Does a Pheromone-like Factor from the Nestling Rabbit stimulate Impregnation and Maturation in the Rabbit Flea?

Abstract

IN nature the rabbit flea, Spilopsyllus cuniculi (Dale), copulates and lays fertile eggs only on the newborn young (1–9 days old), although the female undergoes maturation on the pregnant doe as well as on the nestlings^1,2. It was found that human growth hormone injected into the host can stimulate copulation of the matured rabbit flea feeding on nestlings 10–15 days old³, but it was also realized that several other factors must be involved in controlling pairing and impregnation on the newborn young, apart from the hormones ingested with the blood of the host. We decided independently that rabbit fleas probably matured faster when feeding on nestlings in large litters than on nestlings in small litters. This could have been due to different hormone levels in the blood of large healthy litters, for hormone concentrations apparently vary considerably from litter to litter and individual to individual. Thus fleas regularly mature faster and more effectively on certain pregnant doe rabbits than on others, and, furthermore, the concentrations of corticosterone in the host's blood seem to vary with the season⁴. Other variable factors such as humidity and temperature in large nests could have been responsible for the difference observed, but it also seemed that an environmental or pheromone-like “nestling factor” might be involved. The following preliminary experiments were carried out to investigate this situation.