Seed Preference in the Central American Agouti, Dasyprocta Punctata
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Seed preference in the Central American Agouti, Dasyprocta punctata Kristen Coogan Department of Wildlife and Conservation Biology, University of Rhode Island ABSTRACT Scatter-hoarding animals in the tropics typically depend on a cached supply of reserves when food is scarce. The Optimal Foraging Theory seeks to explain the behaviors these organisms exhibit upon prey selection. It is important for these scatter-hoarding animals to choose the most beneficial food item that will increase their energy intake. Central American agouties (Dasyprocta puncata) were presented with two differently sized seed species, the water apple (Syzygium malaccense) and the peach palm (Bactris gasipaes). A total of 400 flagged seeds were used, 200 of each species in groups of 20 seeds per species, over a 14 day period in Monteverde, Costa Rica. The seeds were collected and observed to determine seed fate. There was a significant difference in seed weight between the two species (T-test, p = <0.0001) and a significant difference between which species D. punctata chose to cache more ((2, p = <0.05), although there was no significant difference between which seed species D. punctata chose to eat (2, p=>0.05). Although D. punctata chose to cache more of the larger seed species, size and weight alone may not be the only measure for seed selection, therefore D. punctata may exhibit other selection processes in order to obtain its energy intake. RESUMEN Los animales dispersores y acaparadores tropicales dependen típicamente del suministro oculto de reservas cuando el alimento es escaso. La Teoría de Forrajeo Óptimo procura explicar las conductas que estos organismos exhiben en la selección de presa. Es importante para estos animales dispersores y acaparadores escoger el artículo de alimento más beneficioso que aumentará su toma de energía. Se presentaron dos especies de semillas de diferentes tamaños, la manzana de agua (Syzygium malaccense) y el pejibaye (Bactris gasipaes) a la guatusa centroamericana (Dasyprocta punctata). Se utilizó un total de 400 semillas marcadas; 200 de cada especie en grupos de 20 semillas por especie en un período de 14 días en Monteverde, Costa Rica. Las semillas se colectaron y fueron observadas para determinar el destino de cada una. Se encontró una diferencia significativa en el peso de las semillas entre las dos especies (prueba de t, p = <0.0001) y una diferencia significativa entre cuál especie D. punctata enterró más frecuentemente (2, P = <0.05). Sin embargo, no hubo diferencias significativas entre las especies que D. punctata prefirió consumir (2, P = >0.05). Aunque D. punctata escogiera enterrar más semillas de la especie más grande, el tamaño y el peso por si solos no pueden ser los únicos criterios para la selección de la semilla; por lo tanto, puede que D. punctata utilice otros procesos de selección para obtener su energía. INTRODUCTION The goal of every organism in their lifetime is to survive and reproduce. Natural selection determines what phenotypes will be represented in future generations based on adaptation to selective pressures in the surrounding environments (Osborne 2000). Organisms must evaluate resource availability, foraging techniques, and survival probability in order to guarantee their survival and fitness. The Optimal Foraging Theory states that an organism will choose a feeding method that will maximize their net rate of energy intake. The net energy is measured as the amount of energy in the food minus the energy expended in finding, investing, and digesting it (Osborne 2000). The Optimal Foraging theory seeks to explain certain behaviors exhibited by organisms upon their diet selection. Selection of prey is made by evaluating both the positive and negative outcomes, and selection may be influenced by prey density, prey size, and nutrient content. Organisms will generally discriminate against food items which will not optimize their survival or fitness and choose those that will be most beneficial to them. Many animals hoard food from local abundance in order to conserve it for future use. In the case of a scatter-hoarding organism, small amounts of food are dispersed among many spatially spaced caches (Jansen et al. 2002). These individuals need to evaluate their food quality, in order to receive the highest net energy gain for the present time, as well as in the future when retrieving their cached reserves. It is beneficial for organisms which exhibit the scatter-hoarding strategy to manage a small supply of large seeds rather than a large supply of small, less nutritious seeds (Jansen et al. 2002). Seed fate is also important in determining the value of the food source. Scatter-hoarding animals typically eat few seeds while caching others. The seeds they store are presumably more valuable than the seeds they are consuming in order for them to be more beneficial in times of scarcity (Hallwachs 1994). The Central American Agouti, Dasyprocta punctata a scatter-hoarding organism. This species is a ground dwelling rodent, mostly active throughout the day, which can withstand disturbed habitats (Wainwright 2002). The diet of D. punctata consists mainly of fruits and seeds that are high in nutrients. Unlike the equally common seminivore- frugivore related species in the area, Agouti paca, D. punctata is unable to store fat. Their solution to seasonal scarcity is to bury seeds in times of plenty and dig them up later (Smythe 1978). Agoutis are therefore classified as a scatter hoarding rodent that usually caches a surplus of seeds in a dispersed pattern due to the uncertainty of retrieval (Smythe 1978). Seeds that have not been recovered have a possibility of germination, thereby also making D. punctata an important disperser for many plants. The aim of this study was to examine what preferences D. punctata exhibited for seed size. Smaller seeds were expected to be eaten due to their less nutritional value, while larger seeds were expected to be cached in order to benefit the agouti in times of scarcity. METHODS This experiment took place from April 23rd through May 7th, 2006 at the Santuario Ecológico located in Cerro Plano, Costa Rica, during the dry season (Fig. 1). The Santuario Ecológico is a secondary, lowland forest that was previously used for banana production and agouties are often observed foraging in a patch of open understory, approximately six meters by two meters in size. Two morphologically different tree seed species were used in choice experiments to determine whether D. punctata commonly took larger seeds over smaller seeds. The two species chosen for this study were Syzygium malaccense and Bactris gasipaes. The value of the seeds picked were placed under three categories based on their fate. The seed fate categories included being eaten, cached, or un-recovered. S. malaccense and B. gasipaes were purchased from a vegetable and fruit stand. The fruit was split with a knife; seeds were removed and then washed and dried. In order to locate the seeds in the field, each seed needed to be marked with flagging tape. Holes were carefully drilled in the center of every seed, and a meter of test nylon fishing line was threaded through the seed and tied. Twelve centimeters of flagging tape were then attached to the fishing line. Seeds were weighed in grams and then each tape was recorded with the seed number and weight. D. punctata bury the seeds not the flagging tape, so cached seeds were easily found. Previous studies have shown that marking seeds in this manner does not alter disperser behavior; however, this cannot be discounted completely (Jansen et al. 2002). Twenty seeds of each species were placed in two separate piles at the site, side by side (Fig. 2). Seeds were placed at 9 a.m. and checked the following morning at 8 a.m. The fates of the seeds were classified into one of the three categories, those being eaten, cached, or lost. Eaten seeds were those with the entire embryo consumed, or if the flagging tape and fishing line were removed from the seed. Cached seeds were those that were removed from the original site and buried shallowly in the nearby soil. Seeds whose flagging was not found were classified as lost. A new supply of seeds was then offered, ensuring that there were twenty of each seed species presented at all times. A t-test was used to determine weight difference in seed species. A Chi-squared test was also conducted in order to test if there was any significant difference in the fate of the seeds. RESULTS A total of 400 seeds were presented to D. punctata, 200 of each seed species. One- hundred and eighty one seeds were found and collected for data. Results of the S. malaccense seeds found, 148 seeds were eaten and 43 were cached. One-hundred and seventy-one of the B. gasipaes seeds were eaten, while 19 were cached (Table 1). The average weight for S. malaccense seeds was 10.78 g while the average weight for B. gasipaes seeds was 3.77 g (Fig. 3). An unpaired t-test showed that there was a significant difference in weight between the two chosen seed species (t = 15.70, p < 0.0001, df = 398). S. malaccense was the larger seed species used, while B. gasipaes was the smaller species. A Chi-squared test was used to determine if there were significant differences between the numbers of eaten or cached seeds between both species (Fig. 4). There was no significant difference in the number of seeds eaten between the S. malaccense and B. gasipaes (Chi-squared = 1.65, p >0.05, df = 1). Conversely, there was a significant difference in the numbers of cached seeds between the two species (Chi-squared = 9.2, p <0.05, df = 1). The Chi-squared test found that D.