Animals live in a complex environment. They experience seasonal changes in weather and food availability and changes in their social environment, just to name a few. Reacting appropriately to these changes is crucial to survive. One important skill when facing unpredictable events is behavioural flexibility, the ability to adapt behavior to novel situations. We can look for behavioural flexibility in animals by testing their ability to abandon a previously learned rule. We presented tree skink (Egernia striolata), a family-living skink from eastern Australia, with a multi-stage visual discrimination task to find out how flexible they are when learning to find food based on colours and shapes.
Why did we choose to test these Australian skinks? Tree skinks live in social groups, small family units including an adult pair and their offspring. Social living has many benefits, but can also be a demanding environment. The challenges faced when living in a complex social environment can influence the general intelligence of individuals (“social intelligence hypothesis”) making them smarter. We wanted to know if social life made these lizards better able to cope with change.
Figure 1. Tree skink making a choice during a trial.
In the first stage of our experiment, we presented lizards with a choice between two stimuli: an X and a triangle. The food dish in front of the X provided a food reward, while the dish in front of the triangle did not. After lizards had learned which shape was the rewarded one, we switched the rule. Suddenly the reward was found in front of the triangle. With this simple change, we were able to find out how fast these lizards can stop looking for the food in front to the now wrong X and change their approach to look for it in front of the triangle. We repeated this simple reversal test two more times with different shapes. Our lizards were able to adjust their choice behavior and learned to respond to the previously unrewarded stimulus in all reversal stages. Contrary to other animals, our skinks did not take longer to learn the reversals compared to the initial acquisition stage.
Starting from the third stage, cue cards included both a shape and a colour (background) dimension. The addition of a second dimension is important because we wanted to know how well lizards pay attention and how they react to a change in dimensional relevance compared to the change in stimulus relevance tested during the reversals. This is called an extra-dimensional shift: Instead of changing to the formerly unrewarded stimulus within the shape dimension, we change to the second previously unimportant colour dimension. This means that lizards have to stop paying attention to the shapes and instead find out which colour is rewarded. Extra-dimensional shifts are considered harder than reversals. Our lizards were able to learn during the extra-dimensional shift, did not, as shown in humans and non-human primates, take longer compared to the intra-dimensional acquisition. Our skinks did not generalise stimuli into colours and shapes but instead most likely viewed each cue card as a single stimulus and learned which two were associated with the reward.
Video presenting a short summary of our methodology.
So what do our results tell us about tree skinks behavioural flexibility? First of all they definitely are flexible to some degree because they performed well in the reversal stages. Unfortunately, our experiment does not tell us how exactly they perceive dimensions and why they learned so fast in the extra-dimensional shift. Our experiment is, however, a first step towards better understanding learning in lizards and how intelligence developed during the course of evolution.
Figure 2. Graphical abstract. Order of stimulus presentation during the eight stages of the discrimination learning experiment. Lizards were able to successfully learn in all stages of the experiment.
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