There isn't a scientifically testable definition of consciousness.
Then perhaps "consciousness" is a useless concept to science.
My simplified definition of consciousness would be 'self aware and capable of thought'. I don't think animals are either of these.
First: self-awareness
A classic set of experiments: An ape is put under anesthesia, then a dot of dye is placed on its forehead (control experiments are done by placing vehicle-without-dye on a set of animals). The ape wakes up and gives no indication of knowing the dot is there. A mirror is placed in front of the ape. The ape immediately reaches up and touches the dot on its own forehead, NOT the image reflected in the mirror. This is self-awareness. The ape realizes that it is looking at itself when presented with a mirror.
Most other animals do not respond the same way (though many other animals may not be capable of this sort of test due to other limitations, even though self-awareness may be present).
Second: capable of thought
I have to ask, do you honestly think that no animal other than humans think?
When an elder chimpanzee in the wild is teaching a young chimp the proper use of hammer and anvil technique to open nuts, is it thinking? When a macaque begins washing its food in salt water, and the practice than proceeds among social lines to the rest of the group, is there thought involved?
Extensive chimpanzee (and other ape) behavior studies have revealed the presence of "culture" - that is, different groups of chimps behave and interact differently, and these behaviors are passed via "nurture" to younger generations. One outstanding example is that a particular group of chimps includes females that make and wear vine necklaces with no practical value. The practice has been maintained within the females of the groups across several generations. Is thought involved with adornment? Furthermore, isn't self-awareness a requirement for adornment practice?
I think the closest we've come is teaching chimps sign language, but linguists say that it isn't actually language when chimps use sign language. Its still just a response to a stimulus, which isn't neccessarily consciousness.
First, much of this line of discussion relies on your definition of language.
However, chimps appear to have their own language separate of human interaction. They use the language not only to relay general emotion, but to organize complex hunting parties.
If a chimp simply responds with sign language in a research context, that might not be considered language. However, chimps that have been taught sign language have taught it to other chimps without human interaction, and chimps use sign language to communicate in the absence of humans. How is this not language?
Seems to me that non-human animals are capable of both self-awareness and thought. Thus non-human animals exhibit "consciousness", (or the definition fails).