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Preferencia en la concentracin de sacarosa en los murcilagos nectarvoros
Sucrose concentration preference in nectarivorous bats
Nectarivorous bats perform vital pollination services for many tropical plant species. While nectarivorous bats have been shown to prefer higher percentages of sucrose in past studies, bat pollinated flowers usually offer a lower sugar concentration in the nectar, equal to 18 percent sucrose on average, thus forcing bats to visit more flowers. Visiting flowers increases outcrossing, pollination, and subsequently, plant fitness. This study attempts to see where bats preferences, if any, exist for sucrose within the natural range of bat
pollinated flowers. For nine nights, bats were observed at Selvatura Park in Monteverde, Costa Rica, where they forage at hummingbird feeders at night. Bats were observed foraging on prepared solutions of
10, 15, 20, 25, and 30 percent sucrose. No significant difference was found in foraging between different concentrations of sucrose, although some trends were apparent, indicating that bats may prefer higher
concentrations of sucrose.
Los murcilagos nectarvoros realizan servicios de polinizacin vitales para muchas especies de plantas tropicales. Aunque se han mostrado que los murcilagos nectarvoros prefieren los porcentajes ms altos de sacarosa en estudios anteriores, las flores polinizadas por los murcilagos por lo general ofrecen una menor concentracin de azcar en el nctar, igual al 18% en promedio, lo que obliga a los murcilagos visitar ms flores. Al visitar ms flores aumenta el cruzamiento, la polinizacin, y posteriormente el xito reproductivo de las plantas. En este estudio voy a observar si hay preferencias por la sacarosa entre el rango natural de las flores que son polinizados por los murcilagos. Los murcilagos fueron observados por nueve noches en el parque de Selvatura, Monteverde, Costa Rica, donde forrajean en los comedores de los colibres en la noche. Forrajearon en cinco tratamientos diferentes: 10, 15, 20, 25, y 30 porciento de sacarosa. No haban diferencias en el forrajeo entre los tratamientos diferentes, aunque haban tendencias que indicaron que los murcilagos prefieren las concentraciones ms altas de sacarosa.
Text in English.
Pollination by animals
Costa Rica--Puntarenas--Monteverde Zone
Polinizado por animales
Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology Fall 2009
Ecologa Tropical Otoo 2009
t Monteverde Institute : Tropical Ecology
Sucrose concentration preference in nectarivorous bats Kyle Bevers Department of Biology, University of Wisconsin Madison ABSTRACT Nectarivorous bats perform vital pollination services for many tropical plant specie s. While nectarivorous bats have been shown to prefer higher percentages of sucrose in past studies, bat pollinated flowers usually offer a lower sugar concentration in the nectar, equal to 18 percent sucrose on average, thus forcing bats to visit more fl owers . Visiting flowers increase s outcrossing, pollination, and subsequently , plant fitness. This , exist for sucrose within the natural range of bat pollinated flowers. For nine nights, bats were obse rved at Selvatura Park in Monteverde, Costa Rica, where they forage at hummingbird feeders at night. Bats were observed foraging on prepared solutions of 10, 15, 20, 25, and 30 percent sucrose. No significant difference was found in foraging between diff erent concentrations of sucrose, although some trends were apparent, indicating that bats may prefer higher concentrations of sucrose. RESUMEN MurciÃ©lagos nectarÃvoros realizan servicios vitales por muchas especie s de plantas tropicales. Mientras murciÃ©l agos nectarÃvoros preferÃan porcentajes altas de sacarosa en estudios en el pasado, flores polinizados por murciÃ©lagos usualmente ofrecen una concentraciÃ³n mÃ¡s bajo de sacarosa en el nÃ©ctar, igual a 18% en promedio, entonces murciÃ©lagos visitan mas flores. Visitando mÃ¡s flores promociona exocrucamiento, polinizaciÃ³n, y Ã©xito reproductivo de plantas. En este estudio voy a observar si hay preferencias por sacarosa entre el rango natural de flores que son polinizados por murciÃ©lagos. MurciÃ©lagos eran observado s por nueve noches en Selvatura Park, Monteverde, Costa Rica, donde forrajean en comedores de colibrÃs en la noche. Forrajearon en cinco tratamientos diferentes: 10, 15, 20, 25, y 30 porciento sacarosa. No habÃan diferencias en forrajeando entre tratamient os diferentes, aunque habÃan tendencias que indicaron murciÃ©lagos prefieren concentraciones mÃ¡s altas de sacarosa. INTRODUCTION Microchiropteran bats are widespread globally and are extremely diverse both morphologically and ecologically (LaVal 2002). The greatest diversity of this suborder occurs in the T ropics, with species richness increasing with decreasing latitudes (Findley and Wilson 1983, Willig and Selcer 1989). Nectarivorous bats are important pollinators in tropical ecosystems (Baker and F rankie 1974) and are excellent examples of the morphological and ecological breadth shown in microchiropterans. In Monteverde, Costa Rica , 33 species of plants are bat pollinated, belonging primarily to the families Marcgraviaceae, Bombacaceae, Bignoniace ae, and Cactaceae (Nadkarni and Wheelright 2000). Nectarivorous b ats and other pollinators are attracted to flowers by a reward, usually nectar, though they need to supplement their carbohydrate diet with insects and pollen, which provides the bats with protein, and subsequently amino acids and nitrogen
(Howell 1974). To ensure maximum cross pollination, then, a flower needs to provide enough of a reward to attract its pollinators, but must limit nutrients so the pollinator is forced to visit other flow ers to fulfill its nutritional needs. The number of floral visits by an individual pollinator is dependent upon its nutritional needs, so it is essential that plants are not so rich in nutrients that they readily satiate the pollinator, but force it to vi sit, and subsequently pollinate, many flowers to gather food (Heyneman 1983). Meta analyses have shown that means of nectar sucrose equivalents of new world bat flowers range between 4.5 and 30 percent with an average of 18 percent sucrose equivalent sug ar content (Baker et al. 1998). While nectar sucrose percentages in nature are not exceptionally high for bat pollinated flowers, Roces et. al (1993) has shown that at least one species of nectar feeding bat, Glossophoga soricina , prefers higher nectar co ncentrations up to 50 percent when presented with a choice between two different sucrose concentrations. These bats prefer sugar concentrations higher than observed in most bat pollinated flowers, and this provides evidence that plants and pollinators hav e coevolved a pollination system in which plants provide enough reward to sustain their pollinators, yet limit the nutrient content so they continue to visit and pollinate other flowers (Roces et al. 1993). The minimum limits of acceptable sugar concentra tion from the point of view of the pollinator should at least cover the expended energy of the visit, while the upper limit of acceptable concentrations for nectarivorous bats is likely determined by increasing viscosity of nectar at higher percentages, an d osmoregulation concerns since nectarivorous bats rarely drink water in a natural setting (Roces et. al 1993). Helversen and Reyer (1984) have shown for Anoura caudifer , that sugar concentrations between 18 and 21 percent provide a balanced energy and o smotic budget, and this is roughly in the middle of the range of nectar concentrations of bat pollinated flowers. This is similar to the 18 percent average shown by Baker et al. ( 1998 ) . Studies in Monteverde have shown that bats visit feeders of higher s ucrose percentage (30% and 40%) more frequently than feeders containing a lower sucrose concentration (10% and 20%) (Sullivan 2004). In this study, a community of nectarivorous bats was presented with solutions of sucrose within the natural range of their bat pollinated flowers to test if a preference existed for higher sucrose concentrations. METHODS Study Site Nectarivorous bats were studied at the JardÃn de ColibrÃs at Selvatura Park in Monteverde, Costa Rica. Here, bats regularly forage on humming bird feeders during the night, and were observed over nine nights between 7:00 and 9:00 p. m. on November 12 16 and 19 22. The site included six identical posts, each of which could hold up to four hummingbird feeders. The feeders themselves consisted of a red plastic bottom with three holes surrounded by replica yellow flowers, and a clear plastic receptacle above, which held a solution of sucrose and water.
Mist Netting Bats were captured to determine species likely visiting the hummingbird feeders using one ten meter mist net between 6:00 and 6:30 p. m on November 22. The mist net was placed between two feeders, where bats were caught and identified to species (Timm, R.M., LaVal, R.K. 1998) Concentration Preference Individual feeders filled wit h a sucrose solution were observed over ten minute periods between 7:00 and 9:00 p. m. from a distance varying from three to five meters away. The study was essentially split in two, and during the first five nights, each of five feeders filled with 400 m L of 10, 15, 20, 25, or 30 percent sucrose (weight solute / weight solution), were placed on separate posts and observed during two ten minute periods. Only five of the six available posts were used, and these were consistent over all five nights. Each ni ght of the first five nights, the feeders were moved to a different post. After each solution of sucrose was afforded one night at each post, I decided to focus my efforts on observing a single post with four feeders attached each night in order to reduce the bias that different posts caused during the first part of my study. I chose to observe the feeders at post 3, as this was the post with both the highest nightly average number of visits, more than double the average of the second highest post, a nd nectar consumption (Fig. 2). During the final four nights, four feeders, consisting of 15, 20, 25, and 30 percent sucrose, were placed on a single post, and each was observed three times for a total of 30 minutes for each feeder between 7:00 and 9:00 p . m . During each ten minute observation period, the number of bats that visited one feeder was counted, and The total number of milliliters consumed by the bats at each feeder over the course of two hours was also recorded for all nine nights. During the final four nights, the feeders were moved to a different one of the four hooks on the same post to account for any bias. No attempt was made to identify bats durin g foraging. RESULTS Mist Netting A total of 16 bats were caught, which included 13 Hylonycteris underwoodi , two Anoura geoffroyi , and one Glossophaga soricina . These three bats belong to the family Phyllostomidae and the subfamily Glossophaginae. Conc entration Preference Sucrose concentration had no effect on either mean nightly visitation ( Friedman test, 2 = 1.9655, df = 3, p value = 0.58) or mean nectar consumption (Friedman test, 2 = 5.6207, df = 3, p value = 0.13) over the course of the entire s tudy . Additionally, concentration had no effect when comparing mean nightly visitation frequency (Friedman test, 2 = 4.2917, df = 4, p value = 0.37) or mean nectar consumption (Friedman test, 2 = 1.9184,
df = 4, p value = 0.75) over the first five night s. There was, however, a significant difference between post position and both visits (Friedman test, 2 = 11.04, df = 4, p value = 0.026) and nectar consumption (Friedman test, 2 = 16.6122, df = 4, p value = 0.0022) during the first five nights of data collection. Visitation and consumption peaked at 20% over nights 1 5, largely due to its position at post 3, the most highly visited site, on night 1, the night with the highest number of visits, where it received 156 visits and the bats consumed 155mL. Average visits for the first five nights showed no real trend with 20% being highest, 15% and 30% being similar and in the middle range, and 10% and 25% both being very low. Posts 3 and 5 showed much higher consumption than posts 1,2 and 4 (Fig. 1). Ther e was also a significant difference between nights in terms of visitations over the first five nights (Friedman test, 2 = 314.2, df = 4, p < 0.0001). Night one had the most visitations by far with a total of 212, while on night 5 there were only 16. Nig hts two through four were fairly similar with visitations of 57, 36, and 64 respectively. There was no significant difference between either visits (Friedman test, 2 = 4.8, df = 3, p value = 0.19) or consumption (Friedman test, 2 = 7.0541, df = 3, p val ue = 0.07) when compared to sucrose concentration over the final four nights, though the comparison between consumption and concentration was nearly significant. Although the data found were non significant, some trends were apparent. Over the final four nights, average visits per night to each feeder rose with sucrose concentration (Fig. 3). Over the same period, the 15 percent sucrose feeder had lower consumption when compared to the other three, albeit not significantly (Fig. 3). The 15 percent conce ntration had the lowest nectar consumption three out of four nights, and the 30 percent concentration had the highest nectar consumption three out of four nights during nights six through nine. DISCUSSION Over the first five nights, there was no signifi cant difference between either visitations or nectar consumption tested over different sucrose percentages. This is likely due to bats preference for two of the five feeders based on position (Fig. 1). The two feeder posts that the bats preferentially vi sited were the two nearest the forest. After switching from observing five feeders at separate posts to observing four feeders attached to a single post, which only had four hooks for feeders, I decided to eliminate the ten percent solution, as it had the lowest number of total visitors over the five nights, and also the lowest amount of nectar consumed over the same period (Fig. 2). Additionally, t here are few naturally occurring bat flowers with nectar below 12 percent equivalent sucrose concentration (B aker et. al 1998). During nights six through nine, I observed no significant difference between sucrose concentration and visitation or nectar consumption (Fig. 3). This may have occurred for several reasons. The bats may not have been able to reme mber where each percentage of nectar was located, or they may not have been able to differentiate between the sucrose solutions that were only five percent apart from each other. To my knowledge, no studies have been carried out to see just how sensitive nectar bats are to changes in sucrose concentration, and this would be an excellent future study. Also, if there were many novel visits each night, bats would not know that the concentrations differed from the usual 20% concentration used in the feeders. The rate of revisits simply may not have
been high enough to see a difference in foraging behavior if the bats could detect differences after consumption, but not before the visit. Some trends were apparent even though the data found were non significant . Over the final four nights, average visits per night to each feeder rose with sucrose concentration (Fig. 2). Also, the 15% sucrose solution saw less consumption, while the 20, 25 and 30% solutions had very similar nightly consumption means. Along wit h the other trends highlighted earlier, these all point to bats preferentially foraging on the higher sucrose concentrations. This may have something to do with 20 percent sucrose equivalent to natural flowers being around the low energy barrier for bats (Helverson, 1993), and it is possible that they may prefer to forage on flowers greater than or equal to this sugar concentration. However, if bats truly cannot distinguish between 15 and 20% sucrose solutions, it would be extremely advantageous for flowe rs to produce nectar around 15%. However, flowers producing nectar with higher sucrose concentrations may confer flower fidelity in nature, since bats may learn which flowers have higher detectable concentrations. More research is needed, especially look ing at lower sucrose concentrations, between five and twenty percent, before definitive conclusions can be drawn. ACKNOWLEDGEMENTS I would like to thank Alan Masters for his support, ideas, and guidance. My gratitude also goes out to Moncho CalderÃ³n for all the time he spent with me both in the field and the lab teaching and helping refine the project. I would also like to thank Anjali Kumar, Pablo Allen, and especially Yimen Araya for their assistance with data analysis and all the help they have offer ed. In addition, the advice of Richard LaVal has been extremely helpful throughout the project, and for that my thanks goes out to him. Finally, I want to extend my thanks to the owners and staff of Selvatura Park for allowing me to use their hummingbird garden as a study site. LITERATURE CITED Baker, H. G., Baker I., and Hodges S. A. 1998. Sugar composition of nectars and fruits consumed by birds and bats in the tropics and subtropics. Biotropica 30(4):559 586. Baker, H. G. and Frankie, Gordon W. 1974. The importance of pollinator behavior in the reproductive biology of tropical trees. Anales del Instituto de BiologÃa. 1:1 10 Findley, J. S., and D. E. Wilson. 1983. Are bats rare in tropical Africa?. Biotropica 15: 299 303. Helversen, O. V. 1993. Adaptat ions of Flowers to the Pollination by Glossophagine Bats. In W. Barthlott et al. (ed.), Animal plant interactions in tropical environments. 159 165 Helversen, O. V. and Reyer, H. U. 1984. Nectar intake and energy expenditure in a flower visiting bat. Oecol ogia 63(2):178 184. Heyneman, Amy J. 1983. Optimal sugar concentrations of floral nectars dependence on sugar intake efficiency and foraging costs. Oecologia 60(2): 198 213. Howell, D. J. 1974. Bats and pollen: Physiological aspects of the syndrome of c hiropterophily. Comparative Biochemistry and Physiology Part A: Physiology 48(2): 263 276. LaVal, Richard K., RodrÃguez, Bernal. 2002. MurciÃ©lagos de Costa Rica / Bats. INBio. Nadkarni, N. M., and Wheelwright, N. T., 2000. Monteverde. Oxford University Pre ss, New York, U.S.A. Roces, F., Winter, Y., Helversen, O. V. 1993. Nectar concentration preference and water balance in a flower visiting bat: Glossophaga soricina antillarun. In W. Barthlott, et al. (ed.). Animal plant interactions in tropical environmen ts. 159 165 Sullivan, Jeremy C. 2000. Preferred sucrose concentration in nectarivorous bats. In Tropical Ecology and Conservation. CIEE, Monteverde, Costa Rica. Fall 2000: 110 117
Timm, R. M., LaVal, Richard K. 1998 A Field Key to the Bats of Costa Rica. The Center of Latin American Studies. No.22: 1 30 Willig, M. R. and K. W. Selcer. 1989. Bat species diversity gradients in the New World: a statistical assessment. Journal of Biogeography 16:189 195. Figure 1 The effect of post position on average milliliters consumed per night and average visits per night. Error bars are standard error of the mean. Figure 2 The effect of sucrose concentration on average milliliters consumed per night and average visits per night over the first five nights. Error bars are standard error of the mean.
Figure 3 The effect of sucrose concentration on average milliliters consumed per night and average visits per night over the final four nights. Error bars are standard error of the mean.