New Phytologist Research Slippery or sticky? Functional diversity in the trapping strategy of Nepenthes carnivorous plants Vincent Bonhomme1, Herve´ Pelloux-Prayer1, Emmanuelle Jousselin2, Yoe¨l Forterre3, Jean-Jacques Labat4 and Laurence Gaume1 1Universite´ Montpellier II, CNRS, UMR AMAP: botAnique et bioinforMatique de l’Architecture des Plantes, CIRAD – TA A51 ⁄ PS2 Boulevard de la Lironde, F-34398 Montpellier cedex 5, France; 2INRA, UMR CBGP, Campus International de Baillarguet, CS 30016, 34988 Montferrier-sur-Lez, France; 3CNRS, Universite´ de Provence IUSTI, Technopole Chaˆteau-Gombert, 13000 Marseille, France; 4Pe´pinie`re Nature et Paysages et Jardin Botanique de Plantes Carnivores, Peyrusse-Massas, France Summary Author for correspondence: • The pitcher-shaped leaves of Nepenthes carnivorous plants have been consid- Vincent Bonhomme ered as pitfall traps that essentially rely on slippery surfaces to capture insects. But Tel: +33 (0)4 67 61 7166 a recent study of Nepenthes rafflesiana has shown that the viscoelasticity of the Email: [email protected] digestive fluid inside the pitchers plays a key role. Received: 4 December 2010 • Here, we investigated whether Nepenthes species exhibit diverse trapping strat- Accepted: 8 February 2011 egies. We measured the amount of slippery wax on the pitcher walls of 23 taxa and the viscoelasticity of their digestive liquid and compared their retention effi- New Phytologist (2011) 191: 545–554 ciency on ants and flies. doi: 10.1111/j.1469-8137.2011.03696.x • The amount of wax was shown to vary greatly between species. Most mountain species exhibited viscoelastic digestive fluids while water-like fluids were predomi- nant in lowland species. Both characteristics contributed to insect trapping but wax Key words: biological trade-off, carnivorous pitcher plant, digestive fluid, leaf wax, was more efficient at trapping ants while viscoelasticity was key in trapping insects Nepenthes, trapping strategy, viscoelasticity. and was even more efficient than wax on flies. Trap waxiness and fluid visco- elasticity were inversely related, suggesting the possibility of an investment trade- off for the plants. • Therefore Nepenthes pitcher plants do not solely employ slippery devices to trap insects but often employ a viscoelastic strategy. The entomofauna specific to the plant’s habitat may exert selective pressures, favouring one trapping strategy at the expense of the other. trapping strategies and stem from their adaptation to differ- Introduction ent arthropod fauna found in their habitats. But it is not Carnivorous plants circumvent the nutrient shortage charac- known whether the trapping strategies of these pitcher plants terizing the habitats they colonize by deriving key nutrients are actually functionally diverse, and whether this diversity is from arthropods which they attract, trap and digest in spe- linked to ecological characteristics of their environment. cialized leaves (Juniper et al., 1989; Ellison & Gotelli, 2001). Nepenthes species are known to vary in their arthropod prey Nepenthes (Caryophyllales: Nepenthaceae) is a climbing and assemblage (Kato et al., 1993; Adam, 1997; Merbach et al., carnivorous plant genus characterized by leaves modified as 2002) and even in their N-sequestration strategies (Moran pitcher traps (Fig. 1). It encompasses > 100 species, mainly et al., 2010), with some outlying species moving away from a distributed in southeastern Asia, with the islands of Borneo purely carnivorous habit by deriving part of their nitrogen and Sumatra as hotspots of diversity (Clarke, 1997, 2001; from leaf detritus (Moran et al., 2003), vertebrate faeces Cheek & Jebb, 2001; McPherson, 2009), and colonize vari- (Clarke et al., 2009; Chin et al., 2010), or from the nutri- ous habitats, including coastal lowlands, cliffs and high- tional service of a symbiotic hunter ant (Bonhomme et al., altitude forests, with a high rate of endemism (Clarke, 1997; 2011). The trapping strategy of strictly insectivorous species McPherson, 2009). Nepenthes species show a great diversity (the vast majority of these pitcher plants) has never been of pitcher morphologies, which could reflect differences in investigated in a comparative study within the genus. Ó 2011 The Authors New Phytologist (2011) 191: 545–554 545 New Phytologist Ó 2011 New Phytologist Trust www.newphytologist.com New 546 Research Phytologist (a) (b) (c) (d) Fig. 1 Experimental designs used to test the effects of pitcher waxiness and fluid viscoelasticity on ants and flies. (a) Ants were handled using a soft tube and allowed to walk freely on the pitcher rim. (b) The jar containing the experimental flies was opened and linked by a gauze mesh to a glass beaker covering the upper part of the pitcher. (c) The photograph shows a Nepenthes pitcher with a waxy zone (pale area, arrow) from which crystalline wax (also see scanning electron microscope view, inset) was extracted using hot chloroform. (d) The extensional rheometry measurements of the digestive fluid were made by high-speed video-recording and analyses of the thinning dynamics of a filament (measure of its diameter D relative to its initial diameter D0) created by vertically stretching a droplet of digestive fluid between two plots 3 cm apart. Filament lifespan was used to estimate the fluid viscoelasticity. Plants in the Nepenthes genus have long been thought to water may have unsuspected elastic properties that result in function as simple pitfall traps relying on slippery surfaces high trapping capacities. Therefore, the viscoelastic charac- that decrease insect adhesion (Juniper & Burras, 1962; ter of the digestive fluid might have remained cryptic in a Juniper et al., 1989; Gaume et al., 2002, 2004; Gorb et al., number of species and could be far more widespread than 2005) and wettable surfaces that cause insect aquaplaning expected in the Nepenthes genus. Interestingly, N. rafflesiana (Bohn & Federle, 2004; Bauer et al., 2009). But in 2007, var. typica bears pitchers with a waxy zone and mainly traps Nepenthes rafflesiana was shown experimentally to use ants during its juvenile phase; but as the plant ages, the waxy another mechanism. It produces a digestive liquid partly layer is lost (Gaume & Di Giusto, 2009) and the upper made up of long-chain polymers, the viscoelastic properties pitchers, which are only produced in the adult phase, con- of which have a marked effect on insect retention (Gaume tain a highly viscoelastic fluid that proves to be very efficient & Forterre, 2007; Di Giusto et al., 2008). Even when against flying insects (Di Giusto et al., 2008). By contrast, greatly diluted by water, the digestive liquid in N. rafflesiana the elongated traps of N. rafflesiana var. elongata keep their has sufficient elastic properties to trap insects (Gaume & waxy layer throughout plant ontogeny and the plant mainly Forterre, 2007). This not only means that the digestive captures ants (Gaume & Di Giusto, 2009). liquid might be crucial for the capture success of this tropi- This casts doubts on the common belief that all cal pitcher plant that is often subjected to heavy rains, but Nepenthes species exhibit the same trapping strategy based also that even species with a liquid viscosity similar to that of on the slipperiness of their pitchers. This study explores New Phytologist (2011) 191: 545–554 Ó 2011 The Authors www.newphytologist.com New Phytologist Ó 2011 New Phytologist Trust New Phytologist Research 547 whether viscoelastic fluids are common among Nepenthes f. ex Burb., N. copelandii Merr. ex Macfarl., N. eymae Sh. species and whether they are produced in addition to or at Kurata, N. glabrata J. R. Turnbull & A. T. Middleton, the expense of a slippery waxy layer. It also investigates the N. gracilis Korth., N. macrophylla (Marabini) Jebb & effects of each of these two retentive devices on the capture Cheek, N. madagascariensis Poir., N. mindanaonensis Sh. of different insect types. Kurata, N. mirabilis var. typica (Lour.) Druce, N. rafflesiana To address these questions, we studied the functional var. elongata Hort., N. tenuis Nerz & Wistuba and N. vogelii diversity of Nepenthes pitcher plants in a sample of species Schuit. & de Vogel. Although, the thickness and length of differing in their geographic origins and habitats. We mea- the waxy layer do not to the naked eye seem to vary when the sured the traits directly involved in the ‘slippery’ and pitcher ages, the fluid viscoelasticities are reported to ‘viscoelastic’ strategies, i.e. waxiness (quantity and density decrease when the pitchers age, some of them becoming of wax coating the inner pitcher walls) and viscoelasticity apparently watery 11–14 d after pitcher opening in N. (relaxation time) of the digestive liquid. We used insect bio- rafflesiana (Bauer et al., 2009). To reduce as far as possible assays to compare the retentive ability of different Nepenthes development-induced heterogeneity between pitchers, waxi- species and measure how efficiently pitcher waxiness and ness and viscoelasticity were measured for each species in the fluid elasticity contribute to the retention of each type of youngest pitcher that had opened in the previous week. prey. Finally, we investigated whether waxiness and visco- elasticity are correlated. Measurement of insect retention ability Experiments comparing the trapping ability of the 12 Materials and Methods Nepenthes species were carried out in April 2007 under homogeneous temperature and hygrometry conditions (26– Studied plants 27°C, 80–90%). Retention rates in the 12 species were One