Tree shrew lavatories: a novel nitrogen sequestration strategy in a tropical pitcher plant

Charles M. Clarke, Ulrike Bauer, Ch'ien C. Lee, Andrew A. Tuen, Katja Rembold, Jonathan A. Moran

Abstract

Nepenthes pitcher plants are typically carnivorous, producing pitchers with varying combinations of epicuticular wax crystals, viscoelastic fluids and slippery peristomes to trap arthropod prey, especially ants. However, ant densities are low in tropical montane habitats, thereby limiting the potential benefits of the carnivorous syndrome. Nepenthes lowii, a montane species from Borneo, produces two types of pitchers that differ greatly in form and function. Pitchers produced by immature plants conform to the ‘typical’ Nepenthes pattern, catching arthropod prey. However, pitchers produced by mature N. lowii plants lack the features associated with carnivory and are instead visited by tree shrews, which defaecate into them after feeding on exudates that accumulate on the pitcher lid. We tested the hypothesis that tree shrew faeces represent a significant nitrogen (N) source for N. lowii, finding that it accounts for between 57 and 100 per cent of foliar N in mature N. lowii plants. Thus, N. lowii employs a diversified N sequestration strategy, gaining access to a N source that is not available to sympatric congeners. The interaction between N. lowii and tree shrews appears to be a mutualism based on the exchange of food sources that are scarce in their montane habitat.

1. Introduction

Nepenthes lowii is unusual among carnivorous plants in that it produces pitchers lacking the features normally associated with arthropod prey capture, the normal mode of nutrition in pitcher plants (Juniper et al. 1989). The pitchers of most Nepenthes species studied to date employ combinations of epicuticular wax crystals, viscoelastic fluids and slippery peristomes to capture and retain prey, most notably Formicidae (Moran 1996; Gaume et al. 2002; Bohn & Federle 2004; Gorb et al. 2004; Gaume & Forterre 2007; Bauer et al. 2008; Di Giusto et al. 2008). Owing to its arrangement of overlapping epidermal cells arranged in radial ridges, the peristome (a collar-like structure that lines the pitcher rim) provides an anisotropic, highly wettable surface that in species such as Nepenthes rafflesiana and Nepenthes bicalcarata is slippery for arthropods when wet (Bohn & Federle 2004; Bauer et al. 2008) and is fundamental to those species’ prey-trapping mechanisms.

Nepenthes produce dimorphic pitchers: ‘terrestrial’ pitchers rest on the ground and are usually ovoid in shape, whereas ‘aerial’ pitchers are funnel-shaped and produced above ground level, supported by attachment of the pitcher tendril to adjacent vegetation (Clarke 1997). Terrestrial N. lowii pitchers are produced only on immature plants, are ‘typical’ in structure and apparently function as arthropod traps (figure 1a), but the aerial pitchers are heavily lignified and the lid is held away from the mouth, which is very broad (figures 1d and 2). The inner surfaces are rough, lack epicuticular wax crystals and are not steeply inclined, while the rim is narrow and the peristome is reduced to a series of small bumps. The lower surface of the lid in both pitcher types is covered with coarse bristles and specialized nectar glands that secrete a buttery, white exudate. Clarke (1997) reported that aerial pitchers of N. lowii from Mount Pagon in Brunei contained large amounts of vertebrate faeces, but no invertebrate prey. This combination of aerial pitcher characteristics and contents indicated a possible interaction with vertebrates and a reduction in the features normally associated with arthropod capture. We conducted field observations and experiments on N. lowii and the animals that visit its pitchers to determine the primary source of foliar nitrogen (N), testing the hypothesis that N. lowii derives a measurable amount of the nutrient from vertebrate faeces.

Figure 1.

(a) Close-up of a terrestrial pitcher of Nepenthes lowii, showing the ridged peristome and waxy bloom on the inner rear surface (epicuticular wax crystals) typical of insect-trapping Nepenthes species. (b and c) Microstructure of terrestrial pitcher peristome, showing large- (b) and small-scale (c) radial ridges, the latter formed by overlapping epidermal cells. (d) Aerial N. lowii pitcher. (e and f) Microstructure of aerial pitcher peristome. Note flattening of the large-scale ridges (e) and partial or complete reduction in small-scale ridges (f), resulting in loss of wettability and trapping capacity. Scale bars, (b and e), 1 mm; (c and f), 200 µm. i, accumulated exudates; ii, freshly deposited Tupaia montana faeces; iii, nectar gland.

Figure 2.

Tupaia montana feeding at an aerial pitcher of Nepenthes lowii.

2. Material and methods

The study was conducted in montane cloud forest on Gunung Mulu, Sarawak, Malaysia, in June 2008 (4.045835 N, 114.924967 E, 1800–2300 metres above sea level). Observations were made to determine the appropriate experimental methods and revealed that terrestrial N. lowii pitchers trap arthropod prey, including Hymenoptera (Formicidae), Diptera, Orthoptera, Coleoptera and Arachnida. In contrast, aerial pitchers do not trap measurable quantities of arthropod prey (although occasional catches are likely), but are visited by mountain tree shrews (Tupaia montana), which feed on the exudate on the pitcher lid and defaecate into the pitchers (see the electronic supplementary material, methods). We then performed observations to document the nature of the interaction between N. lowii and T. montana, focusing on the value of T. montana faeces to N. lowii. Descriptive observations that are relevant to the design and underlying assumptions in our experiments are provided in the electronic supplementary material.

Video cameras (model DCR-HC 46E, Sony Corporation, Tokyo, Japan) in waterproof casings were set up on tripods, each focusing on one aerial pitcher of N. lowii. Recordings were made on three aerial pitchers from different plants. A total of 35 h of recordings were made, at an approximate recording rate of 5 h d−1, between noon and sunset over a period of 7 days.

Stable isotope analyses were performed to determine the origin of foliar N in N. lowii. For full details of the procedures and experimental design, see the electronic supplementary material, methods. The following samples were collected, all from separate plants: laminar (leaf blade) material from the leaf above the youngest functional N. lowii aerial pitcher of mature plants (n = 20); lamina from the leaf above the youngest functional aerial pitcher of Nepenthes muluensis plants (a sympatric ‘typical’ non-faeces-using species, n = 20) and fresh T. montana faeces from aerial N. lowii pitchers (n = 10). δ15N values were determined using a PDZ Europa ANCA-GSL elemental analyser and PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd, Crewe, UK). Using the ISOERROR1_03 two end-member mixing model as a basis (Shearer & Kohl 1989; Phillips & Gregg 2001), we estimated the 95 per cent confidence interval (CI) for N input from tree shrew faeces to mature N. lowii plants based on the following statistic: Embedded Image where δ15Nlowii aerial, δ15Nmuluensis and δ15Nfaeces represent the mean values for laminar material taken from above the newest fully functional aerial pitcher of N. lowii, laminar material taken from above the newest fully functional aerial pitcher of N. muluensis and T. montana faeces, respectively. See methods in the electronic supplementary material for an explanation of the model. All data were tested for homoscedasticity and normality; datasets not satisfying these assumptions were transformed and then re-tested prior to running parametric tests.

For s.e.m., pitchers were excised from pitchers in the field and transferred to glutaraldehyde (4% in 0.1 M sodium phosphate buffer) for fixation. They were then stained with osmium tetroxide, dehydrated via an ethanol dilution series (25, 50, 75 and 96%, in 1 h steps) and critical point dried. The samples were sputter coated with gold and examined in a FEI XL30-FEG scanning electron microscope (FEI Co., Hillsboro, OR, USA) at 10 kV.

3. Results

In a typical interaction, T. montana jumps onto an aerial pitcher to feed on the exudate produced by the glands on the underside of the lid (figure 2, electronic supplementary material, video). The animal then defaecates into the pitcher (figure 1d) and scent-marks the lid using its anogenital glands. Individual T. montana exhibit highly regulated foraging behaviour, moving from one pitcher to another to feed. The scent marking of pitchers identifies them as a valuable resource (Kawamichi & Kawamichi 1979). Tupaia montana routinely visits the same ‘circuit’ of pitchers, spending much of its foraging time on pitcher visits (from analysis of video recordings, mean interval (±s.e.) between pitcher visits = 96 ± 7 min, n = 2100 min (5 h d−1, 7 days)). Although T. montana is frugivorous, it is unable to process fibrous parts of fruits and can only extract and swallow the sugar-rich juices (Emmons 1991). We observed nectar sources to be scarce above 1800 m on Gunung Mulu, but this apparent deficit is offset by N. lowii, the lid nectaries of which produce the most copious quantities of exudate observed thus far among the Nepenthaceae (Clarke 1997).

The structure of N. lowii aerial pitchers facilitates both the feeding of T. montana on the lid exudates and defaecation into the pitchers. The thickened tendrils and robust pitchers can support an animal the size of T. montana (ca 150 g; Emmons 1991) without breaking. The highly reduced peristome enables T. montana to grip the pitcher rim without slipping, while the shape of the mouth and orientation of the lid impede feeding from the side or rear, thereby manoeuvring the animal to sit astride the pitcher orifice, maximizing the likelihood that the animal's hindquarters are positioned over the orifice while it feeds (figure 2).

Using a two end-member isotopic mixing model for 15N, we estimate that between 57 and 100 per cent of foliar N in N. lowii plants possessing aerial pitchers is derived from faeces (95% CI; n = 20). The remainder is derived from other sources, including the soil (via the roots), prey capture and leaf litter, which accumulates in pitchers, particularly when they are old and no longer visited by T. montana (detritus that falls into pitchers that are visited by T. montana is usually knocked out of the pitchers when the animals climb on and off the rim). Foliar N concentrations were significantly higher for N. lowii plants producing aerial pitchers than for N. muluensis, which had no access to faecal material (mean ± 1 s.e. 1.58 ± 0.10 and 1.21 ± 0.07%, respectively; t = 2.90, p < 0.01, d.f. = 38, t-test).

Nepenthes lowii aerial pitchers appear to have lost their slipperiness (electronic supplementary material, table). This change is because of the extensive reduction in the microstructure, resulting in flattening of both the larger radial ridges and small-scale ridges and the relatively gentle incline of the pitcher walls (figure 1df). In contrast, these features are retained in terrestrial pitchers (figure 1ac).

4. Discussion

Immature N. lowii plants are carnivorous, producing terrestrial pitchers that trap arthropod prey in the manner typical of the genus, but the initiation of aerial pitcher production signals the switch to T. montana faeces as the primary N source in this species. The interaction with T. montana enables N. lowii to exploit a N source that is unavailable to the sympatric N. muluensis. A possible explanation for this strategy is that montane Nepenthes are subject to an evolutionary pressure that is absent in lowland habitats: scarcity of ants as potential prey (Collins 1980; Samson et al. 1997). For example, on Gunung Mulu, Formicidae are 28 times more abundant per unit area at the foot of the mountain than at the summit (509 and 18 individuals m−2 at 130 and 2376 metres above sea level, respectively; Collins 1980). Givnish et al. (1984) proposed a cost–benefit model for carnivory in plants, stating that the syndrome would be of most benefit to plants in sunny, moist, nutrient-deficient habitats, such as acid bogs or tropical mountain tops. In habitats that would normally favour plant carnivory (e.g. montane cloud forests, where N. lowii grows), a scarcity of potential prey represents a limitation on the benefits of the syndrome. Nepenthes lowii has responded to this by targeting an alternative N source (T. montana faeces) as soon as the plant produces aerial pitchers.

The association between N. lowii and T. montana appears to be mutualistic, based on the exchange of nutritional resources that are scarce on Gunung Mulu. The importance of the interaction to each species is not yet known, but the geographical range of N. lowii falls entirely within that of T. montana (Payne et al. 1985; Clarke 1997). As faecal inputs by T. montana account for 57–100% of foliar N in mature N. lowii, the association is likely to be of considerable value to the plant. However, T. montana has a wide geographical range in north-western Borneo and is known from sites where N. lowii is absent (Payne et al. 1985), so some populations do not interact with N. lowii.

These results contribute to a growing body of evidence that Nepenthes exploit a range of N sources, including arthropods (Moran 1996; Merbach et al. 2002), leaf litter (Moran et al. 2003) and vertebrate faeces. Nepenthes lowii is the only plant species that is currently known to sequester N from both vertebrate and invertebrate sources.

Acknowledgements

We thank A. Moran for editing the manuscript, Brian and Sue Clark for their assistance at Gunung Mulu National Park, Sarawak Forestry Department for permission to conduct this research and MASWings/Malaysia Airline System for assisting with the transport of equipment. This research was partly funded by a grant from the Cambridge Isaac Newton Trust.

Footnotes

    • Received April 18, 2009.
    • Accepted May 14, 2009.

References

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