> In the latter case, it would have been fairly easy to reason that the interests of the human (seeing a monkey in a zoo) are in no way vital and cannot establish a right

That's a rather poor argument because it is non-specific. By that argument I can not own a potato either because there is no vital need for me to do so.

There is a vital need for you to own a potato though. Humans need to eat plant food for sustenance, so we have moral reason to accept that at least some plants are allowed to be owned. Given that, there is nothing significant to recommend an alternative plant over a potato, so there should be no moral objection to owning a potato.

The point of framing the argument in the terms that konstruktor has, is to change the debate from, "Are apes sufficiently like humans to justify giving them rights?" to "Are apes sufficiently different from non-sentient entities to justify treating them differently." And I think this is a much more persuasive argument -- I think humanity is less willing accept freely that apes are equivalent to humans on some level, than they are to say that the act of viewing an ape for enjoyment does not justify locking up something that is clearly more intelligent and self-aware than your average apricot.

Applying your logic there is a vital need for us to imprison animals for the same reasons (sustenance, as well as utility). We've already made the utility-based decision to cage, raise and domesticate animals.

You could argue that caging an orangutan in particular is not important, however you could just as easily argue that the world might manage if we all stopped growing potatoes.

I see what you are saying, but the same logic implies that there is no vital need to imprison animals, considering that the vast majority of people can get their sustenance from plant based products more efficiently.

However, I would personally argue that a rights based system only works when you have a way to have rights clash and reach compromise positions. An open ended right is not very useful because just about every right becomes impossible to enforce or provide at the extremes. When you provide certain agreed upon rights to both humans and orangutans you can have meaningful discussions about where those rights intersect and how those confluences should be handled, and in this way you avoid the absurd implications of absolute rights.

The vast majority of humanity still depends on domesticated animals for labor and food. Goats are very commonly used to convert inedible wild plants into human ingestible calories in impoverished regions who do not have modern factory farming infrastructure.

Certainly we can find other philosophical approaches which are more reasonable.

I wonder how this reasoning applies to human foetuses ... If the argument that killing is a necessity for free-er sex is judged to be moral, then how could you possibly judge against someone killing for survival ? Hell, how could you possible hold killing for fun against someone ?

(not taking a position, just pointing out the massive inconsistency)

This is an ape, not a monkey.

This probably differs a bit between languages. In Dutch, we call both monkeys and apes "aap", and apes specifically "mensaap" (human-ape).

Relevance? I am sure Dutch zoologists would similarly agree that use of the term 'monkey' is just plain wrong in this instance.

Cladistically, apes are monkeys.

I think you mean primates?

No. Apes (including humans) are more closely related to old world monkeys than new world monkeys are. When you are more closely related to monkeys than other monkeys are, most people would say you are a monkey. Including me.