Project Update: March 2005

Cauritus: during the mist-netting sessions, we were able to capture several bat species, as the one observed in this picture. This rare carnivorous bat, Chrotopterus auritus, had never been captured at my study site, therefore is a new record of bat species for southwestern Costa Rica.

Decaudata: another rare species captured during mist-netting sessions. This vampire bat, Diphylla ecaudata, is suspected to feed mainly on the blood of large birds, such as tinnamous and guans.

Harem: harem male in this picture is located on the far left, and three adult females are located on his right. Note radiotransmitters on all bats. All bats are Artibeus watsoni.

Unmodified: occasionally, young Artibeus watsoni will roost under leaves that have not been modified into tents. This picture shows a subadult female roosting under a cacao plant.

(All photos by Gloriana Chaverri)

The main objectives of this research were to measure the abundance of tent-roosting bats and determine how bat density relates to the abundance and availability of roosting and feeding resources. An additional aim of this study was to understand which plant species are most important for the persistence of tent-roosting bats. Because one of the study species, Artibeus watsoni, is far more common than any other tent-making species in southwestern Costa Rica, research was focused on this species only.

BAT ABUNDANCE. Abundance of the tent-making bat Artibeus watsoni ranged from 0.01 to 1.15 bats per net hour. The lowest densities were found in coastal areas with secondary forests in Corcovado National Park and the highest in Golfito Wildlife Refuge. However, some sites in Corcovado had densities of up to 0.58 bats per net hour, particularly in areas with primary forests.

PLANTS USED FOR ROOSTING. A total of 338 tents representing 25 plant species were used by the tent-making bat Artibeus watsoni during the study period. The most commonly used plant species were Carludovica palmata, Asplundia alata, Heliconia imbricata, and Calathea lutea. Others such as Pentagonia tinajita and Myriocarpa longipes were used only once. It appears that A. watsoni prefers to modify leaves in plants that remain functional for long periods of time, minimizing the need for additional tent construction.

Roosts used by A. watsoni were also used by other tent-roosting species such as Vampyressa pusilla, Uroderma bilobatum, and Artibeus jamaicensis. In one case, a tent that was first used and perhaps modified by V. pusilla was later used by A. watsoni. Tent-sharing has also been observed by others and highlights the important role that a common tent-making species such as A. watsoni may play for the entire tent-roosting assemblage.

The number of plant species used for roosting by A. watsoni in this study greatly exceeds the number of previously reported plant species. Eight new aroids, two new calatheas (C. lutea and C. inocephala), one new palm (C. guagara), and four new families (Moraceae, Rubiaceae, Piperaceae and Urticaceae) were recorded in this study. These results suggest that A. watsoni is the most generalist of tent-roosting species and that methods such as radiotelemetry may be the best way to fully understand the roosting ecology of elusive tent-roosting bats that otherwise would be impossible to locate.

DIFFERENCES AMONG POPULATIONS AT THE TWO STUDY SITES. The roosting ecology of A. watsoni populations in Golfito and Corcovado differed in several ways. First, the population at Corcovado used more plant species as roosts than bats at Golfito, and bats from Golfito exhibited higher fidelity and larger group sizes in tents. These differences between the two study sites may be explained by human activities and their effect on local resources and habitats. For example, the A. watsoni population in Golfito is restricted to only a few areas where roosting resources are available such as lowlands near human settlements, which have been, and continue to be, severely modified for agricultural purposes. Additionally, humans have over harvested plants that are important resources for A. watsoni, such as Asterogyne martiana. In contrast, Corcovado National Park has large interconnected areas of primary and late secondary forests, which have long remained unaffected by human activities. Thus, human modification of habitats in Golfito has reduced availability of native plantsthat would otherwise be frequently used by this bat for roosting.

Another difference between the two study populations that may influence the roosting ecology of A. watsoni is the density of bats. A. watsoni in Golfito has average population densities five times higher than in Corcovado, probably as a consequence of low habitat availability and subsequent population encroachment. As a result, fewer roosting resources at Golfito must be shared among a larger number of bats, leading to higher roost fidelity at this site. Additionally, increased population densities in Golfito most likely result in an increase in group size due to a higher encounter rate and fusion of groups.

During the first three months of this study five new bat species were captured in Corcovado National Park. These species had never been recorded in southwestern Costa Rica, indicating that many other bats may be present but have never been recorded because of a lack of attention to this site. Therefore, this study should alert researchers to the importance of Corcovado National Park as a species’ diversity "hotspot." Hopefully, more research and resources will be directed to this extremely habitat-rich site, aiding in the conservation of several endangered species, such as the jaguar and the white-collared peccary.