Objection 1. It would seem that irrational animals are able to choose. For choice "is the desire of certain things on account of an end," as stated in Ethic. iii, 2,3. But irrational animals desire something on account of an end: since they act for an end, and from desire. Therefore choice is in irrational animals.
Objection 2. Further, the very word "electio" [choice] seems to signify the taking of something in preference to others. But irrational animals take something in preference to others: thus we can easily see for ourselves that a sheep will eat one grass and refuse another. Therefore choice is in irrational animals.
Objection 3. Further, according to Ethic. vi, 12, "it is from prudence that a man makes a good choice of means." But prudence is found in irrational animals: hence it is said in the beginning of Metaph. i, 1 that "those animals which, like bees, cannot hear sounds, are prudent by instinct." We see this plainly, in wonderful cases of sagacity manifested in the works of various animals, such as bees, spiders, and dogs. For a hound in following a stag, on coming to a crossroad, tries by scent whether the stag has passed by the first or the second road: and if he find that the stag has not passed there, being thus assured, takes to the third road without trying the scent; as though he were reasoning by way of exclusion, arguing that the stag must have passed by this way, since he did not pass by the others, and there is no other road. Therefore it seems that irrational animals are able to choose.
On the contrary, Gregory of Nyssa [Nemesius, De Nat. Hom. xxxiii.] says that "children and irrational animals act willingly but not from choice." Therefore choice is not in irrational animals.
I answer that, Since choice is the taking of one thing in preference to another it must of necessity be in respect of several things that can be chosen. Consequently in those things which are altogether determinate to one there is no place for choice. Now the difference between the sensitive appetite and the will is that, as stated above (1, 2, ad 3), the sensitive appetite is determinate to one particular thing, according to the order of nature; whereas the will, although determinate to one thing in general, viz. the good, according to the order of nature, is nevertheless indeterminate in respect of particular goods. Consequently choice belongs properly to the will, and not to the sensitive appetite which is all that irrational animals have. Wherefore irrational animals are not competent to choose.
Reply to Objection 1. Not every desire of one thing on account of an end is called choice: there must be a certain discrimination of one thing from another. And this cannot be except when the appetite can be moved to several things.
Reply to Objection 2. An irrational animal takes one thing in preference to another, because its appetite is naturally determinate to that thing. Wherefore as soon as an animal, whether by its sense or by its imagination, is offered something to which its appetite is naturally inclined, it is moved to that alone, without making any choice. Just as fire is moved upwards and not downwards, without its making any choice.
Reply to Objection 3. As stated in Phys. iii, 3 "movement is the act of the movable, caused by a mover." Wherefore the power of the mover appears in the movement of that which it moves. Accordingly, in all things moved by reason, the order of reason which moves them is evident, although the things themselves are without reason: for an arrow through the motion of the archer goes straight towards the target, as though it were endowed with reason to direct its course. The same may be seen in the movements of clocks and all engines put together by the art of man. Now as artificial things are in comparison to human art, so are all natural things in comparison to the Divine art. And accordingly order is to be seen in things moved by nature, just as in things moved by reason, as is stated in Phys. ii. And thus it is that in the works of irrational animals we notice certain marks of sagacity, in so far as they have a natural inclination to set about their actions in a most orderly manner through being ordained by the Supreme art. For which reason, too, certain animals are called prudent or sagacious; and not because they reason or exercise any choice about things. This is clear from the fact that all that share in one nature, invariably act in the same way.
Source: Summa Theologica (New Advent)