Can lizards count?

170412 ruin lizard full
A ruin lizard deep in thought.
Credit: Petar Milošević

Suppose you were very hungry and you loved to eat apples, and someone offered you a choice between a plate with five apples on it and a plate with just three. You’d probably choose the five apples without hesitation, and most other animals would do the same. Lizards, however, might find the choice a little tougher.

New research on the behaviour of ruin lizards (Podarcis sicula), published today in Biology Letters, casts doubt on previous beliefs about the near-universality of numerical cognition.

The ability of a species to compare relative food quantities and select the largest amount is critical for optimising foraging behaviour and safeguarding survival. To date, this cognitive skill has been observed in all vertebrate species bar reptiles.

An Italian research team from the University of Padova and the University of Ferrara set out to fill this gap in the literature, and investigated the capacity of ruin lizards to select the largest amount of food.

The researchers exposed their lizard subjects to two servings of Musca domestica housefly larvae, with portions differing in either size or number, and observed the lizard’s choice.

The two servings were placed in the arms of a Y-shaped study apparatus. After being released from a tunnel-shaped holding area, the lizards climbed to the top of a ramp, reaching a vantage point from which to view the two servings, before being able to enter the experimental compartment and make their final selection.

Two separate experiments were conducted.

The first exposed the lizard subjects to two platefuls of different sized larvae, while the second presented two servings containing different numbers of equally sized larvae.

The results revealed that, while the lizards are able to select the larger of the two different-sized larvae, they do not seem to have the capacity to choose the option containing the higher number of larvae.

While the observed aptitude for size-discrimination resembles that witnessed in other vertebrate species including chimpanzees, salamanders and guppies, the observed inability of the lizards to distinguish between groups with different numbers of individual components represents a surprising exception.

The researchers do not know why the ruin lizards lack a numerical capacity, but their findings complicate our previous understanding and open space for new research.

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