(I've also been of the opinion, taking Dan Dennett's stance on consciousnesss that there's no "Cartesian meaner" to its logical conclusion — as does Daniel Wegner and others, I'm not alone — that there's no conscious, central, controlled location for free will in humans, as well. That is, there's no "Cartesian free willer" either. But, I digress.)
There's a German-based neuroscientist who agrees with me on the "polarities" angle. But, that's not all.
Bjoern Brembs also says that this free will — free will within constraints — is exhibited by animals, too.
Brembs and others have used mathematical models to simulate brain activity on a computer, finding that what worked best was a combination of deterministic behaviour and what is known as stochastic behaviour - which may look random but actually, in time, follows a defined set of probabilities.
Personally, I actually don't see this as that big of a deal. Given that consciousness itself is understood as being on some sort of continuum, rather than "we conscious humans" vs. "you unconscious animals," how could a will, and a will that is partially free, also not exist, and again, on a continuum?
That then said, I do find it a bit more questionable to extend some degree of free will, as Brembs does, all the way down to the level of flies, just as I would find it questionable to attribute consciousness to an animal with so few neurons.
To talk about a dog, or the old laboratory vertebrate standby, the lab rat, as having some degree or type of free will? Yes. The laboratory insect standby, a fruit fly? Per Carl Sagan, that's an extraordinary claim. I expect more evidence.
I'll stand by for more research; this is surely going to be a hot topic not just for months but for years.