Ultraviolet visual sensitivity in desert iguanas: implications for pheromone detection

Behavioural tests comparing the responses of desert iguanas, Dipsosaurus dorsalis, to conspecific femoral gland secretions and to controls showed that the secretions are relatively non-volatile, but possess pheromonal activity at close range. Low volatility ensures that these secretions remain in the environment longer, but makes signal localization by airborne olfactory cues difficult. Spectroscopic analyses of desert iguana femoral gland secretions revealed that they strongly absorb longwave ultraviolet light. Behavioural bioassays in which lizards were presented with secretion-marked and blank control tiles under both ultraviolet and incandescent light conditions demonstrated the importance of an ultraviolet light source for successful detection of femoral gland deposits. Further tests measuring the lizards' success rate in localizing an ultraviolet-absorbing ink spot under ultraviolet and incandescent light conditions suggested desert iguanas have visual sensitivity in the near ultraviolet part of the light spectrum. The existence of an ultraviolet visual marker provides a possible mechanism for locating these low volatility pheromones.