Are pheromones their own reward?

Hamster vaginal discharge (HVD) contains a variety of intersexual pheromones which are powerful promoters of male reproductive behavior. Males avidly investigate and then consume HVD from females or from artificial sources. We measured the reward strength of HVD and other sapid substances with operant techniques. The highest fixed ratio at which males would bar press, the break point, varied with the level of water deprivation and the volume of water reward. Break points for sucrose rewards, in undeprived animals, were higher than for water and varied with concentration. Water, vanillin and HVD solutions were comparable reinforcers irrespective of deprivation level. Sexual experience did not increase bar pressing for HVD rewards. Moreover, naive animals could not be trained to bar press for HVD. These results contrast with other evidence that HVD is highly palatable and challenge the notion that the pheromones present in it are innately rewarding. However, this conclusion may be mitigated by constraints on learning which limit the utility of operant techniques to evaluate the reinforcing properties of HVD.