Objection 1. It would seem that consent is to be found in irrational animals. For consent implies a determination of the appetite to one thing. But the appetite of irrational animals is determinate to one thing. Therefore consent is to be found in irrational animals.
Objection 2. Further, if you remove what is first, you remove what follows. But consent precedes the accomplished act. If therefore there were no consent in irrational animals, there would be no act accomplished; which is clearly false.
Objection 3. Further, men are sometimes said to consent to do something, through some passion; desire, for instance, or anger. But irrational animals act through passion. Therefore they consent.
On the contrary, Damascene says (De Fide Orth. ii, 22) that "after judging, man approves and embraces the judgment of his counselling, and this is called the sentence," i.e. consent. But counsel is not in irrational animals. Therefore neither is consent.
I answer that, Consent, properly speaking, is not in irrational animals. The reason of this is that consent implies an application of the appetitive movement to something as to be done. Now to apply the appetitive movement to the doing of something, belongs to the subject in whose power it is to move the appetite: thus to touch a stone is an action suitable to a stick, but to apply the stick so that it touch the stone, belongs to one who has the power of moving the stick. But irrational animals have not the command of the appetitive movement; for this is in them through natural instinct. Hence in the irrational animal, there is indeed the movement of the appetite, but it does not apply that movement to some particular thing. And hence it is that the irrational animal is not properly said to consent: this is proper to the rational nature, which has the command of the appetitive movement, and is able to apply or not to apply it to this or that thing.
Reply to Objection 1. In irrational animals the determination of the appetite to a particular thing is merely passive: whereas consent implies a determination of the appetite, which is active rather than merely passive.
Reply to Objection 2. If the first be removed, then what follows is removed, provided that, properly speaking, it follow from that only. But if something can follow from several things, it is not removed by the fact that one of them is removed; thus if hardening is the effect of heat and of cold (since bricks are hardened by the fire, and frozen water is hardened by the cold), then by removing heat it does not follow that there is no hardening. Now the accomplishment of an act follows not only from consent, but also from the impulse of the appetite, such as is found in irrational animals.
Reply to Objection 3. The man who acts through passion is able not to follow the passion: whereas irrational animals have not that power. Hence the comparison fails.
Source: Summa Theologica (New Advent)