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The effect of flower angle on bat pollination of Mucuna urens (F. Papilionaceae) Laura Grieneisen Department of Biology, The College of William&Mary ABSTRACT The purpose of this study is to examine the relatio nship between bat pollination and flower angle in Mucuna urens (F. Papilionaceae). To determine the natural variat ion among M. urens the angles of 100 M. urens flowers were measured with a protractor to the nea rest 5o. The mean angle was -82.45o from the horizon, the mode was -90o, and the range was from -45o to -105o. Approximately one-third of the flowers were tied so that they opened at 90o greater than the natural angle, one-third were tie d to open at 90o less than the natural angle, and one-third were left at the natural angle. Over several nights, the pollina tion status of 383 mature M. urens flowers was observed More flowers than expected were pollinated at the natural angle and fewer flowers than expected were pollinated at the positive and negative angles. (X2 = 63.96, p<0.001, df = 2). This suggests that natural M. urens flower angles are more accessible to bats than other angles. RESUMEN El propsito de este investigacin es examinar la r elacin entre la polinizacin por murcilagos y el ngulo de la flor en Mucuna urens (F. Papilionaceae). Para saber la variedad natural sobre M. urens se midieron los ngulos de 100 M. urens flores con un transport ador al grado cinco ms cercano. El promedio del ngulo fue -82.45 del horizonte, la moda fue -90, y el rango fue de -45 a -105. Se amarraron aproximadamente un 1/3 de las flores para que se a brieron 90 mas que el ngulo natural, se amarraron tambin un 1/3 para que se abrieron 90 menos que e l ngulo natural, y 1/3 se mantuvieron al ngulo natural. Durante algunas noches, se observ el esta do de polinizacin de 383 flores maduras de M. urens Ms flores de las esperadas se polinizaron al ngul o natural y menos flores de las esperadas se polini zaron a los ngulos positivos y negativos. (X2 = 63.96, p<0.001, df = 2). Los resultados sugieren que los ngulos naturales de flores de M. urens son ms accesibles para los murcilagos que otros ngulos. INTRODUCTION Bats offer an efficient method of long distance pol lination in the Neotropics, as a single bat may travel several kilometers per night searchi ng for flowers (Von Helversen and Reyer 1984). Over six hundred species of Neotropica l plants are pollinated by bats (Dobat 1985). It is estimated that two-thirds of ch iropterophilous plants in Costa Rica are pollinated by nectivorous bats in the subfamily Glo ssophaginae (F. Phyllostomidae) (Winter and von Helversen 2001). Glossophagine bats hover in front of flowers much like hummingbirds, and extend their long tongues in to flowers to lap nectar (LaVal and Rodriguez-H 2002). Because they require large amoun ts of nectar, Glossophagine bats are known as high-cost pollinators. They have a met abolic rate that is 15% higher than that of other bats, or similar to that of birds (Ar ends et al 1995). Flower species whose primary pollinators are Glossophaginae often produc e 100 microL of nectar per flower (Winter and von Helversen 2001). Because of the hig h nectar investment on the part of
the plant and high energy expended by a foraging ba t, it is to the advantage of both that flowers be easy to find and access. Although bats use olfaction and sight to find desir ed flowering plants, they determine a flowers exact position via echolocatio n (Stich and Winter 2006). Bat flowers are structured to be acoustically distinct from surrounding foliage. When a bat is flying and projects a vocalization, a leaf might re turn only one call (Simon et al 2003). A specially curved bat flower will return echoes from multiple locations for several calls in a row+/-50o in any direction in the case of Mucuna holtonii as a bat changes position when it flies (von Helversen et al 2003). Many flowers are cauliflorous or on stalks away from the rest of the plant to reduce acoustic inter ference from vegetation and have thicker petals to better amplify echoes (Simon et al 2006, von Helversen et al 2003). A plant genus that includes chiropterophilous flowe rs with unique echolocation and reproductive structures is Mucuna (F. Papilionaceae). Mucuna species in the neotropics appear to have evolved for Glossophagine echolocation because they are morphologically different than those that are polli nated by birds or non-echolocating bats in the palaeotropics (Winter and von Helversen 2001 ). Previous studies conducted by von Helversen & von Helversen (2003) on Mucuna holtonii have shown that bats identify a mature flower by its raised vexillum, a concave pet al that reflects echolocation calls to form an acoustic ne ctar guide. (Figure 1). Other studies have shown that removing, tilting, blocking, or rotating the vexill um reduces the chance a bat will pollinate that flower and substituting an artificially large vexillum increas es the chance (Macedo 2000, von Helversen and von Helverse n 1999, von Helversen and von Helversen 2003). Other factors that have been examined, such as variations in inflorescence height and flower symmetry, have not shown a significant effect on the number of flowers that bats visit in other Mucuna species (Vincent 2003). Although several studies have changed the angle of the vexillum, none has studied the impact of cha nging the angle of the entire flower, which is the purpos e of this study. Since the orientation of vexillum has been s hown to affect the ability of bats to locate and pollinate flowers of M. holtonii I predict that changing the angle of orientation in M. urens will result in a difference in pollination. Specifically, I predict that an extreme deviation f rom the natural mean flower angle will lower the visitation rate. Such a finding would suggest that certain angles are easier to access than other s and that there may be an optimal angle for Mucuna flowers. METHODS Figure 1: An immature M. urens flower (top) and mature flower with raised vexillum (bottom). Photograph by Laura Grieneisen.
Study sites This study was conducted from 22 April to 1 May 200 8 at eight flowering Mucuna urens patches along the trails at El Centro de Educacin Creativa (elevation 1479 meters) and the Estacin Biolgica de Monteverde (elevation 155 0 meters) in Monteverde, Costa Rica. The seven patches at La Creativa were located in secondary growth tropical lower montane wet forest within 10 meters of a trail. The patch at la Estacin was located in front of the Biological Station on the edge of seco ndary growth tropical lower montane wet forest. Study organism: Mucuna urens Mucuna urens (F Papilionaceae) is a woody Neotropical liana that i s common in secondary growth forests in the Monteverde region ( Zuchowski 2005). It is also called ojo de bueyox eye for its large black seeds. The liana can grow up to 70 m long and drape from ground level to 20 m into the canopy (ODell 2000). Mucuna urens has chandelier-like inflorescences that hang from a ~0. 5m long stem. Its flowers are pale green and are bat pollinated. It blooms for several weeks, with one to eight of the 20 or so flowers on each inflorescence flowering each night. This maximizes the spatial memory of bats and allows them to return to the same plant many times (Von Helversen and von Helversen 2003, Winter and von Helversen 2001). Each flower is composed of two fused keel petals, t wo wing petals, and an upper vexillum petal (von Helversen and von Helversen 199 9). A Glossophagine or other Phyllostomidae bat will fly around to inspect diffe rent flowers before hovering in front ot a mature flower and pressing its snout against the front slit on the keel petals. This causes the keel petals to split, the reproductive parts to explosively pop out and dust the bat with pollen, and allows the bat to reach the nectar (von Helversen and von Helversen 2003). This also makes it easy to differentiate between po llinated and unpollinated flowers. Determining average normal angle The angles of 100 unaltered M. urens flowers were measured with a protractor to the nearest 5o to determine the average natural angle of M. urens The flowers were from 25 different inflorescences from all eight patches. Th ey had all matured within the previous 24 hours (vexillum petal had been raised fully) but none had been pollinated. The angle of every mature flower that was found over the cour se of three days was measured until a sample size of 100 was reached. Angle alteration
At each patch of M. urens the mature flowers were determined by checking if the vexillum was raised. A raised vexillum would indicate the flower was mature. Between 3 and 18 flowers (depending on the number mature) in each patch each night were manipulated such that an approximately equal number were oriented at each angle. The angles used were natural, positive (90o greater than the natural angle), and negative (90o less than the natural angle). Odorless white cotton string was used to tie the flowers to face the negative and positive angles. (Figure 2). The stateimmature, mature, or pollinated of a ll the flowers on an inflorescence that contained mature flowers was recorded. Each in florescence was assigned a number and letter and labeled by tying yellow flagging tap e approximately 1 meter away. The following morning each inflorescence was examined a nd whether the individual flowers were pollinated was recorded. This procedure was fo llowed every night until it either started to wilt or was pollinated (Appendix Figure 5). Flowers that broke off of the main stem or were missing the next day were not counted towards the flower total. A total of 383 flowers were documented: 105 flowers at the pos itive angle, 103 flowers at the negative angle, and 175 flowers at the normal angle Flowers ranged in height from 15 cm to 2.5 m from the ground. RESULTS Average natural angle Of the 100 unaltered M. urens flowers whose angles were measured, the mean angle was -82.45o from the horizon. The most common angle was -90o, which composed 48 out of 100 flowers. Angles ranged from -45o to -105o (Figure 3). Figure 2: The three angles of M. urens compared in this study and how the positive and negative angles were tied. Photograph taken by Laura Grieneisen. Negative Angle Natural Angle Positive Angle
Figure 4: The expected versus observed number of po llinated M. urens flowers for positive, negative, and natural angles The bars represent the number of flowers pollinated. (chi-sq uare = 63.96, p<0.001, df = 2).11 4 87 28 27 47 0 10 20 30 40 50 60 70 80 90 100 PositiveNegativeNormalFlower angleNumber of Pollination Events Observed Expected Figure 3: The natural angles of 100 M. urens flowers. -90o is a flower that faces the ground when mature. The bars refer to the number of flowers at a given angle.1 2 4 5 7 0 11 6 4 48 10 2 0 10 20 30 40 50 60 -45-50-55-60-65-70-75-80-85-90-95-105Angle degree from the horizonNumber of flowers Angle alteration Eleven out of 105 flowers of the positive angle wer e pollinated, 4 out of 103 flowers of the negative angle were pollinated, and 87 out of 1 75 flowers at the natural angle were pollinated. More flowers than expected at the natur al angle were pollinated and fewer flowers at the positive and negative angles were po llinated than expected (X2 = 63.96, p<0.001, df = 2) (Figure 4).
DISCUSSION The angles of Mucuna urens had a 60 o range, but over half of the flowers had an angle o f -90o. This natural variation suggests that although the p lants is capable of producing flowers at a range of angles, there may be an optim al angle to maximize bat pollination. When flowers were manipulated, more flowers at the natural angle were pollinated and fewer flowers at the positive and negative angles w ere pollinated. This suggests that bats prefer natural angles to positive or negative angle s and may indicate that certain angles are easier to access than others. Some positive and negative angles were opened, which shows that it is not impossible for bats to pollina te flowers with drastically different angles than natural M. urens It is also possible that bats are accustomed to t he natural angle, but if the same bats were presented with flo wers of different angles for several nights in a row and became accustomed to handling t hem, they would pollinate an equal number of each angle. Further studies could try to determine why more natural angles were pollinated than positive or negative angles. T his study could also be repeated using different angles. Because bats must hover in front of M. urens in order to open it, it is likely that the change in angle made this more difficult. Bats may have been able to locate the flowers acoustically, but were unable to hover in f ront of them to open the keel. In a previous study in which the vexillum was rotated sl ightly, bats were able to locate the flower but were unable to open it successfully beca use they normally use the vexillum to guide them (von Helversen and von Helversen 2003). Although the vexillum was not altered in this study, opening an upside-down flowe r could likewise prove to be difficult. The angle of M. urens flowers may reduce nectar robbing by other animals Covering the buds and sepals in urticating hairs an d angling the flowers down may discourage potential predators. On 26 April after I had finished tying a patch, I observed a hummingbird stick its beak into four positive ang le flowers, but no flowers of other angles in the patch. None of the flowers were polli nated, although it is possible that the hummingbird obtained nectar. To stick its beak into a natural angled flower and reach the nectar, it would have had to tilt its head straight up. The results of this study may help lead to a better understanding of the relationship between flower morphs and bat pollinat ion and what impacts flower choice by bats. Although this study supports the idea that bats prefer flowers at natural angles to altered angles and several possible causes are sugg ested, the reason why is still unknown. Any future studies that could help to elucidate thi s would be fascinating. The shape of other chiropterophilous flowers have been studied, but very little research has been done on the angle of the entire flower. It would be inte resting to study the flower angle of various bat flowers in Monteverde to see if there a re certain angles that are more common than others. ACKNOWLEDGEMENTS Thank you to El Centro de Educacin Creativa and th e Estacin Biolgica de Monteverde for allowing me to conduct research ther e. Thank you to my advisor, Tania Chavarria. Thank you also to Alan Masters, Taegan M cMahon, Pablo Allen, and Richard
LaVal for advice. Many kudos to Garrison, Nikol, Jo sh, Katherine, Cathy, and Dominika for braving urticating hairs so that I didnt fall off the ladder. LITERATURE CITED Arends, A., Bonaccorso, FJ., and M Genoud. 1995. Ba sal rates of metabolism of nectarivorous bats (Phyllostomidae) from a semiarid thorn forest in Venezuela. J. Mamm. 76: 947-956. Dobat, K. 1985. Bluten und Fledermause (Chiropterop hilie). Kramer Verlag, Frankfurt. In: Winter, Y. and O. von Helversen. 2001. Bats as pollinators: foraging energetics and floral adaptations. Cognitive Ecolo gy of Pollination: Animal Behaviour and Floral Evolution Chittka, L. and J.D. Thompson. Ed. Cambridge University Press: London. LaVal, Richard K. and B. Rodriguez-H. 2002. Murciel agos de Costa Rica/Bats INBIO: Costa Rica. Macedo. M. 2002. Function and size optimization in Mucuna urens vexillum. CIEE Fall 2002. ODell, G. 2000. Mucuna urens: A tropical liana. In : Monteverde: Ecology and Conservation of a Tropical Rainforest Nadkarni, N. and N. Wainwright. ed. Oxford University Press, New York. p. 72. Simon, R., Holereid, H. W. and O, von Helversen. 20 06. Size discrimination of hollow hemispheres by echolocation in a nectar feeding ba t. J. Exp. Biol. 209: 35993609. Stich, K P and Y.Winter. 2006. Lack of generalizati on of object discrimination between spatial contexts by a bat. J. Exp. Biol. 209: 48024808. Vincent, C. 2003. The role of height and flower sym metry in bat pollination of Mucuna urens (Papilionaceae). CIEE Spring 2003. von Helversen, D. and O. von Helversen. 1999. Acous tic guide in bat-pollinated flowers. Nature. 398: 759 -760. von Helversen, D. and O. von Helversen. 2003. Objec t recognition by echolocation: a nectar-feeding bat exploiting the flowers of a rain forest vine. J. Comp. Physiol. A. 189: 327 -336. von Helversen, D., Holderied, M. and O.von Helverse n. 2003. Echoes of bat-pollinated bell-shaped flowers: conspicuous for nectar-feeding bats? J. Exp. Biol. 206: 1025 -1034.
von Helversen, O. and H. U. Reyer. 1984. Nectar int ake and energy expenditure in a flower visiting bat. Oecologia. 63(2):178-184. Winter, Y. and O. von Helversen. 2001. Bats as poll inators: foraging energetics and floral adaptations. Cognitive Ecology of Pollination: Anim al Behaviour and Floral Evolution Chittka, L. and J.D. Thompson. Ed. Cambridge Univ ersity Press: London. Zuchowski, W. 2005. A Guide to Tropical Plants of C osta Rica Zona Tropical; Miami, Florida. Appendix Figure 5: The number of positive, negative, and nor mal angle M. urens flowers pollinated each night after the flower matured. Bec ause different flowers matured different nights, 'Nights 1,2, and 3' do not repres ent a specific date.10 0 1 4 0 0 77 8 2 0 10 20 30 40 50 60 70 80 90 1 2 3Number of nights after flower maturesNumber of pollinated flowers Positive Angle Negative Angle Normal Angle
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El efecto de la polinizacin en el ngulo de la flor en Mucuna urens (F. Papilionaceae) polinizado por los murcilagos
The effect of flower angle on bat pollination of Mucuna urens (F. Papilionaceae)
The purpose of this study is to examine the relationship between bat pollination and flower angle in Mucuna urens (F. Papilionaceae). To determine the natural variation among M. urens, the angles of 100 M. urens flowers were measured with a protractor to the nearest 5o. The mean angle was -82.45o from the horizon, the mode was -90o, and the range was from -45o to -105o. Approximately one-third of the flowers were tied so that they opened at 90o greater than the natural angle, one-third were tied to open at 90o less than the natural angle, and one-third were left at the natural angle. Over several nights, the pollination status of 383 mature M. urens flowers was observed. More flowers than expected were pollinated at the natural angle and fewer flowers than expected were pollinated at the positive and negative angles. (X2 = 63.96, p<0.001, df = 2). This suggests that natural M. urens flower angles are more accessible to bats than other angles.
El propsito de este estudio es examinar la relacin entre la polinizacin de los murcilagos y el ngulo de la flor en Mucuna urens (F. Papilionaceae).
Text in English.
Bats--Costa Rica--Puntarenas--Monteverde Zone
Pollination by animals
Murcilagos--Costa Rica--Puntarenas--Zona de Monteverde
Polinizado por animales
Tropical Ecology 2008
Ecologa Tropical 2008
t Monteverde Institute : Tropical Ecology