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Forrajeo de las bromelias selectivas en Cebus capucinus a lo largo de una orilla del bosque
Selective bromeliad foraging in Cebus capucinus along a tropical premontane wet forest edge
Herbivores of the Neotropics are generally highly selective in their foraging behavior. Cebus capucinus (Primates: Cebidae), an omnivore, has been observed tearing apart and chewing on the leaves of bromeliads in the Upper San Luis valley, Costa Rica. This study examines the use of bromeliads by Cebus capucinus. Specifically, I consider the role of plant size, leaf water content and toughness in determining which bromeliads and which leaves C. capucinus selects. Comparing intact to torn bromeliads based on size, this study determined that C. capucinus selects small- and medium-sized plants. Leaves in the intermediate position of the rosette were also found to be favored. Using a penetrometer and a dry weight test, these leaves were shown to have the highest water content and intermediate toughness. Results indicate that leaf water content plays the most influential role in selection. It appears that bromeliads act as important water sources for C. capucinus in the canopy. Bromeliads may actually provide a big proportion of this species water needs, diminishing the need to descend the trees to retrieve water. The finding that C. capucinus is able to consistently forage for leaves with high water content may also confer this species a greater ability to survive in human-impacted habitats; an important skill to have in the changing face of land use in the world today.
Los herbvoros neotropicales son muy selectivos en su comportamiento de forrajeo. Cebus capucinus, un omnvoro, ha sido observado rompiendo y masticando las hojas de bromelias en los Altos del valle de San Luis, Costa Rica. Este estudio examina el uso de bromelias por esta especie de mono. Especficamente, yo considero el papel del tamao de la planta, la cantidad de agua en las hojas y tambin la dureza de las hojas para determinar cules bromelias y cuales hojas fueron elegidas por Cebus capucinus. Una prueba de Mann-Whitney basada en tamao comparo las bromelias vivas y las rotas. Se determin que C. capucinus selecciona a las bromelias de tamaos intermedios. Tambin se descubri que se eligieron las hojas de edades intermedias. Un penetrmetro y una prueba de peso seco mostraron que estas hojas intermedias tienen una dureza intermedia y una cantidad ms alta de agua de todas las hojas de bromelias. El contenido de agua pareci ser el factor ms importante en determinar cules bromelias fueron elegidas por los monos, mostrando que las bromelias tienen mucha importancia en suministrar agua a los C. capucinus en la copa de los rboles, disminuyndoles la necesidad de bajar de los rboles para tomar agua. El hallazgo de que los C. capucinus son capaces del forrajeo constante para conseguir las hojas con alto contenido de agua puede tambin conferir a esta especie una mayor capacidad para sobrevivir en los hbitats afectados por los humanos; una importante destreza de tener en el rostro cambiante del uso del suelo en el mundo de hoy.
Text in English.
Browsing (Animal behavior)
Costa Rica--Puntarenas--Monteverde Zone--San Luis
Hojeada (comportamiento animal)
Costa Rica--Puntarenas--Zona de Monteverde--San Luis
Tropical Ecology Fall 2005
Water sources for Capuchin monkeys
Ecologia Tropical Otoo 2005
Fuentes de agua para los monos capuchinos
t Monteverde Institute : Tropical Ecology
Selective bromeliad foraging in Cebus capucinus along a tropical premontane wet forest edge Vanessa Machen Departments of Biological Aspects of Conservation and Zoology, University of Wisconsin, Madison ABSTRACT Herbivores of the Neotropics are general ly highly selective in their foraging behavior. Cebus capucinus (Primates: Cebidae), an omnivore, has been observed tearing apart and chewing on the leaves of bromeliads in the Upper San Luis valley, Costa Rica. This study examines the use of bromeliads by Cebus capucinus Specifically, I consider the role of plant size, leaf water content and toughness in determining which bromeliads and which leaves C. capucinus selects. Comparing intact to torn bromeliads based on size, this study determined that C. capu cinus selects small and medium sized plants. Leaves in the intermediate position of the rosette were also found to be favored. Using a penetrometer and a dry weight test, these leaves were shown to have the highest water content and intermediate toughness Results indicate that leaf water content plays the most influential role in selection. It appears that bromeliads act as important water water need s, diminishing the need to descend the trees to retrieve water. The finding that C. capucinus is able to consistently forage for leaves with high water content may also confer this species a greater ability to survive in human impacted habitats; an importa nt skill to have in the changing face of land use in the world today. RESUMEN Los herbvoros neotropicales son muy selectivos con lo que se alimentan. Cebus capucinus un omnvoro, ha sido observado rompiendo y masticando hojas de bromelias en los Altos de San Luis. Esta investigacin examina el uso de bromelias por esta especie de mono. Se consider el papel del tamao de las bromelias, la cantidad de agua en las hojas y tambin la dureza de las hojas para determinar cules bromelias y cules hojas fuero n elegidas por Cebus capucinus Una prueba de Mann Whitney basada en tamao compar las bromelias vivas y las rotas. Se determin que C. capucinus selecciona a las bromelias de tamaos intermedios. Tambin se descubri que se eligieron las hojas de edades intermedias. Un penetrmetro y una prueba de peso seco mostraron que estas hojas intermedias tienen una dureza intermedia y la cantidad de agua ms alta de todas las hojas de bromelias. El contenido de agua pareci ser el factor ms importante en determin ar cules bromelias fueron eligidas por los monos, mostrando que las bromelias tienen mucha importancia en suministrar agua a C. capucinus en la capa de los arboles, diminuiendo la necesidad de bajar de los rboles para cobrar agua. Con todos los cambios e n el uso de tierra hoy en dia, el poder de esta especie de seleccionar las hojas de bromelias con el contenido de agua mas alta, encontrado en este estudio, le confiere una ventaja de sobrevivir en habitats afectados por seres humanos. INTRODUCTION Herb ivores forage selectively based on a number of criteria. According to Huntley (1991), these criteria include individuals and plant parts. Folivores herbivores whose diet mainly consists of leaves forage with high discrimination because they must minimi ze the difficulties of processing leaf contents such as cellulose and various toxins. Mammal folivores must therefore forage with extreme care in order to gain sufficient nutrients from their food to survive. Both Alouatta palliata (Primates: Cebidae) and
Bradypus variegatus (Xenartha: Bradypodidae), two well known folivorous mammals in the Neotropics, primarily select young leaves of just a fraction of the plant species that they have available to them (Glander and Rockwood, 1979; Wainwright, 2002). The di et of B. variegatus in fact, is so specialized that this species is rarely able to survive in captivity. Old World mammal herbivore species have also shown high selectivity in order to avoid toxic tissue (Howe and Westley, 1988). Cebus capucinus (Primate s: Cebidae), the white faced monkey, is a neotropical omnivore that focuses mainly on fruits and insects, eating leaf material only marginally (Chapman, 1990). Genus Cebus is known to spend only three to five percent of its time foraging for vegetative mat erial, while allocating up to 60% to foraging for fruits and insects (Terborgh, 1983). C. capucinus has been known to forage selectively for fruits and nuts, choosing the softest of these resources (Terborgh, 1983). However, because this species does not s pecialize on leaves, its foraging behavior as a functional folivore has not been widely studied. It would be expected that since C. capucinus prefers soft fruit, and other Neotropical folivores prefer young, tender leaves, that the leaves that C. capucinus selects would also be young and tender. However, the degree to which C. capucinus is discriminatory in the leaves that it eats and its criteria for selection remain widely unknown. In areas where resources are limited, individuals have been observed feed ing on plant shoots, mainly focusing on bromeliads (Nadkarni, 1984). C. capucinus exploits bromeliads for young plant material, juices, arthropod content, and to a lesser degree, the water collected in their tanks (Wainwright, 2002). Bromeliads that have b een manipulated by these monkeys have been observed in the Monteverde and San Luis areas, with a portion of the core still intact and chewed leaves strewn about it. As bromeliad leaves are unlikely to fall on their own and require force to tear off, torn b romeliads on the forest floor, particularly with chewed ends, are an accurate indicator of C. capucinus activity (Wainwright, 2002). A previous study in the Monteverde area by Friant (2005) showed that these torn bromeliads are more common in lower elevat ions than higher ones. This study was conducted in the dry season, when lower altitudes are drier. The study used the difference in abundances to indicate higher predation rates on bromeliads at lower elevations. However, the study did not take into accoun t relative abundance of intact bromeliads, bromeliads in each location and assessing their macrofauna content, that their content was not a determining factor in select ion of bromeliads. Therefore, C. capucinus selects according to other characteristics of the bromeliad. These findings prompted the present study to focus on the importance of bromeliad characteristics such as the size, toughness, and water content of leav es as they determine the selectivity of C. capucinus foraging. Bromeliad leaves are arranged in a rosette formation. The innermost leaf is the youngest, and each leaf in the rosette sequence increases in age toward the outside. The water content and tough ness of each leaf is likely to vary with age. These factors may aid in determining which leaves from a bromeliad C. capucinus chooses to eat and which it leaves intact. The purpose of this study is to ask a) whether C. capucinus utilizes bromeliads as a r esource in primary or edge habitats more often, b) whether size of bromeliad plant has an effect on the foraging habits of Cebus capucinus c) whether this species selects for a
certain optimal leaf based on toughness and water content, and d) where its op timal leaf occurs in the rosette sequence of the bromeliads. METHODS Study Sites All research was conducted either in a tropical premontane wet forest or on its edge in the Upper San Luis Valley, Costa Rica from 1120 to 1150 meters in elevation (Holdridg e, 1967). Edge habitats were defined as forested areas within 25 meters of a border with a major disturbance, such as a road or pasture. Primary forest habitat data were collected at least 50 meters from any such type of disturbance. Eight species of brom eliad are currently recorded for this area (Haber, 2000). Positive identification requires the inflorescence (Morales, 2000), which, if present, may become disjointed from the core upon manipulation by C. capucinus ; so bromeliads were partitioned by morpho species according to leaf shape and coloration. Only the most predominant was measured and included in the data. Survey of Torn Bromeliads by Habitat Type A brief preliminary study was conducted to measure the relative abundance of torn bromeliads in pr imary forest to edge habitat. Areas of study in each site were determined by identifying ten trees larger than 20cm dbh within ten meters of a path through the site. All trees used for the study could be seen from the path, and the area between the trees a nd the path was carefully swept to encounter all torn bromeliads possible. Tree density was assumed to be equal in primary and edge habitats. In each site, the area containing ten trees was surveyed for all torn bromeliads. One torn bromeliad was defined a s three or more leaves still connected at the base, forming a core, found with one or more disjointed leaves of the same morpho species within two meters of the core. To account for the possibility of C. capucinus tearing apart the entire bromeliad and lea ving no core, all single torn leaves were collected and measured. Any single leaf five centimeters or shorter would be considered as coming from the inner rosette of the bromeliad, discrediting the definition of torn bromeliads used for this study. Surve y of Intact and Torn Bromeliads by Size Size surveys of both intact and torn bromeliads were taken in edge habitats. Three sites of high torn bromeliad densities were identified. Each area contained at least 15 torn bromeliads within a five meter by five m eter area. Most bromeliads at each site were collected on one occasion, but all new torn individuals were collected thereafter, usually accumulating one to five per day. One of four size ranks was assigned to each bromeliad surveyed for both intact and tor n specimens, according to height. Bromeliad height is defined as the length of the longest leaf when standing erect. Four ranks were assigned; extra small (XS: 0 10cm), small (S: 10.1 19.9cm), medium (M: 20.1 35cm), and large (L: 35cm). Seventy tor n bromeliads were collected and measured in three sites of high torn bromeliad concentration. Because unattached leaves cannot be paired with the core from which they came, cores and leaves were counted separately in this survey. Torn cores collected were also examined to determine the number of leaves remaining on the core, as well as the length of the longest remaining leaf on the rosette.
Live bromeliads more than two meters off the ground in trees larger than 20cm dbh within ten meters of torn bromelia d sites were surveyed until 1000 intact specimens were counted and assigned a size rank. In addition, 20 intact bromeliads in the medium size rank were collected. The number of leaves per rosette were counted and recorded, along with the length of the long est leaf in the plant (bromeliad height). The average number of leaves per rosette was recorded for the medium size rank, allowing the average number of leaves removed per bromeliad by C. capucinus to be calculated. Toughness and Water Content of Leaves by Location in Rosette Sequence The 20 intact bromeliads in the medium size rank that were collected were divided into innermost leaves, 5 th 11 th 15 th and outermost leaves. These leaves were used to measure leaf toughness according to increasing age in the rosette sequence. Intact bromeliads were necessary for this analysis as opposed to torn ones to determine the toughness of the section of the leaf that C. capucinus generally eats away. A penetrometer was used to measure the toughness of five leaves of increasing age from each of the 20 bromeliads. The penetrometer consisted of a pin attached to a disk, which was situated over a hole that went through two metal plates. The portion of the leaf to be measured for toughness, an area no more than five milli meters from the base of each leaf, was placed between the two plates under the hole. The bases of the leaves were measured so as to test only the toughness of the area chewed by C. capucinus A container was placed atop the disk and water was added slowly until the leaf could no longer support the weight, and it was punctured. Water volume necessary for leaf penetration was recorded and converted into grams. One bromeliad of the predominant morpho species, medium size rank, and average number of leaves was collected as a representative specimen of the most common bromeliad chosen by C. capucinus Water content per square centimeter of the base of each leaf was measured to predict this value across all bromeliads of the same size in this morpho species. From each leaf in the bromeliad, a one cm by one cm square was cut from the base. Each was labeled with position in rosette sequence and weighed, placed in a dry box with a 50 watt light bulb for six hours, and subsequently weighed again. The difference betwee n wet weight and dry weight gave the water content per square centimeter for the base of each leaf. This value was recorded and compared with toughness values for leaves of increasing age. RESULTS Survey of Torn Bromeliads by Habitat Type The transect ( containing ten trees >20cm dbh) in the primary forest yielded 17 torn bromeliads. The transect in the edge habitat (also with ten trees >20cm dbh) contained 59 torn bromeliads. However, relative abundances of intact bromeliads in the trees in these areas w ere not taken into account in this preliminary study, so the reason for higher abundance in edge habitats could not be confirmed. Survey of Intact and Torn Bromeliads by Size Of eight species of bromeliad recorded for this region, four species were posit ively identified by their inflorescences and commonly seen among the intact bromeliads in the
trees. Three morpho species were identified among the torn bromeliads, only one of which was predominant. Only the predominant was included in the data analysis. Every fallen and torn bromeliad in all three sites along the edge habitat was observed as maintaining an intact core. Further, of all the single leaves observed in study sites, none was fewer than five centimeters, and therefore none was considered as ori ginating from the inner core of the rosette. The survey of the number of leaves remaining on the cores revealed that on average, C. capucinus removes all but 11 leaves from each bromeliad. The average length of the longest remaining leaf on the core was 28 .1cm. The average number of leaves on the rosettes of the 20 intact bromeliads collected was 28, signifying that the average number of leaves removed by C. capucinus was 17, calculated by subtracting the average number of leaves remaining on fallen cores. A Mann Whitney U test comparing the size of intact to torn bromeliads, showing significantly more small and medium torn bromeliads than intact ones (p<0.0001), This demonstrated a clear preference for small and intermediate sized bromeliads over extra sm all and large individuals. Size rank makeup of intact to torn data shown in Table 1. ______________________________________________________________ TABLE 1. A comparison of intact to torn bromeliads. Because there is a significant change in proportion of each size rank from intact to torn (p<.0001), C. capucinus is selectively foraging, disproportionately favoring the small size rank. Toughness and Water Content of Leaves by Location in Rosette Sequence Average toughness was found to increase with age of leaf. W ater content exhibited a sharp increase in the intermediate leaves, and lower values for both innermost and outermost leaves (Figure 2). TORN INTACT Size Rank Number Percent Number Percent TOTAL 70 100% 1000 100% XS: 0 10cm 1 1.4% 411 41.1% S: 10.1 19.9cm 35 50.0% 279 27.9% M: 20 35cm 31 44.0% 302 30.2% L: >35cm 3 4.3% 8 8.0%
FIGURE 1. Distribution of bromeliads by size rank and location. Intact bromeliads in trees are percents from a sample size of 1000, and torn bromeliads are from a sample size of 70. ________________________________________________________________________ FIGURE 2. A comparison of water content per squ are centimeter of leaf base (data taken from zero to one cm away from base) to leaf base toughness (data taken within 5 mm of base) based position of leaf in bromeliad rosette sequence (an indicator of age). Toughness data points are averages from 20 different bromeliads and the toughness (in grams necessary to puncture the leaf) of the innermost (1 st ), 5 th 11 th 15 th and outermost leaf in the rosette sequence. Error bars show the range of toughness for all 20 bromeliads. Innermost requi red an average of 194.5g to penetrate, 5 th : 313.5g, 11 th : 670.5g, 15 th : 793.3g, outermost: 1059g. Water content data is from every leaf of one medium sized representative bromeliad. ___________________________________________________________________ _____
DISCUSSION C. capucinus may use bromeliads for water in the rosette, insects, and small vertebrates that hide between leaves or for the leaves themselves (Wainwright, 2002). At Barro Colorado Island in Panama, 15% of the diet was green leaf tissue ( Freese, 1983), suggesting that C. capucinus is vegetarian to some degree. Though no published studies report bromeliad use by Cebus monkeys (Reid, 1997), and no direct observation of monkeys occurred during the course of this study, despite detailed diet s tudies (Terborgh 1983), biologists in the Monteverde, Costa Rica area report seeing monkeys ripping apart bromeliads and, importantly, chewing on the leaf bottoms where leaves connect to the rosette (Masters, Laval, Pounds, personal communication). It is p ossible that bromeliad use is restricted to premontane and lower montane Cloud Forests that are rich in bromeliads, including less tough, drier, adapted bromeliad species associated with lowland forests. A study by Sorenson and Fedigan (1999) showed that C. capucinus occurs in higher abundances in pristine habitat. Therefore, a higher abundance of torn bromeliads in edge habitat seems to indicate a higher reliance on this resource. Friant (2005) used bromeliad leaves on the ground as an indirect measure o f local C. capucinus activity. She found an increase in torn bromeliad leaves with altitude. Likewise, I found more grounded bromeliads on forest edge than interior. This might suggest greater use of edge in C. capucinus An observational study showed that C. capucinus prefers intact forest to edge or secondary forest (Sorensen et al. 1999), at least in Dry Forest. Therefore, a higher density of bromeliad use by C. capucinus in edge habitat seems to indicate that this resource is more often selected for on the edge. An alternate explanation is that bromeliad density is higher along edge, as it may be at higher altitudes. Edge provides more sunlight below canopy and many bromeliads were just a few meters off the ground on the edge, whereas it was difficult to locate bromeliads from the ground in intact forest. The difficulty of assessing bromeliad density in intact forest limits the use of bromeliads as indicators of C. capucinus activity. C. capucinus seems to forage selectively on bromeliads. Not all speci es reported for the area were represented in my findings, for example. Of those that were, one was overwhelmingly preferred. Moreover, small to intermediate plants were used, above and beyond their relative abundance in the population. Smallest and larger plants were sampled only rarely. Further, the innermost whorls of the rosettes were always found intact. Herbivores usually prefer leaves that are more nutritious than emerging or very young leaves, but not fully toughened (Coley, 1983). Tests of leaf toug hness and water content show that intermediate leaves of medium sized bromeliads have high water content and intermediate toughness. Those in the inner rosette are soft but similar in water content to very tough outermost leaves. Increased water in leave s generally believed to lower nutritional value to herbivores. For an arboreal mammal like C. capucinus however, water may be the main reason for using bromeliad leaves. Water in bromeliad tanks contains debris and bacteria that make this water unpotable. Other sources of water, like tree holes, would have similar problems. Descending to pools or streams may put individuals at undue risk to predators. For C. capucinus the best source of potable water may be intermediate bromeliad leaves which have more than 50% more water than younger and older leaves.
While C. capucinus may rip apart bromeliads to find insects and small vertebrates, they also chew on leaves. As a substrate for small animals hiding between leaves of the rosette, it is difficult to imag ine that some bromeliad species would harbor more animals, as overall arquitecture and size are the same. Yet, C. capucinus clearly prefers certain species of bromeliads to others. Also, larger bromeliads should harbor more animals than intermediate and sm all bromeliads, while C. capucinus prefers small and medium sized bromeliads. Innermost rosettes also are likely to harbor many animals but they are consistently cast off intact. Therefore, it seems that at least some aspects of C. capucinus bromeliad use reflect selectivity for types of leaves that are softer, perhaps less chemically protected and of high water content. A suggested area for further research is selective foraging based on bromeliad species. This is no trivial task, but in fact a long and arduous one. Identifying every species of bromeliad in a forest edge habitat tree may require a longer term study in order to positively identify species by use of inflorescence. The finding that more torn bromeliads occur in edge habitat than in pristine where C. capucinus is actually more abundant, may illustrate a higher reliance on bromeliads in edge habitats due to a lack of other resources. Further research might be directed toward a intact to torn ratio in habitats of varying human impact. This stu dy does not have any method for discerning whether the monkeys are in fact utilizing all of the leaves that do not remain on the cores or whether they select among these removed leaves. However, it is likely that the individuals of C. capucinus select the intermediate leaves, closest to the ones remaining on the core, and discard the outermost leaves. Further research is needed to take this secondary selection under closer examination. The ability of C. capucinus to forage effectively for the leaves with t he highest and manipulation skills. This combination of attributes prepares this species to survive in the face of habitat destruction, climate change, fragmentati on, and other human impacts. Already C. capucinus proves to be more adept in surviving under human compromised conditions than other primate species (Brown, 1990), and these skills will benefit the species even further as humans continue to change the face ACKNOWLDGMENTS I would like to thank Alan Masters for continuous support of my interest in monkeys, no matter how hard it is to find monkey data. Thank you to the Ramirez Villalobos family for suggesting study sites and for providin g a loving environment in which to work. Thanks to Bridgette Suzanne Smith for encouraging me to do what I was truly interested in. I would have been bored out of my mind with moths without you. Thanks to Ollie Hyman and Maria Jost for putting up with thou sands of trivial questions and requests from all of us. And finally, thanks to the rest of our CIEE family for company on all those late nights, smiles in times of stress, and the best semester of my life.
LITERATURE CITED Brown, A.D., 1990. Dietar y Variability in Cebus apella in Extreme Habitats: evidence for adaptability, Folia Primatologica 54(3 4): pp. 187 95. Chapman, C.A. and Fedigan, L.M., 1990. Dietary Differences Between Neighboring Cebus capucinus Groups: local traditions, food availabil ity or responses to food profitability? Folia Primatologica 54(3 4): pp. 177 86. Friant, S.C., 2005. White faced monkey ( Cebus capucinus ) reliance on tank bromeliads at varying altitudes, CIEE Tropical Biology and Conservation Spring 2005, pp. 202 209 Glander, K. and Rockwood, L., 1979. Howling Monkeys and Leaf Cutting Ants: Comparative Foraging in a Tropical Deciduous Forest, Biotropica vol. 11(1), pp. 1 9. Holdridge, L. 1967. Life Zone Ecology: revised edition. Tropical Science Center, San Jose, Costa Rica. Howe, H. and Westley, L. 1988. Ecological Relationships of Plants and Animals, pp. 72 74. Oxford University Press, New York. Masters, A., Masters, K., Laval, R., Pounds, A., September November 2005. Personal communication Morales, J.F. 2000. Bromelias de Costa Rica, Second edition, Instituto Nacional de Biodiversidad (INBio), pp. 9 18. Heredia, Costa Rica Nadkarni, N., 1984. Epiphyte biomass and nutrient capital of neotropical elfin forest, Biotropica 16, pp. 249 256. Haber, 2000. In Nadkarni, N. and Wheelwright, N. (editors), Monteverde: Ecology and conservation of a tropical cloud forest, pp. 504 505. Oxford University Press, New York. Panger, M., 1999. Capuchin Object Manipulation, in Dolhinow and Fuentes (editors). The Nonhum an Primates, pp. 115 120. Mayfield Publishing Company, Mountain View, California. Sorenson, T. and Fedigan, L., 1999. Distribution of three monkey species along a gradient of regenerating tropical dry forest, Biological Conservation Vol. 92(2000), pp. 227 240 Wainwright, M., 2002. The Natural History of Costa Rican Mammals, pp. 62, 135 140, Distributadores Zona Tropical, S.A., Miami, Florida. Terborgh, J., 1983. Five New World Primates: A study in comparative ecology, pp. 48 55, 60, 78 83. Princeto n University Press, Princeton, New Jersey.