Animals and humans are fundamentally different. This is what makes us superior beings and has led to our domination of the earth and our complex ability to communicate, to take one step back in order to take two steps forward, to endure suffering now to ensure a brighter future, to fight for the greater good, protest, have morals, rights and a social contract.
All this stems from the fact that our emotions and rationale, which from now on I will refer to as the higher mind, can override our most basic, primitive instincts, which I will refer to as the lower mind. The majority of animals do not have a higher mind, it takes them long periods of time to learn from mistakes as these mistakes must be embedded deeply into the lower mind in order to change behaviour.
Some animals do have a higher mind - most notably primates who appear to be capable of rational and emotional thought, however there is one significance difference between us and them. The best illustration of this is suicide. Animals do not commit suicide, their basic instinct to survive overrides their emotion and rationale to dispose of themselves (that is, if an animal ever had these thoughts). Normally, humans have the same condition, the addiction to life, the need to live for as long as possible. However certain individuals choose to end their own lives - an example of the higher mind overriding the lower mind.
Humans are the only species in which we see this, this is the reason why we have dominated the earth in such a short period of time. How? It allows us to develop a caring attitude towards other humans. Let us take the example of the Universal Declaration of Human Rights, this gives us the "right to life": "Article 3. Everyone has the right to life, liberty and security of person." What this means essentially, is that it is morally wrong to destroy another's life, limit their liberty (except where that liberty infringes on others' human rights), or breach their personal security.
This requires us to act against natural instincts. Everyone has the immediate instinct of revenge, to harm those who have caused harm to you; why then do we not see hundreds of revenge killings and assaults every single day? It is because people realise that in order to have their rights upheld (not being sent to jail), they must respect the other's rights in an act of reciprocation.
A hippo however, will kill anything which gets between it and its watering hole - it's natural instinct to ensure its own survival (us humans don't go killing everyone in front of us in the queue at McDonald's though, and this is the crucial difference).
The simple fact that an animal is unable to reciprocate the respect some people claim we should show them, and therefore we need not give them rights. Why then do we protect animals from abuse? Well that is a different case altogether - we punish sadism, that is, unnecessary violence or infliction of pain on any being for pleasure. Sadism is a mental disorder and therefore the authorities treat it as such by offering psychological support to the individual whilst segregating them from society, this process being otherwise known as prison or mental sectioning.
The balance between the amount our lives are controlled by our higher and lower minds varies from person to person, as certain actions/habits are often transferred to the lower mind from the higher mind. This is what doctors may call a "muscle memory", carrying out an action repeatedly means will eventually do it "without thinking", in other words, it will just be an instinct (e.g. touch typing).
However, this is often harmful - smoking, drugs and gambling are prime examples. Commonly called addiction, it turns harmful practices into primitive needs of the lower mind. Obviously this causes a whole series of related problems which are only found in humans, the only things an animal is "addicted" to are food, water, shelter and reproduction (isn't it funny how we humans have a separate name for sex addiction "nymphomania", when really it is a basic instinct common to all living organisms).
In the same we do not grant plants rights, we should not grant rights to animals - they are unable to reciprocate any liberties we may give them and as such we are in no way obliged to afford them. It is far more beneficial to humankind to test on and consume animals - we must protect our own survival.
I'll agree that we have no reason to believe that animals attribute rights to each other, but this is a poor justification for the claim that they don't deserve rights. The problem is that you've given no clear reason as to why a being can only be entitled to rights if they are able to afford them to others. A human baby cannot grasp the concept of rights in order to attribute them to another person. If this is the feature that is crucial in deciding whether a being has rights then it seems that you would have to argue that we shouldn't give babies any rights either.
Of course, this is absurd, and, accordingly, it becomes clear that your requirement that a being must have an understanding of the 'social contract' in order to have rights is also absurd. The key issue is that babies, along with almost all animals, have pain receptors. I'd argue that most species also have some kind of primative emotion. Take fear for example. Unless you want to argue that animals have no consciousness at all, it is clear from their behaviour that virtually all animals are capable of distress. As rational, emotional creatures it makes sense that human beings extend our care for other people to animals as well.
Your comparison with plants misses the point completely. Plants don't feel pain and cannot experience fear. Animals, however, can. You say that what sets us apart from animals is our ability to be rational and compassionate, rather than merely selfish. How, then, can you justify your claim that it is acceptable for humans to test on and comsume animals for the reason that it is "far more beneficial to humankind"? This is, I believe, an example of somebody putting their selfish instinct to further the survival of their species before their 'higher' motivation to be compassionate.
In summary, I think that you have failed to grasp one of the most important factors that should determine whether a being should be given rights: whether or not it would consciously suffer if we deny it these rights. That is to say, if an animal is aware of its suffering then it is wrong to knowingly cause pain to that creature.
The real issue here is whether or not that wrong should be treated as more or less significant than the wrong of failing to find treatments for serious medical conditions. Answer that and perhaps you will have given a sufficient justification for your position. Add to that a distinction between different cases, say between medical and cosmetic research, and you might really be getting somewhere.