Under what basis does the Catholic Church consider animals, in particular great apes, not to be persons?

Under what basis does the Catholic Church consider animals, in particular great apes, not to be persons?

Humans being have the gift of free will given directly by God (CCC 1730), as well as an immortal soul (CCC 1703). Only people have the gift of reason, where animals do not have including great apes.

1730 God created man a rational being, conferring on him the dignity of a person who can initiate and control his own actions. "God willed that man should be 'left in the hand of his own counsel,' so that he might of his own accord seek his Creator and freely attain his full and blessed perfection by cleaving to him.

1703 Endowed with "a spiritual and immortal" soul,5 the human person is "the only creature on earth that God has willed for its own sake." From his conception, he is destined for eternal beatitude.

Language is the proof of intelligence, which even great apes do not possess. They simply have a system of communication, as do most animals, but it is no way based on reason. That distinguishes the human race and animals including great apes, possessing a simple animal soul and not an immortal human soul!

Animals, including great ape, are incapable of adoring God as man can with all his heart, mind and soul!

Personalism, a concept promoted by St. John Paul the Great was (and memorize this one because it will be on the test)

A person is an entity toward which the only reasonable attitude is love

Now, to me that seems like a description of another human being, not a one of the lower animals. You cannot substitute love with lunch.

The "Gloomy Dean", Dean Inge of St. Paul's in London in the early 20th century would have had the same propositions, that because animals can X, we should grant them Y. And he did this mixing Christianity and science and coming up with astonishing platitudes that sound more like pessimistic Hinduism.

The same writer makes one or two other very curious remarks; one of the most astonishing being the following: "The lower animals were not made for our sake. So much science can affirm without hesitation." We can affirm without hesitation that science cannot affirm anything of the sort. Science (when used to mean the study of hard fact and not a mere modernist mystery and hocus-pocus) refuses absolutely to affirm anything whatever about what animals were made for; or even, within its own logical operation, whether they were made at all

THE RELATION OF MAN TO THE ANIMALS, G.K. Chesterton - May 26, 1928

G.K. Chesterton didn't like torturing animals, he preferred vivisecting politicians and multi-millionaires to rabbits and apes, but he couldn't see animals being merciful to one another. And it would take mercy to have the rights held by humans - because apes wouldn't even be expected to hold those rights within their community.

Now, maybe reading Tarzan leaves me in relation to the great apes the same as as a Protestant who has never read Belloc, but I do not think apes or dolphins or army ants are model citizens and I cannot (because I was not) be made to love them the way I love my wife, my neighbor, my kids or even my enemies.

Humans have the gift of free will given directly by God [CCC 1730], as well as a soul [CCC 1703]. Only humans have an immortal soul.

Many higher mammals have the traits you speak of: wolves, most domesticated dogs and cats, dolphins and porpoises, and most primates. Sentience is not a function of humanity.

Scripture specifically makes humans special from the time of creation:

Genesis 1:26 And God said, "Let us make man in our image, after our likeness, and they shall rule over the fish of the sea and the fowl of the heaven, and over the animals and over all the earth and over all the creeping things that creep along the earth. 27: And God created man in his image; in his image he created him; man and woman he created them. 28: And God blessed them, and God said, "Be fruitful and multiply and fill the earth and subdue it, and rule over the fish of the sea and the fowl of the sky and over all the beasts that tread upon the earth.

CCC 1704 goes on to say By free will, [man] is capable of directing himself towards his true good.

In these regards, both scripture and Catholic theology recognize the supremacy of humans over all other species on the planet.

I'm not sure if this is a defense, per se, as you have asked, but God was very specific about the supremacy of humans. Of interest, cultures which are not monotheistic have by and large less empathy towards nonhumans than the Church. If anything, Catholic theology supports animal rights and the relief of suffering as long as it is not to the detriment of humans [CCC 2415-2418].

To sum up, sentience, if present, is not a substitute for humanity. (Of note, the CCC specifically prohibits slavery of persons [CCC 2414].) Your argument that "some modern scientists" believe sentience makes a nonhuman a "person" is not a replacement for scripture or dogma.