Quantity discrimination in a lizard

While foraging or during social interactions, animals may benefit from
judging relative quantity. Individuals may select larger prey or a patch
with more food and likewise, it may pay to track the number and type of
individuals and social interactions. We tested for spontaneous quantity
discrimination in the gidgee skink (Egernia stokesii), a family-living
lizard. Lizards were presented with food quantities differing in number or
size and selected the larger quantity of food items when they differed in
number, but not when items differed in size. We show, for the first time,
superior spontaneous discrimination of items differing in number over size
in a lizard species which contrasts with previous findings. Our simple
method, however, did not include controls for the use of continuous
quantities and further tests are required to determine the role of such
information during quantity discrimination. Our results provide support
for use of the parallel individuation system for the discrimination of
small quantities (£ 4 items). Lizards might, however, still use the
approximate number system if items in larger quantities (> 4) are
presented. Overall, we uncovered evidence that species might possess
specific cognitive abilities potentially adapted to their niche in respect
to quantity information (discrete and/or continuous) and the processing
system used when judging quantities. Importantly, our results highlight
the need for testing multiple species using similar testing procedures to
gain a better understanding of the underlying causes leading to
differences across species.