Recently in my participation at Vegan Freak Forums, the topic of first-order and higher-order desires and how non-humans process information has come up. This is a very complicated topic, and has sparked a lot of discussion. I figured I would post one of my lengthier thoughts on this question here:
What happens in both human and non-human minds is generally a matter of mystery. In some respects, I think this question devolves down into "we'll never know for sure!" when we can make some deductions.
For example, let's say you and I have both have to take the same train. We both leave the same house at the same time. We arrive at pretty much the same time. What can be deduced from that (if we have no other evidence available to us) is that we probably took the same route. So, in the absence of other evidence, lots of thinkers have assumed that animals can speak and reason. Hume says and much, and he's far from being alone in the philosophical community for a very long time. But given what we know about human and non-human anatomy (that speech is probably an unfortunate peculiarity of our biology rather than some profound heavenly gift) and from neurological study, we know that it's probably more complicated.
We know this stuff in part because over the mid- to late-nineteenth century, however, there's a shift in the cataloguing of animals species to a shift in understanding them as commodity producers. You actually see the exact same discourses of eugenics applied to humans (with phrenology and all that bullshit) being applied to non-humans. During this period, a wealth of 'knowledge' about non-humans is produced. And this production of knowledge continues unabated over the twentieth C. Obviously, there's a certain amount of bias to this knowledge, but that doesn't mean it's all bad, and there are actually multiple perspectives on this knowledge, what it means, and how to gather it.
What behaviourists, for example, claim is that it's all just stimulus in, response out, and that there is no substantial evidence that any animals, including humans, really reason and act on it (or, more precisely, that 'reasoning' is a conditioned response to stimuli that mimics autonomy). In that sense, humans are like any other animal. The negative effect of behaviourism in the public mind has been to render us all basically automatons. Behaviourism as a popular culture phenomenon is very much like Cartesianism in that sense.
In contrast, ethology takes a more involved approach to answering the question: why do animals do X? by looking at variations across individual animals of the same species (so, why does Fred the Rooster do X when Bob the Rooster does Y) and variations across individual animals (why does Fred do X in response to stimuli A one day but do Y in response to the same stimuli the next day). Ethologists are more like detectives in this regard. We can't really know what's really happening in the mind, but we can gather clues.
Getting back to the train analogy, if we want to understand as much as we can the routes you and I take, if there's more evidence to be gathered that can be gathered noninvasively with respect to you and I, then certainly we should do our best to gather it. Invasive study of you and I, and any non human animals, I take to be a violation of basic rights, and as an abolitionist, I'm opposed to that. But it's possible to study humans and nonhumans in ways that don't use them as property. In any case, what happens in the brain, we do know to a certain degree because of neurological studies on humans and non-humans animals, but what's happening in terms of self-consciousness is still relatively mysterious.
Think of it like a computer. It does stuff in response to actions taken by the user interface (I click on something) and there are things that happen that don't have anything to do with the user interface at all (all that stuff that has to happen with the fan, booting up, for example), and then there are things that we do with the interface that involve a lot of stuff that has to happen that are unseen in order to produce a visible change (for example, starting up a Web broswer). Most often, when something happens in the user interface, it usually sets in motion a whole series of things that happen under the hood. Brains probably work the same way.
I believe (rightly or wrongly) that humans brains have a user interface to varying degrees about varying things, and that we're self-conscious and self-reflecting about that user-interface. That is, we're able to think of it as a user-interface and we take steps to improve it (learning) or unimprove it (watching television). We think, and therefore, we walk, eat, sleep, bone, slack, etc.
The debate (as I understand it) with respect to non humans is largely whether or not their brains have a user interface, or if it's all just stuff that happens in the background, or if some of it happens in the foreground, do they debate their choices, or do they simply make them from a series of propositions of what's possible. For example, do animals experience their foregrounds like watching TV -- a series of visual and aural stimulations to which they can respond, but they're not really directing self-consciously what's happening, or do they experience it more like we do, like interacting in real life, where we reflect and decide what to do in response to that stimuli. One good example of how it's probably more like TV is how poorly human beings can control their own thoughts. Meditation is entirely devoted to clearing and focusing the mind, and many people when they start it are actually quite bad at doing what seems like a relatively simple task.
Which brings us to choice: do they just respond to stimulus without even the TV-level of understanding, or do they smell cat food A, which provides the proposition of food, and smell cat food B, which provides the proposition of better food and exercise some capability for picking. Or, do they smell both cat foods, think about it, consider it as an internal debate, and go for cat food B. I think most nonhumans can do the first, but it's not clear to what degree they can do the second (dolphins, chimps, etc., being possible exceptions). So, most non humans (particularly mammals) almost certainly have first order desires, and brains that are cognitively capable of processing varying propositions about those desires, but higher order desires, maybe in some cases, but probably not in most cases.
What's a higher order desire? It's a desire that's more than just an immediate want that requires that we reflect thoughtfully on X and make a determination based on 'the future' or 'morality' or other abstract concepts. Veganism is a higher order desire. Eating a bucket of Tempation is mostly a first-order desire (unless you're trying to gain weight, in which case, it could be a higher order desire). Do non-humans have more complicated desires in the sense that they can debate about whether to do X or Y internally and choose Y because it affords them a better future? Probably not. For example, my cat probably could determine to eat food A rather than B because one smelled better than the other. He almost certainly couldn't debate internally whether to eat food B because it smelled better but then determine to eat food A because he wanted to loose weight.
I'm not saying there aren't non-human animals that could make a similar, although probably much similar kind of determination -- I'm just saying that for the most part, it seems clear that most non-humans couldn't stand in line at Wendy's for 20 minutes determining whether they want the baked potato or fries. Why nature would punish humans with this kind of 'gift', I seriously don't know, but most non-humans seem to do perfectly well without it.
How do we know they probably don't have internal debates? Well, we don't know it absolutely, but we have some reliable clues. Communication is obviously one good example that it's more than just stuff happening in the background (that animals have at least TV-about-the-world). That they play is another good example. That they seem to mourn the loss of others is yet another. But it's possible that these can all be explained by a number of cognitive capabilities that don't involve reasoning.
Language use, however, seems to indicate that not only is stuff happening in the foreground, but that someone is able to think about what's happening in the foreground as more than just TV. That some nonhumans can understand and respond to signs suggests that at some level there's some self-consciousness, some internal 'discussion' (whether or not they have interior monologues like ours). Tool usage is another good example that there's more than just TV happening. Tools require some basic mastery of the principles of cause and effect (which actually requires quite a bit of brain matter -- again, humans afford a wealth of examples as to how poor they are at understanding cause and affect).
So, how do we make use of this information? Getting back to the train, let's say you, I and my cat all have to take the same train. The behaviourist would be content to say that what happens in my mind, your mind, and my cat's mind is probably very similar. The biology is similar. The simplest explanation is the best explanation. So, we'd all get to the train the same way. Humans just fool themselves into believing they have higher order desires, but it's all basically just first order desires and we pretend it's otherwise. As an approach, behaviourism is largely content to assume similarities between humans and non-humans. When we consider how poorly most adult humans in educated societies reason, and how frequently they do unreasonable things in order to pursue first-order desires, I think there's some strong evidence for this prospect.
But behaviourism does a very poor job of explaining anomalous behaviour in both human and non-human animals as individuals and as species. Why do elephants mourn their dead? Why does my cat like to play? Why do some members of the same species engage in self-sacrifice and others not?
The cognitive ethologist, in contrast, wants to watch us more carefully to see what we did, if there were minor discrepancies in the time required, if one of us was more tired and sweaty upon arrival -- etc. and if the same results were repeated the following day, how other humans might get to the train station, etc. From that, they try to make some determination as to what's happening in the brain. By its nature, cognitive ethology is a more precise approach -- an approach that focuses on assuming and studying the differences and similarities between behaviours -- when it comes to getting at what's really happening when we go to the train station, which is part of the reason it's coming to the fore while behaviourism is falling by the way-side.
Which is the very long way of saying we can understand non-humans best by not assuming that they are naturally similar to us or naturally different from us, but by carefully studying the similarities and differences as thoughtfully as possible.