Mutualisms between plants and animals shape the world’s ecosystems []. In such interactions, achieving contact with the partner species is imperative. Plants regularly advertise themselves with signals that specifically appeal to the partner’s perceptual preferences []. For example, many plants have acquired traits such as brightly colored, fragrant flowers that attract pollinators with visual, olfactory, or—in the case of a few bat-pollinated flowers—even acoustic stimuli in the form of echo-reflecting structures []. However, acoustic attraction in plants is rare compared to other advertisements and has never been found outside the pollination context and only in the Neotropics. We hypothesized that this phenomenon is more widespread and more diverse as plant-bat interactions also occur in the Paleotropics. In Borneo, mutualistic bats fertilize a carnivorous pitcher plant while roosting in its pitchers []. The pitcher’s orifice features a prolonged concave structure, which we predicted to distinctively reflect the bats’ echolocation calls for a wide range of angles. This structure should facilitate the location and identification of pitchers even within highly cluttered surroundings. Pitchers lacking this structure should be less attractive for the bats. Ensonifications of the pitchers around their orifice revealed that this structure indeed acts as a multidirectional ultrasound reflector. In behavioral experiments where bats were confronted with differently modified pitchers, the reflector’s presence clearly facilitated the finding and identification of pitchers. These results suggest that plants have convergently acquired reflectors in the Paleotropics and the Neotropics to acoustically attract bats, albeit for completely different ecological reasons.

Results and Discussion

12 Jandér K.C.

Herre E.A. Host sanctions and pollinator cheating in the fig tree-fig wasp mutualism. 10 Grafe T.U.

Schöner C.R.

Kerth G.

Junaidi A.

Schöner M.G. A novel resource-service mutualism between bats and pitcher plants. 11 Schöner C.R.

Schöner M.G.

Kerth G.

Grafe T.U. Supply determines demand: influence of partner quality and quantity on the interactions between bats and pitcher plants. 13 Müller R.

Kuc R. Foliage echoes: a probe into the ecological acoustics of bat echolocation. 14 Yovel Y.

Franz M.O.

Stilz P.

Schnitzler H.-U. Complex echo classification by echo-locating bats: a review. 11 Schöner C.R.

Schöner M.G.

Kerth G.

Grafe T.U. Supply determines demand: influence of partner quality and quantity on the interactions between bats and pitcher plants. 15 Moran J.A. Pitcher dimorphism, prey composition and the mechanisms of prey attraction in the pitcher plant Nepenthes rafflesiana in Borneo. 10 Grafe T.U.

Schöner C.R.

Kerth G.

Junaidi A.

Schöner M.G. A novel resource-service mutualism between bats and pitcher plants. How mutualisms evolve or how these interactions are maintained is still not sufficiently understood []. Particularly, if partners regularly separate, they require species-specific mechanisms to find each other again. This is also true for the carnivorous pitcher plant Nepenthes hemsleyana (Nepenthaceae), which recently was reported to have a mutualistic interaction with the insectivorous bat Kerivoula hardwickii (Vespertilionidae). This bat fertilizes the plant with its feces while roosting inside the pitchers. The bat droppings enhance the nitrogen intake of N. hemsleyana by 34% on average []. In turn, the pitcher plants provide the bats with roosts that are free of parasites, have a stable microclimate, and offer enough roosting space for one or two bats while at the same time preventing the bats from falling into the digestive fluid due to their unique morphological shape and low fluid level []. Finding and identifying N. hemsleyana pitchers that grow in the dense Bornean peat swamp forests, however, is a challenging task for echolocating bats: they have to distinguish echoes of the pitchers from those of the cluttered surroundings []. The situation is further complicated by the fact that the bats need to distinguish the rare [] N. hemsleyana pitchers from the more common and similarly shaped pitchers of sympatric Nepenthes species, which are unsuitable for roosting [].

7 von Helversen D.

von Helversen O. Acoustic guide in bat-pollinated flower. 9 Simon R.

Holderied M.W.

Koch C.U.

von Helversen O. Floral acoustics: conspicuous echoes of a dish-shaped leaf attract bat pollinators. 16 Marshall A.G. Bats, flowers and fruit: evolutionary relationships in the old world. 17 Boonman A.

Bumrungsri S.

Yovel Y. Nonecholocating fruit bats produce biosonar clicks with their wings. In the Neotropics, a few bat-pollinated plants found an efficient solution to attract bats by developing floral ultrasound reflectors [], which enabled them to exploit the bats’ echolocation system. However, such reflectors have never been described for plants outside the Neotropics, probably because in the Paleotropics, chiropterophilous plants are pollinated by fruit bats (Pteropodidae) that are unlikely to use echolocation for foraging []. We hypothesized that this phenomenon can also be found in the Paleotropics. If so, bat-dependent plants such as N. hemsleyana should have echo-reflecting structures making it easier for bats to localize and identify pitchers. Pitchers lacking such reflectors should be more difficult to find. Additionally, the bats should have echolocation calls that facilitate the finding of targets even within highly cluttered surroundings.

Are the Bats’ Echolocation Calls Suited to Detect Pitchers in Highly Cluttered Space? 18 Douangboubpha B.

Bumrungsri S.

Satasook C.

Wanna W.

Soisook P.

Bates P.J.J. Morphology, genetics and echolocation calls of the genus Kerivoula (Chiroptera: Vespertilionidae: Kerivoulinae) in Thailand. 19 Schmieder D.A.

Kingston T.

Hashim R.

Siemers B.M. Breaking the trade-off: rainforest bats maximize bandwidth and repetition rate of echolocation calls as they approach prey. 20 Siemers B.M.

Schnitzler H.-U. Echolocation signals reflect niche differentiation in five sympatric congeneric bat species. 19 Schmieder D.A.

Kingston T.

Hashim R.

Siemers B.M. Breaking the trade-off: rainforest bats maximize bandwidth and repetition rate of echolocation calls as they approach prey. 20 Siemers B.M.

Schnitzler H.-U. Echolocation signals reflect niche differentiation in five sympatric congeneric bat species. 21 Lazure L.

Fenton M.B. High duty cycle echolocation and prey detection by bats. Bats in the genus Kerivoula generally have relatively short, high-pitched calls [] covering a very large bandwidth, which further increases when they approach an object []. Such a call design is typical for the guild of narrow-space gleaning foragers [] as it facilitates hunting in dense vegetation []. Calls of Kerivoula have also been proposed to facilitate detection of fluttering prey []. 19 Schmieder D.A.

Kingston T.

Hashim R.

Siemers B.M. Breaking the trade-off: rainforest bats maximize bandwidth and repetition rate of echolocation calls as they approach prey. 22 Jakobsen L.

Ratcliffe J.M.

Surlykke A. Convergent acoustic field of view in echolocating bats. 20 Siemers B.M.

Schnitzler H.-U. Echolocation signals reflect niche differentiation in five sympatric congeneric bat species. 23 Brinkløv S.

Jakobsen L.

Ratcliffe J.M.

Kalko E.K.V.

Surlykke A. Echolocation call intensity and directionality in flying short-tailed fruit bats, Carollia perspicillata (Phyllostomidae). 24 Skolnik M.I. Introduction to Radar Systems. 25 Simon R.

Knörnschild M.

Tschapka M.

Schneider A.

Passauer N.

Kalko E.K.V.

von Helversen O. Biosonar resolving power: echo-acoustic perception of surface structures in the submillimeter range. 23 Brinkløv S.

Jakobsen L.

Ratcliffe J.M.

Kalko E.K.V.

Surlykke A. Echolocation call intensity and directionality in flying short-tailed fruit bats, Carollia perspicillata (Phyllostomidae). 25 Simon R.

Knörnschild M.

Tschapka M.

Schneider A.

Passauer N.

Kalko E.K.V.

von Helversen O. Biosonar resolving power: echo-acoustic perception of surface structures in the submillimeter range. 25 Simon R.

Knörnschild M.

Tschapka M.

Schneider A.

Passauer N.

Kalko E.K.V.

von Helversen O. Biosonar resolving power: echo-acoustic perception of surface structures in the submillimeter range. 26 Clare E.L.

Goerlitz H.R.

Drapeau V.A.

Holderied M.W.

Adams A.M.

Nagel J.

Dumont E.R.

Hebert P.D.N.

Fenton M.B. Trophic niche flexibility in Glossophaga soricina: how a nectar seeker sneaks an insect snack. 27 Brinkløv S.

Kalko E.K.V.

Surlykke A. Intense echolocation calls from two ‘whispering’ bats, Artibeus jamaicensis and Macrophyllum macrophyllum (Phyllostomidae). Figure 3 Echolocation Calls and Call Directionality of Kerivoula hardwickii Show full caption (A) Call parameters (n of all analyzed calls = 25) of the last five calls of a pitcher approach (C last ) and the referring call directionality (measured as directivity index [DI]). (B) Spectrogram, power spectrum, and oscillogram of the echolocation calls of K. hardwickii. (C) Beam shape of the calls of K. hardwickii. The high mean peak frequencies in C last resulted in a very high call directionality (blue line; half-amplitude angle = 11°; photographs provided by C.C. Lee). To examine whether the bats’ call design is also suitable for the detection of pitchers, we recorded the echolocation calls of five K. hardwickii individuals upon their approach toward pitchers, selected the last five calls, and analyzed their starting, peak, and end frequency, bandwidth, duration, and pulse interval [] as well as directionality []. The analyzed calls consisted of only the first harmonic with a very short duration, broad bandwidth, and exceptionally high starting frequencies of up to 292 kHz ( Figures 3 A and 3B ). To our knowledge, these are the highest frequencies ever recorded in bats. These high-pitched calls result in a very high call directionality [] ( Figures 3 A and 3C), which facilitate localization and classification of targets in cluttered surroundings as only the object of interest is ensonified while clutter echoes are blended out []. Thus, these calls are well suited to detect targets in highly cluttered space, including pitchers that are partially hidden in vegetation. Interestingly, other bat species interacting with plants that offer reflectors, e.g., Glossophaga soricina, have similar echolocation calls. They are also broadband and high pitched [], except that Glossophagine calls often consist of multiple harmonics and are slightly shorter. Generally, such calls should enable the bats to get a highly resolved acoustic image of targets and facilitate recognition of floral reflectors [] or, in the case of N. hemsleyana, species-specific spectral signatures of the pitchers.

How Do the Bats React to the Ultrasound Reflector of Nepenthes hemsleyana? Figure 4 Behavioral Responses of K. hardwickii to Reflector Modifications Show full caption During behavioral experiments, bats could choose between pitchers whose reflectors were unmodified, enlarged, or (partly or completely) reduced (Wilcoxon rank-sum tests: ∗p < 0.05; ∗∗p < 0.01). (A) Search time for a single pitcher hidden in shrubbery. (B) Final choice of the bats between four simultaneously offered pitchers (see also Tables S1 and S2 ). To test the efficacy of the reflector of N. hemsleyana in attracting bats, we conducted a series of behavioral experiments with wild K. hardwickii in a flight tent. In the first experiment, we tested whether the reflector helps the bats to find pitchers faster in a cluttered environment. We measured the time until the bats (n = 24) approached a single pitcher hidden within shrubbery. In this experiment, the pitchers’ reflector was either unmodified or enlarged or completely removed (n = 8 individual bats per type of pitcher; Table S1 A; Movie S1 ). Bats needed significantly less time to approach enlarged (92.4 ± 58.5 s; W = 2; p < 0.001) and unmodified (182.1 ± 111.0 s; exact Wilcoxon rank-sum test: W = 10; p = 0.02) pitchers than those with removed reflectors (408.8 ± 228.1 s; Figure 4 A). In a second experiment, we tested whether the reflector is decisive for roost identification: we simultaneously confronted a single bat (n = 18) with three types of N. hemsleyana pitchers with modified reflectors (enlarged, partly or completely removed; Table S1 B) and an unmodified N. hemsleyana pitcher as control ( Movie S2 ). Bats approached enlarged pitchers significantly more often than expected by chance (number of approaches per bat = 3.1 ± 3.6; permutation tests, p = 0.005; for explanations, see Supplemental Experimental Procedures ), whereas pitchers with reduced reflectors were approached significantly less frequently than expected (1.0 ± 1.3; p = 0.03; Table S2 ). The number of approaches to unmodified control pitchers did not differ from random expectations (2.1 ± 2.1; p = 0.26). These results confirm that the reflector is crucial for attracting the bats to the pitchers. When it came to the final roost selection, bats predominantly entered pitchers with unmodified reflectors and avoided those that had been enlarged or reduced (p < 0.001; Figure 4 B; Table S1 B). These results suggest that bats are initially attracted by the enlarged reflectors but then do not identify them as N. hemsleyana, possibly because such artificial reflectors do not contain the typical N. hemsleyana spectral cues. To assess the importance of the reflector over other structures of the pitcher in attracting bats and to exclude the possibility that the bats generally avoided roosting in modified pitchers, we conducted further choice experiments. This time, we modified lids or peristomes of N. hemsleyana pitchers but kept the reflectors intact. The bats’ roost choice was not influenced by such modifications ( Table S1 C), demonstrating that bats did not generally avoid roosting in modified pitchers and that other structures of the pitcher were not important compared to the reflector. Taken together, the results of the ensonification measurements and the behavioral experiments provide strong support that the reflector of N. hemsleyana is crucial for the bats to find, identify, and finally enter pitchers.