Fenestration: a window of opportunity for carnivorous plants

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Fenestration: a window of opportunity for carnivorous plants H. Martin Schaefer and Graeme D. Ruxton Biol. Lett. 2014 10, 20140134, published 30 April 2014

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Fenestration: a window of opportunity for carnivorous plants H. Martin Schaefer1 and Graeme D. Ruxton2 1

Research Cite this article: Schaefer HM, Ruxton GD. 2014 Fenestration: a window of opportunity for carnivorous plants. Biol. Lett. 10: 20140134. http://dx.doi.org/10.1098/rsbl.2014.0134

Received: 18 February 2014 Accepted: 8 April 2014

Subject Areas: ecology, behaviour, evolution Keywords: animal behaviour, plant – animal interactions, signal evolution

Author for correspondence: H. Martin Schaefer e-mail: martin.schaefer@biologie. uni-freiburg.de

Electronic supplementary material is available at http://dx.doi.org/10.1098/rsbl.2014.0134 or via http://rsbl.royalsocietypublishing.org.

Faculty of Biology, Department of Evolutionary Biology and Animal Ecology, University of Freiburg, Hauptstrasse 1, Freiburg 79104, Germany 2 School of Biology, University of St Andrews, St Andrews KY16 9TH, UK A long-standing but controversial hypothesis assumes that carnivorous plants employ aggressive mimicry to increase their prey capture success. A possible mechanism is that pitcher plants use aggressive mimicry to deceive prey about the location of the pitcher’s exit. Specifically, species from unrelated families sport fenestration, i.e. transparent windows on the upper surfaces of pitchers which might function to mimic the exit of the pitcher. This hypothesis has not been evaluated against alternative hypotheses predicting that fenestration functions to attract insects from afar. By manipulating fenestration, we show that it does not increase the number of Drosophila flies or of two ant species entering pitchers in Sarracenia minor nor their retention time or a pitcher’s capture success. However, fenestration increased the number of Drosophila flies alighting on the pitcher compared with pitchers of the same plant without fenestration. We thus suggest that fenestration in S. minor is not an example of aggressive mimicry but rather functions in long-range attraction of prey. We highlight the need to evaluate aggressive mimicry relative to alternative concepts of plant–animal communication.

1. Introduction Aggressive mimicry occurs if predators use deceptive communication to deceive their prey. A central element of aggressive mimicry is that prey mis-identify the predator as something benign or even attractive [1]. Aggressive mimicry has received less scientific enquiry than mimicry by prey, and it is often not clearly differentiated from sensory exploitation as alternative strategies of communication if the interests between signallers and perceivers diverge [1,2]. Sensory exploitation occurs if traits are selected that are more effective in stimulating the sensory systems of perceivers, without the implication of fooling cognitive systems associated with object identification that is at the heart of mimicry. It has long been suggested that carnivorous plants use aggressive mimicry to deceive prey by mimicking flowers [3,4], but the current evidence for it is not strong [5,6]. However, pitcher plants may use aggressive mimicry to deceive prey as to the position of the aperture through which they could leave the pitcher [5]. Although 25% of a total of approximately 600 carnivorous plant species are pitcher plants, this hypothesis has not been evaluated against alternative hypotheses (such as prey attraction). Pitcher plants have leaves modified into pitcher-shaped pitfall traps. They belong to three unrelated families: Cephalotoceae, Nepenthaceae and Sarraceniaceae of which the latter two often sport transparent ‘windows’ on the upper surfaces of pitchers called fenestrations (figure 1). Juniper et al. [5] hypothesized that fenestrations function to confuse insects that have flown into the pitcher; thus using mimicry of the true aperture to aid retention of prey. Implicit in this idea is the assumption that the longer a prey is retained in the pitcher, the higher the likelihood of it alighting on or falling into the fluid at the base of the pitcher. Fittingly, fenestrations are more commonly located close to the aperture but on its opposite side and not on the basal part of the pitcher (figure 1). As yet, the physiological and ecological functions of fenestrations are underexplored (but see [7]). Adaptive

& 2014 The Author(s) Published by the Royal Society. All rights reserved.


3. Results 2. Material and methods We obtained 22 S. minor plants from a local supplier (Ga¨rtnerei Carow) and used Drosophila melanogaster (wild-type) as well as two ant species (Lasius niger and Lasius flavus) as prey. Drosophila melanogaster was used because it is a suitable species to address both short-range and long-range attraction. The two ant species were used because ants are the most common prey species of Sarracenia plants, constituting up to 97% of prey biomass [8]. To assess the effects of fenestrations, we painted these areas with either clear varnish (Klarlack, Praktiker, seidenmatt) so that they remained transparent in the control treatment or with green varnish (Buntlack, Praktiker, seidenmatt) so that they became opaque and green in the manipulative treatment (figure 1). Varnish was used in both treatments so that olfactory profiles of both our experimental and control groups were similarly modified. Neither of the two treatments reflected in the UV. Individual plants sport several pitchers, and for each experiment we compared the data of control and manipulated traps within individual plants. Experiments were conducted from 10.00 to 16.00 with temperature varying from 18 to 258. The illumination was variable from sunny to overcast. We found no effects of time of the day, temperature or illumination on the results. We matched pairs of pitchers on a single plant by age and size, then randomly allocated one to our control (clear varnish) treatment and one to our manipulation (green varnish) treatment.

A total of 141 Drosophila individuals (70% of those tested) entered the pitcher from the lip (mean latency 2.03 min + 0.28 (s.e.)). There were no effects of treatment or plant individual on either the latency or the number of flies entering the pitcher (GLMM, all effects p . 0.2). The retention time of flies within the pitcher (3.39 min + 0.32) did not depend on treatment or plant individual (GLMM, both effects p . 0.5). A total of 78 (55% of flies that had entered the pitcher) were caught by the plants. Again, there was no effect of treatment or plant individual upon survival of prey (GLMM, both effects p . 0.5). Likewise, fenestration had no effect on L. niger and L. flavus ants. A total of 42 ants entered the pitchers. They had no preference for pitchers with fenestration nor did they stay longer in those pitchers (GLMM, all effects p . 0.3). A total of 20 ants were caught by the plants, but there was no effect of treatment or plant individual upon survival of ants (GLMM, both effects p . 0.6). Hairs curved downward inside the pitcher apparently impeded upward movement of prey. Once inside the pitcher, both L. niger and D. melanogaster moved significantly more often downwards than expected by chance (Binomial-test both p , 0.01). We found evidence that fenestrations function to attract prey towards the plants. Control pitchers attracted

Biol. Lett. 10: 20140134

communicative function appears plausible; however, aperturemimicry is not the only possible mechanism by which fenestrations might influence prey capture. Here, we tested three possible ecological functions of fenestrations in the pitcher plant Sarracenia minor: attraction to the plant from afar, attraction into the pitcher and retention within it. Only the latter is a clear prediction of aggressive mimicry. If fenestrations function in aggressive mimicry of the aperture, we predicted that prey (entering the pitcher) should be more frequently captured or retained longer in pitchers with fenestration compared with pitchers without fenestration. Yet, as fenestrations could also have an attractive effect of luring prey into the trap (e.g. by increasing light levels within the trap [7]), we tested for that effect as well. Additionally, fenestrations may exploit sensory biases of the prey towards contrasting plants. We thus tested whether insects alighted preferentially on pitchers with fenestration compared with pitchers of the same plants without fenestration.

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Figure 1. Fenestration is located close to the aperture of the pitcher trap. The pitcher on the right is a control with fenestration covered by transparent varnish, whereas fenestration has been covered by green varnish in the manipulation treatment ( pitcher on the left). (Online version in colour.)

To test for the effect of fenestration on luring prey from the lip of the pitcher (where nectar is naturally produced) into the traps, we let 200 D. melanogaster crawl singly from an Eppendorf tube onto the lip of the aperture of a pitcher (i.e. 100 per treatment and five successively per individual pitcher). The animals had no access to food for approx. 30 h and were anaesthetized with CO2 at least 24 h before the experiment and put singly into an Eppendorf tube at room temperature. A 24 h recovery time after anaesthetization is recommended for behavioural experiments [9]. They were not manipulated in any other way nor immobilized. We repeated the experiment with 40 ants of both L. niger and L. flavus which were not anaesthetized. All insects were unconstrained; thus, they could move in any direction from the lip including flying away from it (D. melanogaster only). We compared the number of flies and ants entering the trap and the latency (up to 15 min) to do so between treatments. To test for the effect of retention, we compared between treatments the time that individuals of each species spent inside the pitcher, and the survival rate of individuals that had entered the trap. We tested for these effects with separate linear mixed models for latency, retention time and survival as dependent variables and treatment (clear or green varnish) as a fixed effect and plant individual as a random effect. We pooled both ant species when analysing their survival, given that we found no difference between the two species. Analyses were done with the package lme4 in R. To test for the effect of fenestration on attracting potential prey to the plant, we positioned 22 S. minor plants, each with one control pitcher and one manipulated one, singly into a cage (30 30 18 cm) that held 50 – 90 Drosophila individuals. We then compared the number of Drosophila individuals alighting on each pitcher within 5 min with a paired t-test. This test was only conducted with D. melanogaster. Capture success was high in our experiments. Hairs on the inside of the pitchers were curved downwards. We hypothesized that they hindered prey from leaving the pitcher. To test that function, we cut a hole into the pitcher (below the fenestration) and positioned singly 15 L. niger and 15 D. melanogaster within an Eppendorf tube on that hole. Using Binomial tests, we analysed whether they preferentially moved downwards.


8 6 4 2 0 no fenestration

Figure 2. More Drosophila flies alighted on pitchers with fenestration compared with pitchers of the same plant without fenestration. Black bars indicate medians, boxes show interquartile range and whiskers the 10th and 90th percentile. significantly more flies than manipulated ones ( paired Wilcoxon test, V ¼ 118, p ¼ 0.047; figure 2).

4. Discussion In contrast to the prediction of aggressive mimicry, fenestrations did not increase the retention time of D. melanogaster and ants within pitchers nor did they lure them from the lip into the trap. We therefore conclude that fenestrations in S. minor do not function to confuse prey inside the pitcher about its exit location. Fenestration also did not lure more prey into the pitcher by increasing light levels inside of it. Instead, the main function of fenestrations in S. minor may be to attract potential prey from afar; in our experiment, more Drosophila flies landed upon pitchers with fenestration compared with those without fenestration. The attractive function of fenestrations is plausible given that well-studied insects like bees and bumblebees show biases, sometimes innate, towards contrasting floral displays [10]. Whether the typical prey of pitcher plants exhibits such biases is unknown. An alternative explanation appears unlikely, i.e. that prey are attracted to fenestrations because they are associated with higher nectar rewards or with more attractive volatiles. First, in our experiments we used a within-individual comparison that minimizes difference in scent and nutritional rewards. Second, our manipulation

Acknowledgements. We thank Heide Teubner for help in conducting experiments, which conformed to local laws. We particularly thank Simcha Lev-Yadun, David Wilkinson and one anonymous referee for very insightful comments on earlier versions.

References 1. 2.

3. 4.

5.

Stevens M. 2012 Sensory ecology, behaviour, and evolution. Oxford, UK: Oxford University Press. Schaefer HM, Ruxton GD. 2009 Deception in plants: mimicry or perceptual exploitation? Trends Ecol. Evol. 24, 676 –685. (doi:10.1016/j.tree.2009.06.006) Wickler W. 1968 Mimicry. Mu¨nchen, Germany: Kindler Verlag. Bessie´re J-M, Gue´roult M, Lim LBL, Marshall DJ, Hossaert-McKey M, Guame L. 2010 Flower-scent mimicry masks a deadly trap in the carnivorous plant Nepenthes rafflesiana. J. Ecol. 98, 845– 856. (doi:10.1111/j.1365-2745.2010.01665.x) Juniper BE, Robins RJ, Joel DM. 1989 The carnivorous plants. London, UK: Academic Press.

6.

7.

8.

9.

Schaefer HM, Ruxton GD. 2011 Plant-animal communication, p. 320. Oxford, UK: Oxford University Press. Moran AJ, Clarke C, Gowen BE. 2012 The use of light in prey capture by the tropical pitcher plant Nepenthes aristolochioides. Plant Signal. Behav. 7, 957 –960. (doi:10.4161/psb.20912) Green ML, Horner JD. 2007 The relationship between prey capture and characteristics of the carnivorous pitcher plant, Sarracenia alata Wood. Am. Midl. Nat. 158, 424 –431. (doi:10.1674/00030031(2007)158[424:TRBPCA]2.0.CO;2) Greenspan RJ. 1997 Fly pushing: the theory and practice of Drosophila genetics. Cold

Spring Harbor, NY: Cold Spring Harbor Laboratory Press. 10. Lunau K, Fieselmann G, Heuschen B, van de Loo A. 2006 Visual targeting of components of floral colour patterns in flower-naive bumblebees. Naturwissenschaften 93, 325–328. (doi:10.1007/s00114-006-0105-2) 11. Merbach MA, Zizka G, Fiala B, Maschwitz U, Booth WE. 2001 Patterns of nectar secretion in five Nepenthes species from Brunei Darussalam, Northwest Borneo, and implications for ant–plant relationships. Flora 196, 153– 160. 12. Jackson RR, Cross FR. 2013 A cognitive perspective on aggressive mimicry. J. Zool. 290, 161 –171. (doi:10.1111/jzo.12036)

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fenestration

changed only a small part of the natural surface of pitchers in each treatment. Our experiments help to tease apart aggressive mimicry from other forms of (aggressive) communication in plant carnivory. Our experiments suggest that fenestrations can also influence attractiveness and/or conspicuousness of pitchers in a way that can enhance prey capture rates. Overall, we conclude that fenestrations may be selected because of their effects on long-range attraction rather than aggressive mimicry of pitcher apertures leading to enhanced prey retention. Typically, capture efficiency of carnivorous plants is low, for example ranging from 0.34 to 1.6% in five Nepenthes species [11]. Because hairs inside the pitcher effectively function to retain prey, we suggest that pitchers have been selected to exploit the senses of potential prey in ways that enhance visitation rate not retention efficiency. Mimicry is an intuitively appealing idea; but this attractiveness can lead to premature assumption of the existence of mimicry in specific systems without careful consideration and elimination of simpler, more parsimonious, explanations. Our work should not be interpreted as evidence that fenestrations do not ever function in aggressive mimicry. Indeed, fenestration in Nepenthes aristolochioides influences capture rates through increased attraction of prey into the pitcher [7]. While the study reported in [7] did not differentiate the frequencies with which Drosophila flies flew into pitcher with shaded versus unshaded fenestration from variation in the likelihood that they failed to emerge from the pitcher, it clearly showed that fenestration can be involved in short-range attraction of prey. Given that fenestration can apparently function in long- and short-range attraction, we recommend exploration of the concept of aggressive mimicry in concert with other mechanisms by which sensory and cognitive exploitation of potential prey of carnivorous plants might occur. Recently, Jackson & Cross [12] argued that aggressive mimicry holds exceptional potential for advancing our understanding of animal cognition. However, to explore that potential most effectively, we must first demonstrate the importance of aggressive mimicry in candidate study systems.


no. Drosophila alighting

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