Egg incubation effects generate positive correlations between size, speed and learning ability in young lizards

Previous studies have suggested that body size and locomotor performance are targets of Darwinian selection in reptiles. However, much of the variation in these traits may derive from phenotypically plastic responses to incubation temperature, rather than from underlying genetic variation. Intriguingly, incubation temperature may also influence cognitive traits such as learning ability. Therefore, we might expect correlations between a reptile's size, locomotor speed and learning ability either due to selection on all of these traits or due to environmental effects during egg incubation. In the present study, we incubated lizard eggs (Scincidae: Bassiana duperreyi) under 'hot' and 'cold' thermal regimes and then assessed differences in hatchling body size, running speed and learning ability. We measured learning ability using a Y-maze and a food reward. We found high correlations between size, speed and learning ability, using two different metrics to quantify learning (time to solution, and directness of route), and showed that environmental effects (incubation temperature) cause these correlations. If widespread, such correlations challenge any simple interpretation of fitness advantages due to body size or speed within a population; for example, survivors may be larger and faster than nonsurvivors because of differences in learning ability, not because of their size or speed.