| dcterms.abstract | <dd>Interspecific competition is the most common form of interaction described for coexisting organisms. Because of shared resource requirements, each of two species faces fitness costs in the presence of the other. Such competition can be reduced when species diverge in niche use. If, however, sympatric species maintain ecological similarity, heterospecific aggression should be high and interspecific dominance relationships may structure access to preferred resources. Across animal species, body mass has often been found to equate to dominance, providing large-bodied species with priority of access. Large group size may also lead to a competitive advantage. Nevertheless, because dominant species cannot simultaneously occupy all resources in their home range, subordinate species may survive by using either lower-quality or unoccupied resources. Thus, subordinate groups avoid potentially dangerous encounters with dominant species; these encounters can directly reduce reproductive success in subordinate species and, in extreme cases, be fatal. As more studies become available it seems that interspecific aggression may have a much higher explanatory value for individual species' behavior than previously assumed for primate ecology. Likely because this topic requires data for multiple habituated, sympatric groups of different species, it has, however, rarely been investigated in the past. </dd> <dd>This dissertation investigated coexistence in a community of three primate species living at Way Canguk, Bukit Barisan Seletan National Park, Sumatra: the ecologically-similar siamangs (<italic>Symphalangus syndactylus</italic>) and agile gibbons (<italic>Hylobates agilis</italic>) and the ecologically-dissimilar mitered langurs (<italic>Presbytis melalophos</italic>). Four major research topics were addressed: (1) in what ways do these sympatric species overlap and differ in dietary niche use, (2) what determines interspecific dominance and what are the costs of being a subordinate species, (3) how do alternative mechanisms - other than niche partitioning - promote species coexistence, and (4) how does interspecific competition impact sleeping strategies? </dd> <dd>Because siamangs are about twice the mass of agile gibbons and mitered langurs, they are expected to be dominant over the other two species. Alternatively, if group size is a better determinant of dominance, then langurs should be dominant. Because large body size also increases locomotor costs, it was predicted that agile gibbons would travel faster and farther than siamangs, while siamangs would feed longer in larger, more-productive patches. Agile gibbons were furthermore expected to avoid encounters with siamangs, resulting in lower daily energy intake and higher energy expenditure than siamangs. </dd> <dd>Following 9 months of preparatory work during which 1 mitered langur, 2 siamang, and 2 agile gibbon groups were habituated, systematic data were collected on 4 siamang, 2 agile gibbon, and 1 mitered langur groups from November 2008 through October 2009 with the support of 4 local assistants. During 3,298 contact hours, data were collected on food intake, availability, and nutritional composition (249 food items), as well as interspecific encounters, activity budgets, ranging, and sleeping site use (226 agile gibbon, 223 siamang, and 48 mitered langur nights). The dietary niche of each species was described based on 282 all-day follows (151 days for siamangs, 95 for agile gibbons, and 26 for mitered langurs). The context and outcome of all interspecific encounters (n = 289) were used to assess dominance ranks. Interspecific comparisons of hylobatids' foraging strategies were made based on 269 all-day follows (161 siamang days and 108 agile gibbon days) and 2,817 siamang and 1,161 agile gibbon feeding bouts. </dd> <dd>As expected, observations of feeding behavior (Chapter 2) suggested that the potential for interspecific competition was much higher between siamangs and agile gibbons than between hylobatid species and mitered langurs. As simple-gutted species, both hylobatid species predominantly fed on ripe fruits and figs and about equally used young leaves and flowers to supplement their diets. In fact, dietary overlap at Way Canguk was high compared with other populations where siamangs and small-bodied gibbons occur in sympatry. Mitered langurs (colobines with complex digestive anatomy adapted to a fibrous diet), in contrast, spent the majority of feeding time on leaves and supplemented with unripe fruits and flowers. </dd> <dd>Interspecific dominance relationships (Chapter 3) were mediated by body mass and possibly not group size. That is, siamangs were dominant over both agile gibbons and mitered langurs. The much heavier siamangs initiated and won almost all encounters with agile gibbons (98%) and mitered langurs (100%). However, encounters between ecologically-similar siamangs and agile gibbons were much more frequent and aggressive than between mitered langurs and either hylobatid species. Encounter locations were non-random, occurring more often in shared-food than non-food locations. Agile gibbons incurred energetic costs and, rarely even physical wounds as a result of lost encounters with siamangs. Perhaps as a tactic to evade detection and reduce harassment by siamangs, agile gibbons frequently became motionless and remained in dense vegetation when approached by siamangs (passive avoidance). Taken together, agile gibbons seem to be at a real disadvantage in the system and their persistence requires an alternative explanation to classical niche partitioning. </dd> <dd>I examined two potential mechanisms of hylobatid coexistence (Chapter 4), under which dominant and subordinate species are expected to differ in foraging strategies within a heterogeneous habitat. Subordinate agile gibbons were hypothesized to either 1) be fugitive species (i.e., they more rapidly reach and consume foods than dominant siamangs) or 2) use different, lower-quality feeding patches as competition refuges. Results revealed that agile gibbons are not fugitives, but likely use competition refuges. In support of the second hypothesis, agile gibbons fed in less productive patches, for shorter bouts, and at lower intake rates than siamangs. Additionally, agile gibbons minimized their energy expenditure by spending a much higher percentage of time resting each day. Due to their use of lower-quality feeding patches and the risks inherent in encounters with siamangs (Chapter 3), agile gibbons may struggle to meet their energy requirements. If that is the case, then the use of competition refuges would not facilitate their persistence and, in fact, coexistence of siamangs and agile gibbons may not be locally stable. </dd> <dd>In addition to being subordinate and (seemingly) at an energetic disadvantage, agile gibbons were found to use a sleeping strategy unexpected for arboreal primates (Chapter 5). Compared with sympatric mitered langurs and siamangs, agile gibbons used shorter, smaller and more densely-vegetated sleeping trees. These characteristics are opposite to what is usually preferred by primates (i.e., large, emergent trees with open crowns), and should increase predation risks for agile gibbons. Agile gibbon's use of suboptimal (i.e., vulnerable to predation), yet well-concealed sleeping trees suggests that the subordinate species puts a priority on avoidance of detection by dominant siamangs. This would be an extreme, as of yet described, reaction to interspecific competition. </dd> <dd>Overall, this dissertation suggests that sympatry with siamangs at Way Canguk is very challenging for agile gibbons. A clear mechanism for their stable coexistence has yet to be identified. Thus, it is possible that this population may only persist through periodic re-population from a nearby source population. | |
