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Efectos de objetos extraos frente a la presencia de depredadores en el comportamiento de forrajeo en los murcilagos nectarvoros
Effects of novel object versus predator presence on nectarivorous bat foraging behavior
Bat foraging behavior has been shown to be affected by the presence of predator models near feeding sites. A previous study found significantly lower visitation at feeders with a model owl or snake next to it but no difference was found between the two predator models. In this study, foraging behavior was observed by recording bat visitations to hummingbird feeders with different treatments. The treatments
consisted of a control feeder and feeders with either a shoe or a snake model nearby. Four treatment conditions were observed: control versus snake, control versus novel object, control versus control, and
novel object versus snake. The novel object did not significantly affect visitation but visitation was significantly less at the feeder with the snake model.
En estudios previos se ha mostrado que el comportamiento de forrajeo de los murcilagos es afectado por la presencia de modelos de depredadores en las cercanas a los lugares de alimentacin. Un estudio previo encontr que el nmero de visitas a bebederos con un modelo de bho o serpiente en la cercana fue significantemente inferior pero no se encontraron diferencias significativas entre los modelos de los depredadores. En este estudio, el comportamiento de forrajeo de los murcilagos fue observado mediante el conteo de visitas a bebederos de colibres con diferentes tratamientos. Las diferentes condiciones consistieron en una muestra testigo y bebederos con un zapato, o con un modelo de serpiente prximos a ellos. Cuatro condiciones fueron observadas: Testigo contra serpiente, testigo contra objeto extrao, testigo contra testigo y objeto extrao contra serpiente. El objeto extrao no afect significativamente el nmero de visitas, pero el nmero de visitas fue significativamente inferior en el bebedero con el modelo de la serpiente.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone--Santa Elena
Costa Rica--Puntarenas--Zona de Monteverde--Santa Elena
Tropical Ecology Summer 2005
Ecologia Tropical Verano 2005
t Monteverde Institute : Tropical Ecology
Effects of Novel Object versus Predator Presence on Nectarivorous Bat Foraging Behavior Lizeth Daz Department of Biology, University of Illinois in Chicago ABSTRACT Bat foraging behavior has been shown to be affected by the presence of predator model s near feeding sites. A previous study found significantly lower visitation at feeders with a model owl or snake next to it but no difference was found between the two predator models. In this study, foraging behavior was observed by recording bat visitati ons to hummingbird feeders with different treatments. The treatments consisted of a control feeder and feeders with either a shoe or a snake model nearby. Four treatment conditions were observed: control versus snake, control versus novel object, control v ersus control, and novel object versus snake. The novel object did not significantly affect visitation but visitation was significantly less at the feeder with the snake model. RESUMEN En estudios previos se ha mostrado que el comportamiento de forrajeo de los murcilagos es afectado por la presencia de modelos de depredadores en las cercanas a los lugares de alimentacin. Un estudio previo encontr que el nmero de visitas a bebederos con un modelo de bho o serpiente en la cercana fue significantement e inferior pero no se encontraron diferencias significativas entre los modelos de los depredadores. En este estudio, el comportamiento de forrajeo de los murcilagos fue observado mediante el conteo de visitas a bebederos de colibres con diferentes tratam ientos. Las diferentes condiciones consistieron en una muestra testigo y bebederos con un zapato, o con un modelo de serpiente prximos a ellos. Cuatro condiciones fueron observadas: Testigo contra serpiente, testigo contra objeto extrao, testigo contra testigo y objeto extrao contra serpiente. El objeto extrao no afect significativamente el nmero de visitas, pero el nmero de visitas fue significativamente inferior en el bebedero con el modelo de la serpiente. INTRODUCTION Predator prey interaction s affect populations at many different levels. Studies have found a significant decrease in feeding activities of small birds in the presence of predator models (Alcock 1984). Similar studies have found significantly lowered hummingbird visits to feeders w ith a predator model present independently of the caloric reward (Flowers 1998), and reduced visitation rates of insects to flowers of Clibadium leiocarpum (Tomon 1995). Altringham (1996) notes that bat specialized predators are few (including hawks, owls, and snakes, particularly arboreal snakes). Occasional bat eaters include: monkeys, mustelids, raccoons, oppossums, cats, other bats, raptors, snakes, frogs, fish, and a few large arthropods, mainly spiders. Only birds and snakes appear to be regular preda tors on bats that affect their populations (Altringham 1996). Specialized bat predators include the bat falcon ( Falco rufiguloris ), the brown tree snake ( Boiga irregularis ), and owls, which prey on bats on a regular basis (Findley 1993).
Heuer (2004) stud ied the effect of predator avoidance on the feeding habits of nectarivorous bats at hummingbird feeders. Heuer found that fewer bats fed at feeders where either a fake owl or snake were present, as compared to a control group with nothing present at the fe foraging behavior of nectarivorous bats might be influenced by predator presence. indicating that it may jus t be a novel object that reduced visitation. Heuer did not test the effect of a novel object on bat visitation. The observations in the current study were done at a guild level, because of the difficulty in distinguishing between species in flight at the f eeders. The current study were indeed recognizing and avoiding predators, or if they were just reacting to a strange object near the feeding site. The present study attempted to gain further understanding on the impact of predator avoidance on the foraging behavior of bats, since a compromise must be made between conflicting factors, i.e. caloric reward vs. potential predation. METHODS Data collection occurred at Selvatura, near the Santa Elena reserve, between July 8th and August 1st. Bat visits were recorded at hummingbird feeders in the hummingbird garden. Data were collected between 7:00 p.m. 9:00 p.m. for seven evenings. A rubber snake (1.55 m long, 0.03 m th ick) was rubbed with snake scent obtained from a snake skin in order to imitate a snake both visually and olfactorily. The owl model was excluded from this study as no significant difference was found between snake and owl e was chosen as a novel object to compare to the snake and control. For each observation, two feeders (six meters apart) were monitored, and visitation counted during five minute intervals. Four treatments were observed: control versus snake, control vers us shoe, shoe versus snake and control versus control. The treatment objects were placed on the feeder stands at approximately 0.25 m from the feeders. The feeders were illuminated with mag lights in order to view the bats and count visits. In order to co ntrol for preset feeder preferences and influences from surrounding objects, snake, shoe and control feeders were rotated after each set of observations. In order to avoid feeding time dependent patterns, the order of the treatments was rotated on a nightl (2004), but the owl model was replaced with the shoe. A feeding visit was counted as a pause in front of the feeder holes, regardless of the duration of the pause, or if the bat landed on the feeder or not. All bats that approached the feeder, whether or not they were new individuals, were counted as visits. Moonlight and weather conditions were also recorded for each observation period. An ANOVA test was used to compare the dif ferences in means between the three significant differences among the treatments. ANOVA tests were also used to compare number of visits by night, weather conditions, and time. Paired t tests were used to
compare feeder visitation for the four different conditions: control versus snake, control versus shoe, shoe versus snake and control versus control. Unpaired t tests were used to compare feeder visitations under dry versus rai a light shower was occurring), and to compare the visitations to the snake treated feeder under dry versus rainy conditions. RESULTS 30, five minute observations were conducted for the snake versus control treatm ent; 34, five minute observations were conducted for the shoe versus control treatment; 30, five minute observations were conducted for the shoe versus snake treatment, and 12, five minute observations were conducted for the control versus control treatmen t. Visitation was significantly different between the treatments (ANOVA, F = 7.22, between visitation to the control group (N = 39 x = 467.923 136.765) versus the shoe tr eatment (N = 31 x = 471.194 145.244). Significantly fewer visits were made to the snake PLSD, P = 0.0012) (Fig. 1). The number of bat visits was significantly higher at the con trol feeders as compared to the snake treated feeders (Paired t test: t = 4.055, P = 0.0014). No significant difference in the number of bat visits was found between the control feeders and the shoe treated feeders (Paired t test: t = 0.561, P = 0.5832). T he control feeders were found to be not significantly different (Paired t test: t = 0.785, P = 0.4765). The number of bat visits was significantly higher on the shoe treated feeders as compared to the snake treated feeders (Paired t test: t = 5.064, P = 0 .0002). No trend was found in visitation rates over the seven nights of observations but visitation varied significantly by night (ANOVA, F = 4.44, P = 0.002. The number of visits was significantly higher on the 3 rd and 4 th ificant difference in visitations was found between weather conditions (ANOVA, F = 1.960, P = 0.134). Time of observations and number of visits were not significantly correlated (Pearson, r = 0.268, P = 0.0621). No significant difference between visitation s on dry versus rainy conditions was found (Unpaired t test: t = 1.683, P = 0.0990). The snake treatment visits were analyzed separately comparing dry versus rainy conditions and no significant difference was found (Unpaired t test: t = 0.586, P = 0.5632). The bat species present at the study site were Anoura geoffroyi, Glossophoga commissrisi, and Glossophoga soricina
DISCUSSION The species found at the study site belong to the family Phyllostomidae, and the sub family Glossophaginae. The member s of this sub family are small, nectarivorous bats with adaptations for a nectar diet, such as a long, narrow snout with sensory hairs and a long tongue. The members of this sub family are important pollinators in tropical forests and they may behave like hummingbirds in many ways (LaVal 2002). Bats showed significantly higher feeding activity on the presence of the novel object and the control group as compared to the snake model near the feeders. Feeding activity was not significantly affected by the pre sence of a novel object (the shoe), as compared to the control feeding site. These results indicate that bats avoid specific predators, and are not only responding to foreign objects near their feeding site. tly with successive nights as a result of feeding disturbance avoidance. Mag lights may have caused a decrease in visitation rates to the feeders because bats appeared to slightly shift to the dark feeder in all occasions. Weather conditions did not affect the number of bat visits significantly, but dry, dark, windless observation conditions tended to show slightly higher feeding activity. Rain did not appear to be a determinant factor for bat visits. The conditions nstances when light showers were occurring, as heavy rain caused bats to stop visiting the feeders completely. Further studies with higher numbers of observation nights under different weather conditions may help determine the effects of weather on nectar ivorous bat foraging behavior. No significant differences between rainy and dry conditions on visitation numbers at the snake treatment feeders may indicate olfactory cues are not important as light rain increased the strength of the snake smell. The effec t of rain on the snake treatment feeder was found to be lower than the effect of rain in general. Further studies how heavily nectarivorous bats rely on their sense of smell versus echolocation and sight for detecting predators. A study that includes a scented snake and an unscented snake would be useful for this purpose. The time of the night was not found to be significantly influential on bat visitations to the fee ders. There was a slight increase in bat visitations as time passed on the nights when observations were made, further research on the peak activity hours of nectarivorous bats could help in gaining a deeper understanding of a model of nectarivorous bat op timal foraging. AKNOWLEDGEMENTS Antes que nada, gracias a mi familia, los kero ms que a mis ojos. Este proyecto va dedicado a ellos, en especial a Danny, a quien doy gracias por haber nacido. Gracias a mis amigos y a todos los que creyeron y siempre ha n credo en mi. Gracias a Lili, Maru, Lali, Martha, Maria y Cristian y todo el staff de la estacin por ser tan chidos. Thanks to Carlos, for helping and supporting me so much, y recuerde que somos compaeros de guerra en la jungla! I would also like to th ank the Selvatura staff for letting me use their feeders and their hummingbird garden, and for being so caring to me. I would like to thank Richard for helping me with identifying the bat species. Nathaniel and Maria, thank you for attending my every panic attack. Maria, thanks for taking the pictures. Thanks to all my wonderful classmates, who have been
following up with this project since the twilight walk, and were very interested, yet too lazy to go with. Y finalmente, gracias Javi por toda tu ayuda, y por ser tan tuanis. Y que viva el Rockanrol! Figure 1: Comparison of the mean number of bat visits in treatments of A: Control (N = 39, x = 467.923 136.765) versus B: Shoe (N = 31, x = 471.194 145.244) versus C: Snake (N = 28, x = 354.964 118.5 48). Each count consisted of a five minute observation period. Error bars represent one standard deviation. The mean number of visits were significantly different among treatments (one way ANOVA, F = 7.222, P = 0.001). The control and the shoe were not si gnificantly different from each other but the snake treatment had significantly
Table 1. Observations summary table Night Total # v isits Total # visits Total # visits Visits with snake Visits with snake ----------------Dry Wet Dry Wet 1 10 553 10 553 0 1028 0 2 6 044 1 308 4 736 0 962 3 10 323 10 323 0 1839 0 4 11 309 11 309 0 1454 0 5 8 392 0 8 392 0 2059 6 10 389 10 389 0 1366 0 7 8 362 8 362 0 0 1231
LITERATURE CITED Alcock, J. 1984. Animal behavior: An evolutionary approach. Sinaur Associates, Inc. Sunderland, Massachussetts. pp. 270 Altringh am, J. D. 1996. Bats: biology and behaviour. Oxford University Press, New York, New York. pp. 185, 211, 221 222. Findley, J. S. 1993. Bats: a community perspective. pp. 44 45. Cambridge University Press, Cambridge, Great Britain. Flowers, T. 1998. Trade offs between caloric gain and anti predator defense in hummingbirds (Tochilidae). Tropical Ecology and conservation. CIEE, Spring, pp. 150 162. Heuer, R. 2004. The effects of predator presence on nectarivorous bat foraging behavior. Tropical Ecology and C onservation. CIEE, Summer, pp. 88 95. LaVal, R. J. 2002.Murcilagos de Costa Rica BATS pp. 134. Editorial INBio, Costa Rica. Tomon, T. J. 1995. Predator recognition by insects foraging at flowers of Clibadium leiocarpum (Asteraceae). Tropical Ecology an d conservation. CIEE, Summer, pp. 236 244.