The Function of Shudder Displays in the Lizard Sceloporus jarrovi

Rapid head-nod displays, called shuddering, have previously been considered a courtship display used only by males in iguanid lizards. Field observations of Sceloporus jarrovi indicate both sexes shudder during the non-breeding season although the display retains its courting function in breeding males. A new hypothesis is proposed, that the display represents a high level of excitement and a potential for attack. Intruder retreat or imitation of a female will prevent the attack. The selective regime which favored the change in display function can only be conjectured from existing comparative data.