apparently have the smallest size at weaning relative to adult size (see here).
Marmota and Cynomys both contain species that live socially. All species of Cynomys are social and form larger colonies than Marmota, of which there are some non-social species (in line with the common name of 'prairie dog', the name Cynomys translates as 'dog-mouse'). These two genera form a clade relative to the other Marmotini, but it is debatable whether sociality has appeared multiple times within the clade or whether it has appeared once and then been lost in the non-social marmots - the nested position of the solitary Marmota monax within marmots suggests to me that the latter may be more likely (Cardini et al., 2005).
Callahan, J. R., & R. Davis. 1982. Reproductive tract and evolutionary relationships of the Chinese rock squirrel, Sciurotamias davidianus. Journal of Mammalogy 63 (1): 42-47.
Cardini, A., R. S. Hoffmann & R. W. Thorington Jr. 2005. Morphological evolution in marmots (Rodentia, Sciuridae): size and shape of the dorsal and lateral surfaces of the cranium. Journal of Zoological Systematics and Evolutionary Research 43 (3): 258-268.
Mercer, J. M., & V. L. Roth. 2003. The effects of Cenozoic global change on squirrel phylogeny. Science 299: 1568-1572.