Humans recognize numbers rapidly and precisely up to four (“subitizing”; Kaufman, Lord, Reese, & Volkmann,
1949), but we also estimate larger numbers in a rapid fashion, although there with increasing errors. A typical characteristic of the numerosity system is that the errors depend on the set size, in accordance with the Weber–Fechner law (Gallistel & Gelman,
1992). Whether these characteristics support the idea of two clearly distinct subsystems or reflect different operation modes of a general number system is still under debate (Feigenson, Dehaene, & Spelke,
2004; Piazza, Mechelli, Butterworth, & Price,
2002; Ross & Burr,
2010). Not only has estimation of numerosity been reported for human adults (Whalen, Gallistel, & Gelman,
1999), but it has also been shown that even 6-month-old infants were able to perform a number-distinction task (Xu & Spelke,
2000). Investigation of the development of mathematical competence and the ability for numerosity estimation in children suggests that mathematical ability is correlated with acuity in numerosity estimation (Halberda, Mazzocco, & Feigenson,
2008). But number estimation is not restricted to humans with mature cognitive abilities—it has also been found in infants and animals (Brannon,
2006; Nieder, Freedman, & Miller,
2002), and recently even in invertebrates (Gross et al.,
2009).