Lions and tigers and ...aphids
Catnip is an extract of the catmint plant Nepeta cataria which elicits a dramatic response in domestic cats 22 23, lions 24 and tigers. Initial attraction is followed by behaviour which is very similar to the cat mating ritual. This involves chin and cheek marking, followed by rolling on to the back and body rubbing 25. In fact it is very difficult to grow the catmint in a suburban garden because the cats constantly roll on it and destroy it! The active constituent of catnip is (4aS,7S,7aR)-nepetalactone 11 which elicits the catnip response at concentrations as low as ppb to ppt. Nepetalactone (11) and other stereoisomers 26 are found in many Nepeta species and are formed biosynthetically 27 from nerol (or a closely related monoterpene) by terminal hydroxylation, oxidation and cyclisation 28.
Cats respond equally well to both enantiomers of nepetalactone, although there are two reports that racemic nepetalactone is weakly active or inactive to cats 29. Frustratingly, the group which tested enantiomerically pure material did not test the racemate. If all of the data from the various groups is correct, this would be the first case where both enantiomers have activity and the racemate does not 30. However response to catnip is rather variable and highest in cats of reproductive age; hence a mundane explanation is more likely.
Aphids are small insects which feed on the sap of plants through fine tubes. They reproduce asexually during the summer, but sexually during the winter 31. The females release a male attractant pheromone from their hind legs. A mixture of nepetalactone (11) and the nepetalactol 12 32 is the female released sex pheromone of the vetch aphid, Megoura viciae 33 and the wheat aphid, Schizaphis gramium. These compounds also act as an attractant for the black bean aphid, Aphis fabae 34. Females of several Cryptomyzus species 35 release both the lactone 11 and the lactol 12 (ratio 1:30). Males are only weakly attracted by this mixture, which suggests that further components are required for full activity. The lactol 12 alone is the sex pheromone of the damson-hop aphid, Phorodon humuli 36. However, aphid parasitoids are more strongly attracted by the lactone 11 37. It is very common for closely phylogenically related species to use mixtures with the same components, but with different ratios or extra components which give species specificity and maintain reproductive isolation.
Catmint is a perennial plant with soft grey or green leaves, which are ideal for aphid attack. It is possible that nepetalactone 11 is released to attract aphid parasitoids. The effect on cats is more difficult to explain and the lack of enantioselectivity (but not the sensitivity) suggests that it is mimicking some other effector. It is conceivable that catmint seeds are dispersed from the cats' coat and that the advantage of this mechanism of dispersal is sufficient to compensate for the damage done to the plant by cats.
Cats have an array of marking glands which have been barely investigated. In particular glands on the front and side of the head are used in "affectionate" behaviour in which cats rub the cheek, shoulder or sometimes the whole body against each other, inanimate objects or human legs. The same behaviour is also exhibited by lions towards rocks and trees but a rather different "behaviour" is usually displayed to human legs!