Mole-rat harderian gland secretions inhibit aggression

The harderian gland secretions of mole-rats, Spalax ehrenbergi, usually released by self-grooming, include odorous substances which are sex dependent. Male secretions were the most attractive to both sexes, while female secretions were attractive to males but not to other females. The rate of attacks by females towards intact males was higher than towards males whose harderian gland had been removed. However, grooming by intact male mole-rats decreased the rate of attacks by their opponents, while grooming by males without harderian glands did not; thus the male harderian secretions appear to have appeasement qualities. Grooming by females with and without harderian glands failed to reduce aggression. Unlike intact males, those without harderian glands had almost no volatiles on their fur, and thus are probably not considered to be a threat to conspecifics. Gas chromatography spectra showed that substances of harderian origin were added to the fur during grooming. Some of these substances remained on the fur long after the animal ceased grooming, and appear to give the animal its specific odour, but some volatile substances peaked briefly after grooming, and were probably responsible for the decline of aggression that occurred after grooming. Although grooming has long been considered to be a displacement activity, we suggest that in the mole-rat its performance is too risky to be merely this, and it has acquired the meaning of appeasement through the release of chemical cues.