Can lizards count? Testing numerosity in an Australian skink using spontaneous quantity discrimination

Quantity discrimination is a cognitive trait that allows animals to process numerical information. Although well studied in mammals, birds and fishes, little is known about quantity discrimination in nonavian reptiles. Quantity discrimination is typically measured through spontaneous binary choice experiments or by training animals to discriminate quantities through conditioning, but rarely are both methods used in tandem. We examined spontaneous quantity discrimination with and without positive reinforcement in the eastern water skink, Eulamprus quoyii . We offered a series of binary choices: one versus four, two versus four, three versus four, two versus eight, four versus eight and six versus eight same-sized crickets for number discrimination and 0.3 versus 1.2 cm, 0.6 versus 2.4 cm, 0.6 versus 1.2 cm, 1.2 versus 2.4 cm, 0.9 versus 1.2 cm and 1.8 versus 2.4 cm for size discrimination tests. We quantified number discrimination performance using crickets of the same size that varied in number and size discrimination performance using two crickets that varied in size. These tests helped determine which core system the skinks use and assessed the effect of positive reinforcement. In the spontaneous experiment, we found that skinks could only discriminate between one versus four crickets and 0.3 versus 1.2 cm. However, when rewarded, skinks successfully discriminated one versus four crickets in number and between all ratios in size, except for the highest ratio of 1.8 versus 2.4 cm. These results indicate a possible difference in discrimination method when faced with number or size and showed that training can improve an animal's discrimination capacity. • First use of digital stimuli to study quantity discrimination in nonavian reptiles. • Water skinks show improved quantity discrimination following positive reinforcement. • Water skinks discriminate between size better than number of prey items. • Approximate number system may be used by water skinks for quantity discrimination.