From a too-long review of Are We Smart Enough to Know How Smart Animals Are? by Frans de Waal, entered into Scott Siskind's book review contest, I extract this:
Ethology also has some really interesting lessons about how important various practical matters and methodology can be when it comes to what your field can (and can't) produce. For example, it turns out that a surprising amount of useful data about animal cognition comes from experiments with dogs. Is it because dog brains have some interesting physical structures? No, not really that different from a comparably sized mammal. Is it because they are social animals and so have a lot of the same cognitive lego blocks as we do? Maybe a little. The main reason is because they will sit still for an fMRI to be the goodest boy (and to get hot dogs). Turns out sitting still for several minutes in a giant, whirring machine isn't something most animals (including chimps) are that into. So we use dogs.
On the other side of that coin, elephants are clearly very smart, but we've done surprisingly little controlled experiments or close observation with them. Why? At this point, I bet you can guess: because they are huge. They're damn inconvenient to keep in the basement of the biology building, they mess up the trees on alumni drive, and undergrads kept complaining about elephant-patty injuries while playing ultimate on the quad. More seriously, it takes a big, expensive facility to keep captive elephants, and there aren't that many wild habitats. We are missing out on a lot of potentially valuable insights because they are really inconvenient. I don't say this as a moral judgment, but to point out that the same thing might be true elsewhere, and we'd do well to keep an eye out.
As an archaeologist I encounter a related problem all the time: we know a lot about some cultures because they left understandable remains in easy-to-access places, whereas other cultures remain obscure because they didn't use distinctive tools or bury people with grave goods or whatever, or because they lived in a place like the Amazon where not much survives.
Many Americans have a basic knowledge of the artistic tradition of southwestern native peoples, because they painted designs on their pottery that survive and are easily recognizable. But what do you know about native artistic traditions in the northeast? Likely not much, because their pottery was unpainted and the artistic works they invested the most in were made of feathers and porcupine quills.
Our knowledge in every field suffers from gaps like this, and some of the biggest questions in science are about how well what we do know can be extended into the places where we know nothing.