Here's a paper from the University of Cincinnati on yawning. According to the author:

Most animals with backbones yawn and even some without. Dogs, cats, rats, snakes, fish, birds and humans all yawn. Fetuses at 11 weeks after conception yawn in utero.

She describes most of Lucy's yawns here:

It's a not-always-so-subtle cue to spouses, co-workers, and bosses for attention, sympathy and a respite due to tiredness, stress, over-work or boredom.

And sometimes she's just sleepy.