Quick! How many dots?
You can do that fast, right? You don’t even have to count.
In comparison, as fast as you can, how many dots?
That’s much trickier, isn’t it? Slower. You have to count.
The first, “at a glance” way of determining the number of things is called subitizing.
A new paper by Bisazza and colleagues takes a look at these abilities in guppies. Guppies, like many other fish, have a behaviour that is sensitive to numbers of things: joining a school of other fish. Bisazza used this behaviour to test guppies abilities to distinguish quantities.
And, for an added twist, they looked at how the behaviour changed over time. Little (one day) guppies were able to tell 3 from 4... but not 4 from 8. Or even 4 from 12! “Four” seems to be the breaking point. Adult guppies, however, can tell 4 from 12. This suggests that these fish have a mechanism for distinguishing small numbers, but not larger ones, even when the differences between the numbers are substantial.
The guppies abilities to distinguish sizes gets better as the guppies get older; they hit adult abilities at about 40 days of age. Interestingly, though, practice matters. Guppies reared in pairs improve in their abilities to distinguish large from small more slowly than guppies reared in tanks with many companions.
You might argue that this doesn’t represent counting in any real sense – this could just be amount of “visual space” that objects represent. That is, guppies distinguish large from small. The researchers tried to control for that, using a tank set-up so that the lone fish being tested could only see one other fish at a time. That seems to eliminate “size” as a proxy for counting.
While Bisazza and collegues emphasize the evidence for there being a small number subitizing system and another for larger numbers, I’m actually most fascinated by the entire social aspect of this. I wonder if the improvement in discrimination would be seen in other situations. Would fish that could discriminate large from small schools also be able to discriminate large from small numbers of food items, say?
P.S.—There are 23 dots in the second picture.
Bisazza A, Piffer L, Serena G, & Agrillo C. 2010. Ontogeny of numerical abilities in fish. PLoS ONE 5(11): e15516. DOI: 10.1371/journal.pone.0015516
Photo by Tartaruga33 on Flickr. Used under a Creative Commons license.