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Dureza de las semillas en Persea americana (Lauraceae) y Dioclea reflexa (Papilionaceae) y el comportamiento de alimentacin en las guatusas (Dasyprocta punctata)
Seed toughness in Persea americana (Lauraceae) and Dioclea reflexa (Papilionaceae) and feeding behavior in agoutis (Dasyprocta punctata)
Agoutis are a common study subject because of their caching behavior and for their role in seed dispersal in the tropics (Wainwright 2002). This study examined the relationship between the hardness of seed coats and their rate of consumption by conducting trials at eight known agouti territories in Monteverde, Costa Rica using softened and untreated seeds. Trials were conducted separately for each seed species. A significant difference was found between the number of hard and soft seeds of Persea americana and Dioclea reflexa left behind at the sites. According to statistical tests, agoutis seem to prefer softer seeds of P. americana over hard seeds (Wilcoxon Matched Pairs test, p = 0.0858) and no preference in D. reflexa (Wilcoxon Matched Pairs test, p = 0.715). This difference can be explained by the much more noticeable softening of P. Americana when compared to D. reflexa, which may have allowed a shorter seed handling time.
Las guatusas son un objeto comn de estudio por su comportamiento de almacenamiento de semillas y porque cumplen un papel importante en la dispersin de semillas en los trpicos (Wainwright 2002). Este estudio examino la relacin entre la dureza de las semillas y su tasa de consumo haciendo pruebas en ocho sitios en Monteverde, Costa Rica, usando semillas suavizadas y no tratadas.
Text in English.
Central American agouti
Las guatusas de Centro Amrica
Tropical Ecology 2007
Ecologa Tropical 2007
Dureza de las semillas
t Monteverde Institute : Tropical Ecology
Seed toughness in Persea americana (Lauraceae) and Dioclea reflexa (Papilionaceae) and feeding behavior in agoutis ( Dasyprocta punctata) Maria Peterson Department of Zoology, Univ ersity of Wisconsin-Madison Abstract Agoutis are a common study subject because of thei r caching behavior and for their role in seed dispersal in the tropics (Wainwright 2002). This study examined the relationship between the hardness of seed coats and their rate of consumption by condu cting trials at eight known agouti territories in Monteverde, Costa Rica using softened and untreated se eds. Trials were conduc ted separately for each seed species. A significant diffe rence was found between the number of hard and soft seeds of Persea americana and Dioclea reflexa left behind at the sites. According to statistical tests, agoutis seem to prefer softer seeds of P. americana over hard seeds (Wilcoxon Matched Pairs test, p = 0.0858) and no preference in D. reflexa (Wilcoxon Matched Pairs test, p = 0.715). This difference can be explained by the much more noticeable softening of P. Americana when compared to D. reflexa which may have allowed a shorter seed handling time. Resmen Las guatusas son objeto comn de estudio por su comportamiento de almacenamiento de semillas y porque cumplen un rol importante en la dispersin de semillas en los trpicos (Wainwright 2002). Este estudio examin la relacin entre la dureza de las semillas y su tasa de consumo haciendo pruebas en ocho sitios en Monteverde, Costa Rica, usando semillas suavizadas y no tratadas Se hicieron pruebas separadas para cada especie de semillas. Se encontr una difere ncia significativa entre el nmero de semillas duras ysuavizadas de Persea americana y Dioclea reflexa en los sitios. Las gu atusas parecen tener una preferencia para las semillas suavizadas de P. americana y no presentan preferencia en D. reflexa. Esta diferencia se puede explicar porque la suavidad es ms notoria en P. americana que en D. reflexa lo que puede tener un efecto en el tiempo de manejo de la semilla. Introduction Seed dispersal is a challenge for many tropi cal trees and plants, which must utilize creative ways to make sure their seeds are spread around the forest. One common dispersal mechanism for plants that fruit and be ar large seeds is the relationship with the agouti ( Dasyproctus punctata ), which exhibits a scatterhoarding behavior (Wainwright 2002). Agoutis are diurnal mammals that are exclusively ground-dwel ling and feed on an assortment of food items, including fruits, se eds, fungi, insects, and crabs (Wainwright 2002). They are most active during the late af ternoon and prefer to i nhabit forested areas with dense undergrowth (Emmons 1997). They tend to cache extra seeds throughout the rainy season when food is plentiful so that they will have a suffi cient amount when food is scarce (Wainwright 20 02). They also have been known to eat the sweet fruit of some trees when ripe, but cache the hard seed wh ich is sometimes initially inedible (Smythe 1970). Agoutis locate th eir caches through visual and ol factory cues, but usually do not recover all of their caches and therefore help distribute seeds of many tree species, including those of Persea americana and some Dioclea reflexa species (Murie 1977).
Plant species have evolved different mech anisms to deal with seed predation, including toxic compounds and tough seed coats (Janzen 1971). Previous studies indicate that seed preference may be based on these deterrents or other factors including nutritional content or handling time (Meiners 1997). In th is study I will examine if agoutis show a preference in the seed toughness of P. americana and D. reflexa I expected that there would be a difference in the number of normal versus softened seeds that were eaten. Based on previous research, it is my prediction that agoutis will eat more of the seeds that have been softened. They will prefer softer seeds because of the shorter handling time and the higher effectiveness of caching hard seeds, which will be less prone to decay than softened seeds. Materials and Methods This study was conducted from mid-July to early August of 2007 for two weeks in the area of Monteverde and Santa Elena in Costa Rica. Eight known territories of D. punctata located in this area were used to conduc t experimental trials. These territories were mostly located on forest edges in fair ly open areas, although two were located in closed forest. Both Dioclea reflexa and Persea americana were used in separate trials. In order to test the effect of s eed toughness, half of the seed s to be used in the upcoming trial were soaked for 48 hours in water prior to use in a trial. Thr ee trials were conducted using P. americana while two trials were conducted with D. reflexa For each of the eight D. punctata territories, ten control seeds and ten soaked seeds of P. americana were placed in two separate piles about one meter apart from each other with untreated seeds placed on the left and softened seeds placed on the right. The same procedure was used for the D. reflexa trials, with the exception that five control and five soaked seeds were used at each site. The sites were checked after 48 hours to inventory the number of seeds that were uneaten (Appendix 1). After re cording how many seeds remained, new seeds were placed of either P. americana or D. reflexa such that the trials alternated between each seed species. Statistical analysis in cluded non-parametric Wilcoxon Matched Pairs tests, used to determine if there was a signi ficant difference between the types of seeds eaten or left at the sites. Seeds were assu med to be either eaten or cached if removed from the site. Results For the duration of the study, a total of 480 P. americana seeds and 160 D. reflexa seeds were placed at the eight sites. Overa ll, the removal of softened seeds was more prevalent for P. americana than for D. reflexa (Wilcoxon Matched Pair s tests). There was a tendency for the agoutis to leave behind fe wer softened seeds than tough seeds in P. americana (Wilcoxon Matched Pairs test, p = 0.0 858, n = 24, df = 22). There was no difference in preference betw een softened and untreated D. reflexa seeds (Wilcoxon Matched Pairs test, p = 0.715, n = 16, df = 14). While neither p-values were significant for either seed species, it can be seen from Figures 1 and 2 that there was a definite difference between the number of soft ened seeds and hard seeds left of P. americana. With further trials or the addition of more s eeds, a significant diffe rence may be found for the seed type preferred by agoutis in P. americana Discussion
Agoutis appeared to prefer softer seeds to the hard seeds for P. americana because they became softer in 48 hours of soaking in water than D. reflexa The entire seed became much easier to crack open in addition to the meat of the seed itself becoming softer, as observed during the trials that were conducted. Further research could be done to determine exactly why the softer seeds are preferred. It could be a difference in the handling time or even so mething as simple as a change in the palatability of the seed, as noted in previ ous studies (Meiners 1997). A shorter handling time would be preferred by agoutis because it would expose them to less predation by shortening the amount of time they would need to spend sitting in one place and chewing a seed or fruit (Meiners 1997). A softer seed, or a seed that has been softened naturally by rain or other conditions, would decreas e the amount of time necessary for seed consumption and thus might be preferred over tougher seeds. In contrast to P. americana, there was shown to be no difference between the number of untreated and soft seeds of D. reflexa left at each site. It was observed during the experiment that the soaking of D. reflexa seeds did not soften them as much as P. americana The D. reflexa seeds had a much tougher exocarp, which made them less permeable to water. The fact that there wa s not a difference between the untreated seeds and the soaked seeds is shown by the fact that the agoutis did not a ppear to distinguish between the two types (Figure 2). D. reflexa seeds appeared to have been chosen either randomly or without preference w ith regards to seed softness. Another factor that may have had an effect on the number of seed s consumed at each site was the decreased amount of seeds placed at the sites wh en compared to the amounts left of P. americana (Tables 1 and 2). As seen in the tables, some of the sites had all of the seeds removed of both types, which may have skewed the results. More trials or the use of more seeds may confirm or reject the idea that agoutis have no preference for seed toughness in D. reflexa These results show that the agoutis may be choosing the food they eat based on its seed toughness, among other factors. Their scatter-hoarding behavi or has often been studied due to its effect on seed survival rates and plant redistri bution (Wainwright 2002), so studies on preference with regards to seed toughness may illuminate more aspects of this behavior. For instance, agoutis may choose to cache seeds with harder seed coats because they are able to remain fresher than seeds with thin seed coats, which might decay rapidly (Janzen 1971). If this were th e case, it should be common to find varieties of soft seeds more prone to predation immedi ately after falling from the tree or after many days of rain, which would soften some seeds further. Further studies could be conducted regarding caching behavior that would examine the seed toughness of the cached seeds in relation to seed s that are eaten at a site. Acknowledgements I would especially like to thank my advisor Tania Chavarra for her guidance, along with Karen Masters, Pablo Allen, and Camryn Pennington for their help with statistical analysis and my overall project design. I am very grateful to all of the landowners who allowed me to conduct my research on their properties, including Turid Forsyth, Bruce Young, Bob Law, Frank Joyce, and John Trostle. I also wish to express my gratitude for the opportunity to work at the Estacin Biolgica de Monteverde and in the beautiful Monteverde/Santa Elena ar ea with my eleven classmates wh o helped keep me mostly sane.
Literature Cited Emmons, L.H. 1997. Neotropical Rainforest Mammals A Field Guide The University of Chicago Press, Chicago pp. 229-230. Freeland, W. J. 1974. Strategies in herbivory by mammals: the role of plant secondary compounds. The American Naturalist 108 (961): 269-289. Janzen, D.H. 1983. Dasyprocta punctata Costa Rican Natural History. The University of Chicago Press, Chicago pp. 463-465. Janzen, D.H. 1971. Seed predation by animals. Annual Review of Ecology and Systematics 2: 465-492. Meiners, S.J. 1997. Selective predation on the seeds of woody plants. Journal of Torrey Botanical Society 124 (1): 67-70. Murie, J.O. 1977. Cues used for cache-finding by agoutis ( Dasyprocta punctata ). Journal of Mammology 58 (1): 95-96. Smythe, N. 1970. Relationships between fruiting seasons and seed dispersal methods in a neotropical forest. The American Naturalist 104 (935): 25-35. Wainwright, M. 2002. The Natural History of Costa Rican Mammals Distribuidores Zona Tropical, S.A. pp. 178-180.
Figures Std. Dev. Std. Err. Mean 7,8 8,2 8,6 9,0 9,4 9,8 10,2 10,6 11,0 CONTROLSOAKED Figure 1. A Wilcoxon Matched Pairs Test comparing the amount of Persea americana seeds that remained uneaten over thre e trials at eight different agouti ( Dasyprocta punctata ) territories. For this test, p = 0.0858 which was almost significant, thus signifying a tendancy to leave behind less soaked seeds than control seeds. For these trials, n=24 and the degrees of freedom = 22. (T=8, Z=1.718). Std. Dev. Std. Err. Mean 1 2 3 4 5 6 7 CONTROLSOAKED Figure 2. A Wilcoxon Matched Pairs Test comparing the amount of Mucuna sp. seeds that remained uneaten over two tr ials at eight di fferent agouti ( Dasyprocta punctata ) territories. The p-value for this test was 0.715, which indicates that th ere is no statistical difference between the amount of control or soaked seeds that remained uneaten. For these trials, n = 16 and the degrees of freedom = 14. (T = 4, Z = 0.365).
Appendix 1 Table 1. The data collected from each of the eight agouti sites for Persea americana for three separate trials. The number of seeds left after 48 hours out of the ten seeds placed at the beginning of each trial was recorded fo r both the untreated seeds and the seeds that had been soaked for 48 hours previously. site control (7/21) soaked (7/21) control (7/26) soaked (7/26) control (7/30) soaked (7/30) Station 10 6 10 10 10 10 Turid Forsyth 10 9 10 10 10 10 Bruce Young 10 10 10 10 10 10 Bajo del Tigre 9 10 9.5 10 10 10 Bob Law 10 8.5 9 10 10 10 Frank Joyce 10 10 10 9 10 10 John Trostle 10 10 10 10 10 10 Conservation Association 9 6 9 6.5 10 10 Table 2. The data collected from each of the eight agouti sites for Mucuna sp. for two separate trials. The number of seeds left after 48 hours out of ten seeds placed at the beginning of each trial was recorded for both the untreated seeds and the seeds that had been soaked for 48 hours previously. site control (7/23) soaked (7/23) control (7/28) soaked (7/28) Station 5 5 5 5 Turid Forsyth 5 5 0 0 Bruce Young 5 5 5 5 Bajo del Tigre 4 5 5 5 Bob Law 0 0 0 0 Frank Joyce 5 5 4 5 John Trostle 5 5 5 0 Conservation Association 4 5 5 5