Oaks and walnuts were common on the slopes and ridges south of Pinal de Amoles, and there were squirrels. They struck me as looking and behaving exactly like North America's Gray Squirrels except that they looked as if they'd been crouching in orangish French dressing; their bellies and the backs of their legs were bright, rusty-orange. The squirrels were Red-bellied Squirrels, SCIURUS AUREOGASTER, which you can see (with a black color morph of the same species) at http://www.greglasley.net/redbelliedsquirrel.html.
Red-bellied Squirrels are distributed from northeastern Mexico to northern Guatemala and have been introduced into Florida. As far as I can tell, other than their French dressing, the only big difference between them and northern Gray Squirrels is that they bear litters of one to two young anytime during the year, while birthing among northern Gray Squirrel is seasonally synchronized.