What we call "intelligence" is to some degree down to nurturing and skills development.
Now, here's something interesting. When I was living in the Wicklow mountains I spent many hours wandering the fields and observing the cows and sheep - yes really! In the Spring, when the lambs were about, I observed how curious and inquistive they were and how they would approach and be quite ready to socialize with humans, if given the chance. But their mothers always intervened and prevented them from doing so (for obvious reasons, protecting their young etc). After a month or two, the lambs ceased to approach and ignored the humans, having clearly lost all interest in them and indeed become wary of them.
Another thing I observed was one individual sheep whom Mr Atch and I christened 'the curious sheep' :) Mr Atch and I were sitting by the river bank one day when a sheep approached. They didn't usually come that near so we stayed very still and didn't look directly at it, just watched it out of the corner of our eyes, waiting to see how far it would venture. It stood about 25 feet away from us and gazed out over the river. Then it turned its head, stared at us and literally sidled along the bank, sideways steps, so that it was a bit nearer, We glanced round and it turned its head in an instant, staring out over the river again. This tit for tat, continued until it was about 5 feet away and then when we glanced round at it, it got a bit panicked and ran off. It was very comical! But the interesting thing was that it was clearly curious about us, a quality adult sheep don't normally display. So the potential for intellectual development is present in all sentient beings.
And there were the pheasants. Now they have a death wish. Anybody who lives in the countryside has experienced pheasants running across the road into oncoming traffic. Well, one day we were driving along and up ahead we saw a pheasant hop out of the hedgerow, look right and left, proceed to cross the road, glance round again, saw us approach, decided, 'no, I'm not gonna make it in time' nipped back to the hedge and waited till we'd passed to cross :D
Just to conclude with a human example, one day I was sitting on a Dublin bus, top deck of course :) and a little boy of about four, nearer to four than five, was sitting nearby with his grandfather. The child was asking numerous very intelligent questions to which the only response was "I don't know". For example, "Grandad, why don't we have seatbelts on the bus?" "Grandad what are those birds over there, what are they called?" After a while that child will simply stop asking questions and after another while will stop wondering about things.............if a child asks you about a bird and you don't know what it is, then say "I don't know what that one is called. We must see if we can find out. We could have a look on the internet and see if we can find a picture of one that looks like it or maybe we could go to the library and get a book about birds". And then actually DO it. Never tell a child 'we'll do such-and-such' if you don't really intend to. If you've never wondered why we don't have seatbelts on the bus, then say "Goodness me, that's a very interesting question. I never thought about it before. What do you think yourself, do you have any ideas about it?" and then have a discussion. That's how you develop whatever intelligence the child has and you can expand your own mind in the process.