If a predator is looming on the horizon, agamid lizards, Trapelus pallida, prefer to run off in the opposite direction. But sometimes it is too cold to run, so as a last resort the lizards will stand their ground,behave aggressively, and bite. Scientists suspected that there might be a physiological reason for this shift in behaviour: the ability of a muscle to generate force, which is important during biting, is less temperature dependent than power output and contraction speed, which are important during running. This implies that lizards could defend themselves by biting at all temperatures, relying on running at higher temperatures only.

To test if physiological differences in lizards' leg and jaw muscles would explain the shift in behaviour, Anthony Herrel and his colleagues measured lizards sprinting down a 2 m track at different temperatures, finding that as temperature increased, the lizards ran faster(p. 1762). When they measured the lizards' biting force at a range of temperatures, they found that they could bite with a similar force at all temperatures, showing that force was less affected by temperature than muscle power and contraction speed.

To find out how temperature affected the largest muscle in the leg, and another in the jaw, they measured both muscles' contraction properties at different temperatures. In both muscles, maximum force production was not affected that much by temperature, while power and contraction speed improved with warmer temperatures. However, the jaw muscle was better at producing maximal force at all temperatures than the leg muscle. This suggests that the properties of the muscles may limit how the lizards can behave at different temperatures, and explain why they are more likely to run when it's warm and bite when it's cold.

Herrel, A., James, R. S. and Van Damme, R.(
). Fight versus flight: physiological basis for temperature-dependent behavioral shifts in lizards.
J. Exp. Biol.