Five years ago I wrote a post about chimp infanticide and added "Another interesting aspect is that chimp infanticide appears somewhat harder to explain with simple evolutionary principles than is the case with monkeys. That fits my own little theory that behaviour of smart animals is hard to predict...."
With my usual, blisteringly fast analytical skill, I've come up with another theory after five years. Chimp behavior isn't necessarily harder to predict than mouse behavior, it's just that it's harder for humans to predict chimps because the relative difference in our intelligence is much smaller. By analogy, a chess grand master (human) can easily predict the outcome of a chess move by a low-ranked player (mouse) but not so easily playing against a slightly less proficient chess master (chimp). They're a lot closer to our league.
All the more reason why we shouldn't treat chimps, other great apes, and maybe a few other animals as belonging in the same ethical category as mice.
Unsettling video of a young chimp playing with the mummified body of an infant below. An adult is clearly disturbed and wants no contact with the corpse, and the mother eventually scares off the juvenile and takes back the corpse. If you ignore that macabre aspect, though, you'll notice the chimps using stone hammers and anvils to crack open nuts, one of the prime examples of chimp tool use and one of the few that many of us have done as well.