Inhibitory interactions between multimodal behavioural responses may influence the evolution of complex signals

Most animal displays make use of multiple sensory modalities (visual, acoustic and chemical signals) to convey similar information. Although multimodal displays may allow producers to use displays in a wider variety of social and physical contexts, it is difficult to explain their evolution because of the likely increased costs to senders and receivers. In this study, playback experiments in two contexts were used to study the behavioural responses to visual (headbob displays) and chemical (femoral pore secretions) signals in sagebrush lizards, Sceloporus graciosus. Lizards in the field tended to headbob in response to headbob displays and to engage in chemical exploratory behaviour when presented with chemical secretions. Territorial residents produced fewer headbob displays and head-turns in response to a combined signal produced by a robotic lizard than they did to either signal presented alone. This inhibition was confirmed in the laboratory, where presentation of a visual stimulus alone decreased chemical exploratory behaviour and presentation of a chemical stimulus alone decreased the number of headbob displays produced. The absolute cost of this interaction between sensory modalities may be low because the two behavioural responses are redundant, both allowing the receiver to acquire additional information, either by engaging a second lizard in a bout of interactive visual displays or by absorbing more of their scent. Thus, in sagebrush lizards, multiple signals may be evolutionarily maintained because behavioural responses to different sensory modalities are redundant and, hence, the cost of negative interactions between those responses is low.