Do those experiments show definitively that babies and animals understand that 1 + 1 = exactly 2 and nothing else? Or do they understand that 1-ish + 1-ish is more than 1 -- because this isn't that much different from what anumeric groups understand. Less and more are different from discrete arithmetic, strictly speaking.
For instance, 1 fish + 1 fish might be > 3 fish, if the first two fish we're talking about are 20 and 40 lbs respectively, and then the second set of three fish are ten pounders each. In the "real world", hunter-gatherers deal with continuous mass problems at least as often as they deal with standardized, discrete units.
Did these babies and animals understand that 1 + 1 = 2 and only 2, and not 2.5? Or 3?
The abstract idea of arithmetic is something on the scale of reasoning about an idealized circle, say, which doesn't exist in nature on a day-to-day basis.