What I believe is true but cannot prove

The Edge's World Question Center started the year by asking 'What do you believe is true even though you cannot prove it?'. My interest was piqued by George B. Dyson's response:

During the years I spent kayaking along the coast of British Columbia and Southeast Alaska, I observed that the local raven populations spoke in distinct dialects, corresponding surprisingly closely to the geographic divisions between the indigenous human language groups. Ravens from Kwakiutl, Tsimshian, Haida, or Tlingit territory sounded different, especially in their characteristic "tok" and "tlik."

I believe this correspondence between human language and raven language is more than coincidence, though this would be difficult to prove.

This answer relates to my own belief that animals are less different from humans than we tend to allow, in terms of both intelligence and culture. This conviction was inspired by one of my favourite books, Frans de Waal's The Ape and the Sushi Master, and 2004 was a particularly good year for studies challenging some of the perceived barriers between animals and people.

First, in May, there was a report that ravens follow the human gaze and likely have a theory of mind. Then, in July, we discovered that yawning is contagious for chimps, suggesting that they are able to understand their own and others' state of mind. And, finally, in December there was evidence of Capuchins using tools to dig up tubers:

Several species are known to use 'tools', such as the birds of prey that dash their hard-shelled prey on to rocks to crack them open. But the latest case of tool use differs from many of these examples because it may be based on an understanding of cause and effect.

Were there any other examples that I missed?