We've known for a while that chimps can accurately mentally measure pouring liquid, now some research shows that old world monkeys are really pretty good at counting as well, showing the rudiments of numeracy. BBC News gives a good overview of the research, but this is the gist of it:
In basic tests using both food and non-food items, macaques understood the concept of relative quantities. Choosing plates with greater numbers of pebbles over ones with lesser numbers, which they learned represented food they'd receive if they chose the larger quantity. The twist in it, which also confounds young children, is that their desires for the food overwhelmed their judgement if the food itself was presented--the chimps chose the greater quantity with less accuracy in these situations. More on Monkeys:
Self-Doubting Monkeys & Really Clever Sheep. Who Knew?!
Chimpanzees Even Smarter Than We Thought - Can Mentally Measure Pouring Liquid's Volume