Olfactory stimulation by catnip is a well-known method to provide enrichment for a cat’s living environment. Although it has been long known that not all cats respond to catnip, only limited data are available about plants that may be used as an alternative. This study was designed to compare the responsiveness of cats to catnip, silver vine, Tatarian honeysuckle and valerian root. The sample size (n = 100) and the comparison of responses to four different plant materials make this study the most comprehensive on this subject thus far. In addition, this is the first study to document responses of domestic cats to silver vine fruit galls, Tatarian honeysuckle and valerian root, and those of tigers and bobcats to catnip and silver vine.

Nearly all domestic cats responded to at least one of the plant materials tested, with the largest numbers of cats responding to silver vine (79%) and catnip (68%). Our results indicate that cats predominantly responded to the powder of silver vine fruit galls, and much less frequently to the wood of the silver vine plant. In contrast, Katahira and Iwai [21] found that 28 out of 31 cats (90%) responded positively to a dried silver vine branch. This discrepancy may be explained by the methodology of their study. Experiments were performed on laboratory cats housed in small cages (39 × 51 × 33 cm) that were offered a dried branch of silver vine. Approaching and trying to bite the branch within three minutes was considered a positive response, and no negative controls were used in their study.

The percentage of non-responders to catnip in our study is in line with findings from previous studies: 35% [9], 28% [10] and 18% [11]. Pooling the data from these studies gives a total of 53 non-responders out of 170 cats (31%), which is very similar to the 32 out of 99 cats (32%) that did not respond to catnip in our study (Fig. 9). Also in line with our observations, a similar percentage of cats with mild or partial responses to catnip was reported by Palen and Goddard [10] (8 out of 43 or 19% compared to 16 out of 99 or 16% in our study).

Fig. 9 The absolute number and percentage of cats that responded positively to catnip in three previous studies [9–11] (left). The pooled results from these three studies are similar to our findings (right) Full size image

Our observation that tigers did not respond to catnip agrees with results of previous studies. Todd found that 15 of the 23 tigers (65%) tested did not respond at all, six (26%) only sniffed, licked and shook their head, and two tigers (9%) showed a partial response including chin and cheek rubs [6]. None of the tigers rolled over. Hill et al. found that three out of four tigers did not respond to catnip, whereas one showed a mild or partial response [7]. Results from these studies also indicated that all other members of the genus Panthera (lions, jaguars, leopards and snow leopards) tested, did respond positively to catnip. Todd and Hill et al. also tested catnip on bobcats [6, 7] and their findings were contradictory, possibly because only two bobcats were tested in each study.

Although data specific for cats are not available, all four plant species used in this study are generally considered safe and not toxic or addictive for cats and humans. Cats have been exposed to silver vine and catnip, in a variety of different forms, for many years. We could only find one anecdote related to the potential addictive properties of silver vine [26]. In the 1970s, the smell of an unspecified form and concentration of silver vine was reportedly preferred by large cats (species not specified by the author) kept in cages in a zoo in Japan over food and sexual intercourse. However, their response to the silver vine was one of excitement and joy; no data were presented supporting any negative health effects. The scientific value of their claim that silver vine is addictive recently has been disputed by others [27], and there is currently no scientific data supporting any addiction potential, nor are there any other negative anecdotes known to the authors, despite the widespread use of silver vine in Japan. A recent literature review on the use of silver vine concluded that its enrichment potential has been largely overlooked, and the authors encouraged further investigation of feline behavioral responses to silver vine [27]. The Cat House Inc. has sold thousands of pieces of Tatarian honeysuckle since 1991, and has never received a report of adverse reactions (J. Wegiel, personal communication, April 2016). None of the four plants used in this study are listed as poisonous or toxic by the Animal Poison Control Center (SafetyCall International, Bloomington, MN, USA) or the American Society for the Prevention of Cruelty to Animals (New York, NY, USA). All plant materials used in this study are commercially available, and are sold as (part of) toys or enrichment products specifically for cats. Finally, inquiry to the Pet Poison Helpline (SafetyCall International) (22 June 2016; case number 1849511) taught us that valerian root is generally considered safe, and that no toxicity has been reported from exposure to silver vine or Tatarian honeysuckle wood. The berries of Tatarian honeysuckle (not used in this study and not believed to have a stimulating effect on cats) may cause mild gastrointestinal irritation because of the presence of terpenoids.

The results of our chemical analysis suggest there are significant differences in the concentrations of compounds known or claimed to have a stimulating effect on cats between the various plant materials tested. However, interpretation of Table 3 is somewhat hampered by lack of knowledge of the limit of detection for these compounds for cats, and if there is a dose-response relationship. Our findings suggest that cis-trans nepetalactone is the dominant and possibly the only compound in catnip to which cats respond. The higher concentrations of actinidine in silver vine, Tatarian honeysuckle and valerian root may explain, in part, the responsiveness of cats that did not respond positively to catnip to these plant materials. In addition, cats that only responded to silver vine may specifically detect iridomyrmecin and isodihydronepetalactone in silver vine fruit galls (Table 3). Indeed, 25% (19/77) of the cats that responded to silver vine did not respond to Tatarian honeysuckle or valerian root, despite similar concentrations of actinidine in these plant materials. In normal silver vine fruit, we detected actinidine at very low concentrations (6 μg/g) compared to the fruit galls (Fig. 8), Tatarian honeysuckle, and valerian root. Sakan et al. also detected actinidine in leaves of the silver vine plant [17], but did not report the concentration they detected. Like normal silver vine fruit, leaves did not elicit a response in any of the cats we tested, so it is likely that the concentration of actinidine in silver vine leaves is also very low. While actinidine has previously been identified in silver vine and valerian root [17–20], this is, to the knowledge of the authors, the first study demonstrating that Tatarian honeysuckle contains actinidine, and that the amount is comparable to concentrations found in silver vine fruit galls and valerian root (Table 3). The low levels of cis-trans and trans-cis nepetalactone, iridomyrmecin and isodihydronepetalactone in Tatarian honeysuckle suggest that actinidine may be the only compound in the wood that cats respond to. Indeed, we found large overlaps between the plant materials that contained high concentrations of actinidine: 93% (41/44) and 88% (43/49) of the cats that responded positively to valerian root or Tatarian honeysuckle also responded to silver vine (Fig. 5a). However, because of technical limitations, we were unable to identify all of the known or claimed active compounds, which leaves room for the possibility that other compounds are involved as well. This possibility is further supported by the observation that despite similar concentrations of all the measured compounds in Tatarian honeysuckle and valerian root, only 28 of the 65 cats (43%) that responded positively to Tatarian honeysuckle or valerian root responded positively to both these plant materials. The popularity of silver vine fruit galls among cats may be explained by the relatively high concentrations of several compounds: actinidine, iridomyrmecin, isodihydronepetalactone and its isomers, although it is unknown if the latter have a stimulating effect on cats. The observation that fewer cats responded positively to silver vine wood than to silver vine fruit galls suggests that lower concentrations of these compounds are present in the wood and that the concentration of these chemicals in the plant material as well as the detection threshold of the cat contribute to the outcome of the cat’s response.

Evidence that certain compounds have an apparently euphoric effect on cats is scarce. In the early 1940s, McElvain and colleagues performed a bioassay using seven adult lions to demonstrate that nepetalactone is the active compound in catnip, but they did not discriminate between the cis-trans and trans-cis form of nepetalactone [16]. Results from subsequent studies that investigated which of the two nepetalactone isomers causes the catnip response suggest both isomers do, but the design of these studies and the reporting thereof were of poor quality [22, 23]. While Sakan and colleagues claimed that certain compounds in silver vine have cat-attracting or stimulating properties, evidence was not presented [17–19]. The limited information available about bioassays performed by Sakan and colleagues was written in Japanese [18]. They stated: “We mainly used cats and a compound was considered active when one or more of the following were observed: express interest in the sample, lick the sample, salivate, exhibit the Flehmen response, rub their neck against something, rub their back against the ground and move around, become entranced, or fall asleep. Actinidine and matatabilactone (comment by the authors: at the time matatabilactone was believed to be a mixture of iridomyrmecin and isoiridomyrmecin [19]) showed strong activity in lions, tigers and panthers, and actinidine even showed some activity in dogs.” Other than this, no data were provided to support their claims. Well-controlled studies in which cats are exposed to isolated, pure compounds and scored according to relevant and well-defined behavioral characteristics are required to conclusively establish which compounds cause the apparently euphoric response of cats to the various plant materials.

The precise mechanism by which allomone production in silver vine and other plants is stimulated and regulated is unclear. It is known that some gall-inducing insects promote growth of plant tissue to help provide them with food and shelter [28]. It seems that in response to the invasion of the fruit by the larvae, silver vine plants produce and secrete volatile compounds trying to repel the matatabi fruit gall midge. Alternatively, these compounds could function by attracting midge predators or parasitoids. Because nepetalactone has been found to be an insect repellant [29, 30] and all the cat-stimulating compounds are similar in structure (Fig. 1), the former may be more plausible. Although few in number, we did see some cats respond positively to silver vine wood, whereas no cats responded to silver vine leaves or normal silver vine fruit. We did not perform chemical analysis of silver vine wood tissue. Actinidine, iridomyrmecin, isoiridomyrmecin, dihydronepetalactone, isodihydronepetalactone and neonepetalactone were first identified in silver vine leaves and silver vine fruit galls [17, 19]. It was not specified if these leaves were from silver vine plants with or without fruit galls. In addition, it is unknown if the plant’s response to the local attack by the matatabi fruit gall midge larvae is local or systemic, and thus if allomone concentrations can increase in leaves and wood of the plant. It would be interesting to learn whether cats respond differently to silver vine wood from plants bearing galled fruit as compared to wood from vines without fruit galls.

Each of the four plant materials tested in this study can be a valuable addition to a cat’s environment, whether indoor or outdoor, but some come with a few disadvantages. Many cats respond to catnip, which is inexpensive and easy to obtain or grow. Silver vine, although perhaps even more popular among cats than catnip, is more expensive and difficult to obtain; it is predominantly available online and comes from East Asia. Producing silver vine fruit galls in areas outside of the natural habitat of silver vine is practically impossible because it would require the presence of Pseudasphondylia matatabi. The life span of the adult fly is one to two days. Females of P. matatabi lay their eggs in the flower buds of silver vine in May. The larvae spend the summer in the fruit galls, but silver vine is not flowering when the flies emerge from the fruit galls in autumn. Because of their short life span, the gall midge needs to use a different host plant during the winter season to survive [31]. To this day, this host has not been identified (Dr. J. Yukawa, Entomological Laboratory, Kyushu University, Fukuoka, Japan, personal communication, June 2016). Although inexpensive and easy to obtain, valerian root comes with a strong smell that is not appreciated by everybody. Tatarian honeysuckle wood may be the most difficult plant material to obtain, but the wood comes in many different sizes, and will last a lifetime.

The various plant materials tested may be used to increase playtime of cats, which is especially valuable for less active, obese or under-stimulated cats. They also can be given to cats for distraction when left home alone for a long time. In animal shelters, these plant materials could be used by staff or volunteers to help socialize cats or to increase the chance of rehoming more timid adult cats. Indeed, playfulness of a cat was identified as one of the most important selection criteria used by potential adopters when choosing a cat [32]. We have observed several fearful, withdrawn cats, brought into the sanctuary days or weeks before we tested them, that were attracted by the plant materials and responded seemingly euphorically after encountering them. These cats would normally hide when staff or volunteers were present. This suggests that trap-neuter-return programs also may benefit from using these plant materials. Indeed, a previous study already suggested that catnip may be effective for luring feral cats [33]. The plant materials also might be used for training purposes, where exposure to these stimuli may be used as a reward instead of food, which is most often used.

Recent studies, including a systematic review, have demonstrated the lack of evidence that synthetic feline pheromones (e.g., Feliway) can relieve stress in cats [34, 35]. It would be worthwhile to investigate if the plant materials studied here can be used to alleviate distress in cats, for example during medical procedures, transportation or boarding. Before these plant materials can be used for anything other than enrichment, it is critical that multiple well-controlled, independent studies with sufficient sample size have demonstrated their effectiveness.