by Greg Mayer
A trip to San Diego last month gave me the opportunity to check out the squirrel situation there. The native vegetation is coastal sage scrub and chapparal, with oaks and pines in higher and wetter areas, and desert to the south and west—very different from the deciduous woodlands of eastern North America which so many squirrels call home. The vegetation of San Diego has been greatly modified by man, however, with introduced species now dominating the cityscape– Eucalyptus (from Australia) and exotic palms (from everywhere) are omnipresent.
On the second day of our visit, we hit squirrel pay dirt in Balboa Park, a large urban park with little or nothing of its natural vegetation remaining. Appropriately enough, the squirrel was an introduced species, the fox squirrel, Sciurus niger. They were first recorded in San Diego in 1929.
This pair was just outside the park’s visitors center. In the picture below, you can see the underside of the tail clearly. In Wisconsin, the shadings of the tail are the most reliable way to tell gray squirrels (Sciurus carolinensis) from fox squirrels– gray squirrels have white-tipped tail hairs, giving a ‘halo’ effect. Fox squirrels are very variable geographically, however, and this character may not work in all places, and I’m not sure where the San Diegan fox squirrels came from. (In a post here at WEIT a few years ago, I identified some squirrels from Texas as gray squirrels, but, given fox squirrels’ variability, I am now unsure of their identification.)
A third fox squirrel, also outside the visitors center, sat in a classic squirrel pose.
On another visit to Balboa Park a couple of days later, I was able to get up close and personal with this guy, and needed to use flash, hence the red-eye.
The eastern gray squirrel (Sciurus carolinensis) was also introduced to Balboa Park, in 1924, but has apparently disappeared. During an 8-day stay I saw fox squirrels only in Balboa Park, not in other parts of the city. The native western gray squirrel, Sciurus griseus, is found only in the mountains in San Diego County.
Cities in the eastern half of the United States often have tree squirrels as a prominent element of their urban fauna. In much of the northeastern quadrant of the country, the “city squirrel” is the eastern gray squirrel, Sciurus carolinensis. From personal observation, this species is the city squirrel of Boston, New York, Washington, DC, Chicago, Milwaukee, and Madison, WI. As you head to the south and west, however, another species of tree squirrel, the somewhat larger fox squirrel, Sciurus niger, becomes a possibility. It is the city squirrel of Springfield, IL, and, quite exceptional for being so far to the north and east, Ann Arbor, MI. (On one of my first visits to that city, I remarked to someone that the city had the biggest gray squirrels I’d ever seen, and they replied, yeah, that’s because they were fox squirrels!) Both of these eastern species have been widely introduced elsewhere, the gray squirrel even overseas, including Great Britain and Ireland.
To see native squirrels, we had to go to San Ysidro, the southernmost part of the city of San Diego, on the Mexican border. As we walked to the border crossing there, we saw, on the slope to the east of the pedestrian approach, a modest colony of California ground squirrels, Spermophilus beecheyi. The white patch on the shoulder, extending somewhat in a line on the flank, is diagnostic; and in the out-of-focus individual in the background you can, paradoxically, see more clearly the dark area from the head onto the nape that separates the two shoulder patches. Note that someone seems to have thrown them a peanut (chewed on shell at left).
Ground squirrel tails tend to be less bushy than tree squirrel tails, but this species’ tail is fairly bushy. From the angle in the picture below, the squirrel even looks a fair amount like an eastern tree squirrel, since you can’t see its shoulder and nape.
In the next picture, we see a broader view of the colony, with scant, scrubby vegetation, including cactus; there were some scelrophyllous shrubs, as well. Bonus question: How many squirrels can you spot?
The ground squirrels also occurred just over the border in Tijuana.
There are many species of ground squirrels in western North America, but none in the east. (The easternmost is the thirteen-lined ground squirrel, which reaches the Midwest.) Many of these ground squirrels have more geographically or habitat restricted distributions. The white-tailed antelope squirrel, Ammospermophilus leucurus, is a desert species, which in San Diego County is found only in the eastern part of the county. The one below is in an exhibit hall, Coast to Cactus, at the San Diego Natural History Museum, devoted to the various biotic communities of southern California. It is a model, not an actual specimen.
And, to enlarge this account to include all of my encounters with incisor-enhanced critters in San Diego, here’s a Desert cottontail, (Sylvilagus audubonii), also in Balboa Park. Despite the common name, this is a common rabbit in a broad range of habitats, including urban-suburban ones, in San Diego.
The San Diego Natural History Museum has been a leader in providing online access to its publications, collections, and exhibits, and for mammals there are a number of resources, and I’d point in particular to the various guides listed here, their checklist of the mammals of San Diego, and Suzanne Bond’s annotated list. They have recently published a truly excellent atlas of the mammal of San Diego, which I was able to consult while in San Diego, but not while writing this post, so if you’re interested in the exact details of the introductions and distributions of the species mentioned, you should consult the Atlas.
Bond, S.I. 1977 An annotated list of the mammals of San Diego County, California. Transactions of the San Diego Society of Natural History 18(14):229-248. pdf
Reid, F.A. 2006. A Field Guide to Mammals of North America. 4th ed. Houghton Mifflin, Boston. (The Peterson series mammal guide, and an excellent one; by browsing it, you can get a good overview of mammal distributions in the U.S. and Canada.)
Tremor, S., D. Stokes, W. Spencer, J. Diffendorfer, H. Thomas, S. Chivers, and P. Unitt. 2017. San Diego County Mammal Atlas. San Diego Natural History Museum, San Diego. Museum Store