@Harp
Though I agree with your post, I must disagree here:
“and we’re often only guessing when we say how intelligent an animal is, based on brain size relative to body size.”
I believe the ethologists deserve more credit for their hard work. There have been extensive studies into for example whether chimpanzees are aware of what other people can see and hear, or whether they can infer what other people are trying to do even if those don’t actually succeed, whether they can be trained to understand numbers like we do, or whether baboons and macaques understand the family relationships between other monkeys.
I wrote a paper including this recently.
If you can be bothered, I’d advise Call & Tomasello (2008: Does the chimpanzee have a theory of mind? 30 years later, Trends in Cognitive Sciences, volume 12, number 5, pp. 187–192) for an overview into the main findings of the research of theory of mind in chimpanzees carried out in the previous thirty years. It’s pretty impressive.
Admittedly most research of intelligence seems to have been into (other) primates, mainly because they’re most interesting, but this should go to show that we do in principle have some advanced tests of intelligence beyond brain to body size ratio.