animal interests

I've been planning something rather grand, but it aint gonna happen, besides this is blogging...

How do we know that animals do actually have interests and are not simply sophisticated machines? Insects have primitive nervous systems, they react to stimulus in a way that tries to avoid harm, does this mean they feel pain? Not necessarily, for example if an insect damages a leg it doesn't shield it or favor other legs, it doesn't seem to be aware of the damage at all. Even some plants can release chemicals when they are eaten to 'warn' other plants which then become bitterer. This doesn't mean they it suffers, just that it has evolved mechanisms to help it survive. It isn't difficult to imagine a complicated robot programmed to avoid harm that does not feel pain. Fetuses go through a phase where they react to stimulus (like retracting away from sharp objects) but do not feel pain. When they start feeling pain is the subject of fierce controversy. Mammals show signs of suffering because they do favour injured limbs and distress lingers so long as the injury is a problem. They can also be affected psychologically, take a trip to the SPCA, some dogs are terrified of humans no matter how gentle they are. It's not a wild leap to guess that they were physically abused by humans in the past.

There is a more subtle reason why we can guess that animals (particularly mammals) suffer from pain in similar ways to humans. Their nervous systems are similar to ours. Senses (like touch) are expensive (in terms of food) to maintain and repair. Any gadget a species has must be useful for it to be retained, for example brains are very expensive (our brains make up a small percentage of our mass, but uses about 40% of our food), in the past couple thousand year's domestic animals haven't needed their brains as much and they have predictably shrunk. Animals wouldn't have or maintain complex nervous systems if they didn't use them and since they look and function much like ours, they suffering from pain must be similar.