A behavioral bioassay for analysis of rabbit nipple-search pheromone

To suckle, newborn rabbits are completely dependent on a pheromone present on the doe's ventrum and also apparently contained in the milk. Pups presented in a standardized test with 30 microliters of fresh rabbit milk on a glass rod reacted with vigorous search-like movements in 93%, and with grasping of the rod in 70% of trials, but did not respond to control presentations of other odorants. The pups were found to be most reactive 1-3 hours before the once daily nursing, and although they rapidly habituated to repeated testing, a break of 30 minutes was sufficient for the recovery of responsiveness. Even milk diluted by as much as 1:10(4) elicited significantly more responses than control substances. However, when left at room temperature it lost this behavior-releasing quality within about 20 minutes, but retained it for several months when stored at -40 degrees C. Rabbit milk thus appears to represent a ready source of nipple-search pheromone, and the behavioral response of pups to be reliable enough to serve as a bioassay for further pheromonal analysis.