We have a large number of chipmunks, probably mostly Eastern Chipmunk, Tamias striatus, in our yard in Massachusetts. There are different sizes and a large variation in markings. The behavior appears similar, so the gender isn't obvious.
They all have names, including Long-tail Chip; Short-tail Chip; Chubby Chip; Skinny Chip; Dark-striped Chip; Light-striped Chip; Pointy-head Chip; Speedy Chip; Lazy Chip; Silly Chip; Brave Chip; Shy Chip. They're all Smart Chip, because they're brilliantly adept at getting into bird feeders and tall pots. They can also reach, and eat their way though, plastic bags of food and nuts which are 6 feet up on shelves in the garage.
Without picking them up, is there a way to tell a male from a female?
Update as of April, 2017: Four weeks ago a very pregnant chipmunk popped up out of a hole in the snow. We memorized her markings to the best of our ability. We saw her again a few days later. Since then, we've been feeding her near that hole every day. She dutifully gathers up the seeds and corn and pops back down. Since the babies stay in the nesting burrow for six to eight weeks, she'll probably keep feeding them until then. In the meantime, many other chipmunks are now very active in the yard. There's a wide variety of sizes, shapes and colors, and there are holes everywhere. The only way we know about the female is that we saw her when she was pregnant.