I do not mean rationally attuned (e.g. as to a knowledge of God and his laws), but emotionally/spiritually attuned – more likely to feel and follow (and be startled by) the spirit, without knowing why.

You don’t think animals have free will? That surprises me. I would argue that the agency of animals has a much smaller sphere than that of humans because of their limited knowledge and understanding, but I certainly don’t view them as deterministic. I would not subscribe to Mark’s idea that they are more spiritually attuned either. I think the scriptural idea that we needed a knowledge of good and evil to be able to exercise agency is explained quite well by comparison to animals who do not have a well developed sense of morality. This limits their ability to develop morally, which is one of the most important ways for each of us to be developing.

We can have a never ending, roughly exponential increase in capacity, but note that an exponential function is never zero. One cannot create intelligence out of nothing, nor evolve from nothing. You have to start somewhere, and since intelligence is the driving force of creation, we must start with intelligence, and some form of rationality or intentionality (goal directed behavior).

So when I am using the term sentience in this post I am referring to the progression of an eternal intelligence from pre-rationality into rationality — I just used the terms pre-sentience and sentience to describe that. I think the Garden of Eden narrative shows that progression in all of our intelligences.

And that he created man, male and female, after his own image and in his own likeness, created he them;
And gave unto them commandments that they should love and serve him, the only living and true God, and that he should be the only being whom they should worship.
But by the transgression of these holy laws man became sensual and devilish, and became fallen man.
(D&C 20:18-20)

It is worth noting of course that God created his own image, i.e. God created his own (second) nature. Otherwise he is not a creator at all. The transgression of the law is the transgression or corruption of what God intended. e.g. the natural man is an enemy to God and has been since the fall of Adam and ever will be save he yield to the enticings of the Holy Spirit [i.e. the intent of God in his creation]. Compare Ether 3:8-9.