Counting bees

I suppose I am a little surprised about this, too:

Math Bee: Honeybees Seem To Understand The Notion Of Zero

The details:

Howard trained one group of bees to understand that sugar water would always be located under the card with the least number of symbols. "They could come and see two circles versus three circles, or four triangles versus one triangle, or something like that," she explains.
The bees quickly learned to fly to the card with the fewest symbols, an impressive feat.
But then they got another test: The researchers presented the bees with a card that had a single symbol — and a blank card that had nothing on it.
The bees seemed to understand that "zero" was less than one, because they flew toward the blank card more often than you'd expect if they were choosing at random — although they weren't that good at distinguishing between the two.
It got easier for them when they had to compare zero with a larger number. "When we showed them zero versus six, they did that at a much higher level than zero versus one," Howard says. "So what tells us is that they consider zero as an actual quantity along the number line. They're actually better at doing zero versus six because those two numbers are further apart."

The reaction:

"This is quite amazing, in my view, that bees can really do it," says Andreas Nieder, a scientist who studies how animals' process the idea of "nothing" and was not part of the research team.
He says zero was discovered relatively recently in human history, and was essential in the development of both mathematics and science. "It's a hard and very abstract concept," Nieder says. "It is a sort of eccentric uncle in the number family."
Previous experiments have shown that honeybees have some facility with numbers, because they were able to count landmarks as they foraged around for a sweet reward. But in these tests, the insects couldn't count very high — only to about four.

Update:   Now that I think about it...isn't a simpler explanation that the bees were just learning the rule "the less cluttered a card appears, the more likely a reward"?    If so, can you interpret this as understanding "zero"?   I mean, a blank card is less cluttered than anything with symbols on it, and the more symbols, the more obviously less cluttered is the blank card.  

Is this a case of over-interpretation of a result?