Mini-Review Mini-Review Signaling & Behavior 5:10, 1187-1189; October 2010; © 2010 Landes Bioscience between tree shrews and pitcher Perspectives and avenues for future research

Charles Clarke,1,* Jonathan A. Moran2 and Lijin Chin1

1School of Science; Monash University Sunway Campus; Bandar Sunway; Selangor, ; 2School of Environment and Sustainability; Royal Roads University; Victoria, BC Canada

Key words: , tree shrew, sequestration, mutualism, animal-plant interactions

Three of Nepenthes pitcher plants from engage cause the pitchers to overflow, thereby losing digestive enzymes in a mutualistic interaction with tree shrews, the and the products of their activities. The lid is often brightly basis of which is the exchange of nutritional resources. The coloured, has many glands on its surfaces and plays an plants produce modified “toilet pitchers” that produce copious important role in prey attraction.2 amounts of exudates, the latter serving as a food source for tree shrews. The exudates are only accessible to the tree shrews The degree of development and/or modification of each when they position their hindquarters over the pitcher orifice. pitcher component varies substantially among (and even within) 2,6,7 Tree shrews mark valuable resources with feces and regularly Nepenthes species and recent research has demonstrated that defecate into the pitchers when they visit them to feed. Feces unique modifications to pitcher structure possessed by several represent a valuable source of nitrogen for these Nepenthes species play important roles in specialized nutrient acquisition species, but there are many facets of the mutualism that are yet strategies.8-12 One such species, Nepenthes lowii, demonstrates a to be investigated. These include, but are not limited to, seasonal remarkable nitrogen sequestration strategy, in which mountain variation in exudate production rates by the plants, behavioral tree shrews (Tupaia montana) defecate into its pitchers while ecology of visiting tree shrews and the mechanism by which feeding on exudates secreted by glands on the inner surface of the the plants signal to tree shrews that their pitchers represent a pitcher lid. Feces accounts for 57–100% of foliar nitrogen in this food source. Further research into this extraordinary animal- species13 and N. lowii “toilet pitchers” are ineffective arthropod plant interaction is required to gain a better understanding of the benefits to the participating species. traps. The large orifices and reflexed, concave lids ofN. lowii pitchers induce T. montana to sit astride the pitcher whilst feed- ing, facilitating fecal deposition. Chin et al.14 found that two other montane species from The Nepenthes comprises approximately 120 Borneo, and Nepenthes macrophylla, also trap species, with the centre of diversity lying in the perhumid trop- tree shrew feces. Detailed analysis of trap geometry revealed that ics of . All species are vines or subscandent shrubs these two species and N. lowii share a unique arrangement of that produce highly modified organs (“pitchers”) which typi- trap characteristics that was not detected by earlier studies on the cally attract, trap, retain and digest arthropods for nutritional genus. This involves the production of pitchers with very large benefit. The pitchers of almost all Nepenthes species share the orifices, large, concave lids that are reflexed approximately 90° same physical components,1 including the pitcher cup, the peri- away from the orifice and lid glands that produce copious exu- stome and the lid. The pitcher cup usually consists of two main dates.14 The distance from the front of the pitcher orifice to the sections: an upper zone which is often covered with wax crystals inner surface of the lid precisely matches the head + body length and anisotropically-oriented semilunate cells2,3 that assist in the of T. montana, resulting in the tree shrews’ food source being capture and retention of prey; and a lower portion, which con- positioned behind the pitcher orifice and ensuring that the ani- tains fluid and is lined with digestive glands.2,3 The peristome is mals’ hindquarters are positioned over the orifice while they feed a ridge of hardened tissue that lines the orifice. Its anisotropic, on the lid gland exudates. wettable surface plays a key role in prey capture.4,5 In most spe- Thus, N. lowii, N. macrophylla and N. rajah are all engaged in cies, the lid is a broad, flat structure which overhangs the orifice a mutualism with T. montana, the basis of which is the exchange and prevents the entry of rainwater which, if unimpeded, can of nutritional resources that are scarce in these species’ habitats. The interaction with T. montana is facilitated by trap geometry, *Correspondence to: Charles Clarke; Email: [email protected] but all three Nepenthes species produce pitchers that differ sub- Submitted: 06/22/10; Accepted: 06/28/10 stantially in structure, apart from the shared characteristics out- Previously published online: lined above.14 Through a series of modifications to trap structure www.landesbioscience.com/journals/psb/article/12807 and geometry—none of which appears to have compromised DOI: 10.4161/psb.5.10.12807 their ability to trap arthropod prey—N. rajah and N. macrophylla

www.landesbioscience.com Plant Signaling & Behavior 1187 benefit from a highly specialised nitrogen sequestration strategy T. montana still visited pitchers of N. rajah during May (Chin et that is not available to congeners other than N. lowii. al.14), but very few fecal pellets were deposited inside them. Tree Although Clarke et al.13 demonstrated that N. lowii derives shrews mark valuable resources using feces,16 so it is feasible that nutritional benefit fromT. montana feces, there are many facets during periods of decreased nectar production, T. montana alters of the association that have yet to be investigated and the dis- its foraging behavior to utilize alternative food resources, result- coveries of Chin et al.14 give rise to a number avenues for further ing in decreased rates of defecation into N. rajah pitchers. research, several of which are discussed below. N. lowii, N. rajah and N. macrophylla are virtually confined The behavioral ecology of T. montana with respect to to montane habitats above 1,800 m altitude, but the geographical Nepenthes has not been studied in detail. We do not know range of T. montana extends well beyond that of the “toilet pitch- whether individual tree shrews defend valuable pitchers against ers” and includes a number of sites that are substantially lower other animals or whether such resources are shared. However, than 1,800 m.17 Given this, why are the toilet pitchers not found video footage, showing T. montana scent-marking a toilet pitcher at lower altitudes? Large, fleshy fruits with small seeds (such as of N. lowii after feeding from it, supports the former scenario figs) comprise a major component of the diet ofT. montana,18 but (Clarke et al.13 and Suppl. video). It is not known whether or plants that produce these are relatively scarce in alpine and upper how, the plants signal to tree shrews that their pitchers provide a montane equatorial habitats.19 This could limit the distribution nutritional resource (or even how valuable that resource is—the of toilet pitchers in two ways. First, the lack of fleshy fruits at composition and nutritional value of the lid gland exudates has high altitudes might make toilet pitchers a valuable resource for not been determined). When newly-formed pitchers first open, T. montana in upper montane habitats. Furthermore, the density their tissues generally remain soft for several days while they of arthropods at high altitudes is considerably less than in the undergo rapid expansion during the final stages of development.1 lowlands.20 This exerts selective pressure on Nepenthes to adopt During this period, the pitchers are incapable of supporting a tree non-carnivorous nutrient acquisition strategies.13 Accordingly, shrew without suffering significant damage, yet few pitchers of the production of very large, specialized pitchers that receive a N. lowii, N. macrophylla or N. rajah that we observed exhibited steady input of feces may provide a net benefit for the plants, signs of such damage. One possible explanation for this is that but only at high altitudes. Second, at lower altitudes, fleshy fruits the plants signal the tree shrews to indicate whether or not indi- (and arthropods) are more abundant and at these sites the ben- vidual pitchers are “open for business.” This might be achieved efits of producing toilet pitchers may be reduced or even negated, using variations in color: Tupaia spp. are dichromatic, with sen- hence their absence from smaller . sitivity maxima at ca. 440 and 550–560 nm15 and the pitchers of Through their unique pitcher characteristics and trap geom- all three feces-trapping Nepenthes species utilize combinations etries, a number of Nepenthes species derive supplementary of green, red, yellow, orange and purple pigments, which change nutrition from a wide variety of arthropod groups, leaf litter as individual pitchers age.1 In N. lowii, the inner surfaces of the and animal feces.6,10,13,14,21,22 It is arguable that no other plant feces-trapping pitchers are uniformly dark purple when mature, family has such a complex and diverse array of interactions with but when they first open, they are unevenly covered with purple, animals. Recent discoveries add to a growing body of evidence pink and green patches. The production of copious lid gland exu- to suggest that Nepenthes demonstrate adaptive radiation with dates in N. lowii appears to commence after the pitchers have regard to nutrient sequestration strategies (see Chin et al.14 for a hardened and the uniform dark purple color has developed on more detailed discussion). The findings of Chin et al.14 provide the inner surfaces. the strongest support for this hypothesis to date; in addition, The study by Chin et al.14 was based on a series of three field they provide the first plausible explanation for the extraordinary trips to northern Borneo that were conducted in March, April size of N. rajah pitchers. This iconic species is the world’s larg- and May 2009. The first of these two visits took place during the est and was first described 150 years ago. The wet season and heavy rain fell on most days throughout these population studied by Chin et al.14 grows at a site on Mount months. In contrast, May was unusually dry. During this period, Kinabalu that has been visited by tourists since 2001 and has many N. rajah plants exhibited signs of stress due to lack of water, been regularly examined by scientists and Parks staff including wilting or senescence of developing pitchers and inflo- for more than 30 years. Despite this, the association between rescences. This coincided with an apparent (but unquantified) N. rajah and T. montana remained undetected until we employed decline in the number of pitchers that received tree shrew feces: remote survey methods to record pitcher visitors. To date, this whereas such pitchers were relatively easy to locate during our technique has been used on just five species of Nepenthes4,5,13,14 visits in March and April, they were rare during May. Furthermore, and in each case, remarkable insights into the interactions most pitchers that received copious amounts of feces in March between animals and Nepenthes have been gained. The poten- and April received none in May. The reasons for this are unknown tial for further discoveries using this method is therefore high and may involve changes in the foraging behaviour of T. mon- and through new and innovative experimental methodologies tana or perhaps a reduction in the quantity and/or quality of the now being employed, we anticipate many more exciting discov- lid gland . Through video recordings, we found that eries in the near future.

1188 Plant Signaling & Behavior Volume 5 Issue 10 References 9. Moran JA, Merbach MA, Livingston NJ, Clarke CM, 15. Jacobs GH, Neitz J. Spectral mechanisms and color Booth WE. Termite prey specialization in the pitcher vision in the tree shrew (Tupaia belangeri). Vision Res 1. Clarke CM. . , plant —evidence from stable 1986; 26:291-8. Sabah, Malaysia: Natural History Publications (Borneo) isotope analysis. Ann Bot 2001; 88:307-11. 16. Kawamichi T, Kawamichi M. Spatial organization and 1997. 10. Moran JA, Clarke CM, Hawkins BJ. From carnivore to territory in tree shrews (Tupaia glis). Anim Behav 1979; 2. Lloyd FE. The carnivorous plants. New York, NY USA: detritivore? Isotopic evidence for leaf litter utilization 27:381-93. Chronica Botanica 1942. by the tropical pitcher plant . Int J 17. Payne J, Francis CM, Phillipps K. A field guide to the 3. Juniper BE, Robins RJ, Joel D. The carnivorous plants. Plant Sci 2003; 164:635-9. of Borneo. Kota Kinabalu, Malaysia: Sabah UK: Academic Press 1989. 11. Merbach MA, Merbach DJ, Maschwitz U, Booth WE, Society 1985. 4. Bonn HF, Federle W. aquaplaning: Nepenthes Fiala B, Zizka G. Mass march of termites into the 18. Emmons LH. Frugivory in treeshrews (Tupaia). Am pitcher plants capture prey with the peristome, a fully deadly trap. Nature 2002; 415:37. Nat 1991; 138:642-9. wettable water-lubricated anisotropic surface. Proc Natl 12. Merbach MA, Zizka G, Fiala B, Merbach D, Booth 19. Smith AP. Tropical Alpine Plant Ecology. Ann Rev Ecol Acad Sci USA 2004; 101:14138-43. WE, Maschwitz U. Why a carnivorous plant cooperates Syst 1987; 18:137-58. 5. Bauer U, Bohn HF, Federle W. Harmless nectar source with an —selective defense against pitcher-destroying 20. Collins NM. The distribution of soil macrofauna on or deadly trap: Nepenthes pitchers are activated by rain, weevils in the myrmecophytic pitcher plant Nepenthes the west ridge of Gunung (Mount) Mulu, . condensation and nectar. Proc Royal Society B 2008; bicalcarata Hook.f. Ecotropica 2007; 13:45-56. Oecologia 1980; 44:263-75. 275:259-65. 13. Clarke CM, Bauer U, Lee CC, Tuen AA, Rembold 21. Kato M, Hotta M, Tamin R, Itino T. Inter- and Intra- 6. Clarke CM. Nepenthes of Sumatra & Peninsular K, Moran JA. Tree shrew lavatories: a novel nitrogen specific variation in prey assemblages and inhabit- Malaysia. Kota Kinabalu, Sabah, Malaysia: Natural sequestration strategy in tropical pitcher plants. Biol ant communities in Nepenthes pitchers in Sumatra. History Publications (Borneo) 2001. Lett 2009: 5:632-5. Tropical Zoology 1993; 6:11-25. 7. Phillipps A, Lamb A, Lee CC. Pitcher plants of Borneo, 14. Chin L, Moran JA, Clarke C. Trap geometry in three 22. Moran JA. Pitcher dimorphism, prey composition and nd 2 edn. Kota Kinabalu, Sabah, Malaysia: Natural giant montane pitcher plant species from Borneo is a the mechanisms of prey attraction in the pitcher plant History Publications (Borneo) 2009. function of tree shrew body size. New Phytol 2010; in Borneo. J Ecol 1996; 84:515-25. 8. Clarke CM, Kitching RL. Swimming and pitcher 186:461-70. plants: a unique ant-plant interaction from Borneo. J Trop Ecol 1995; 11:589-602.

www.landesbioscience.com Plant Signaling & Behavior 1189