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Mike Bernstein, Clint Barnett, Chris Gutshall,
Anthony Smith
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ABSTRACT
There are 15 species of tent-making bats in Costa Rica. Artebius
watsonii specializes in the Astrogyne martiana palm. The bats use
the leaves of the palm for shelter from predators, wind, weather,
and sun. The objective of the study was to determine if there are
certain characteristics of the leaves which make them more likely
to be used by a bat. Measurements of leaves with bat tents were
compared to control leaves without tents. This study showed that
Artebius watsonii bats seek out higher leaves, perhaps for greater
shelter and avoidance of predators. In addition, they constructed
tents in leaves facing in a southerly direction. This preference
in south-facing leaves could be related to local weather patterns.
THERE
ARE FIFTEEN SPECIES OF tent-making bats in Costa Rica (Kunz
& McCracken 1993). These bats will usually specialize on one
certain species of plant. Plants that have been used for bat tents
include both large and small understory palms, cyclanths, epiphytitic
lianes, bananas, small saplings and epiphytes. The leaves of these
plants are altered for roosting sites by the bats. The bats cut
the leaves by chewing the lateral nerves and interconnecting tissues
extending from the midrib. After the leaves have been cut, they
fold down creating a shelter for the bats. These bat tents will
provide the bats with shelter from wind, weather, predation and
sun. Actual and potential predators of these bats include false
vampire bats, owls, squirrel monkeys, night hawks, wildcats, Double-toothed
Kites and snakes (Kunz & McCracken 1993). The highest diversity
of tent-making bats is found in wet tropical regions.
It is not known if bats choose some leaves over
others to construct their tents. There are many characteristics
of leaves that can be expected to influence the decision of the
bats. Our study evaluates some of these possible characteristics,
such as direction, height, length and width of the leaf, and diameter
of the petiole. We hypothesized that bats would construct their
tents in one direction more frequently than another because of local
weather conditions (micro-climates). Other leaf characteristics
about which we formed hypotheses were the height of the leaf, length
and width of the leaf, and diameter of the petiole. We hypothesized
that bats would prefer higher leaves for tent construction. We also
hypothesized that bats would favor larger leaves with thicker petioles
because of their greater strength and stability.
METHODS
We conducted our study in the lowland rain forest in the Golfito
Wildlife refuge near the town of Golfito, Puntarenas province, Costa
Rica. We focused our study on the Astrogyne martiana palm. It is
in the leaves of this palm that the dwarf fruit-eating bat (Artebius
watsonii) seeks shelter. The bat symmetrically bites along the leaf,
making it fall in an A-frame tent, in which the bat sleeps upside
down.
In order to test our hypothesis that bat tents
would be more common on one side of the palm than the other, we
recorded the compass readings for each tent encountered. A Chi-square
test was used to test if the direction of the tents was significantly
different.
In order to test our hypothesis that leaves with bat tents would
be higher than those without bat tents, we compared the leaves with
bat tents to a control leaf of the same age (petiole growing from
approximately the same height on the meristem) on the same plant
which did not possess a bat tent. We used a paired t-test in order
to analyze the difference between the heights of the leaves with
tents and their corresponding untented leaves of the same age.
To test our hypothesis that leaves with bat tents would be larger,
wider, and have thicker petioles, we measured the length and width
of the leaves, and the diameter of the petiole in centimeters. To
analyze this data we performed paired t-test on each leaf characteristic
for the leaves with tents and their corresponding control leaves.
RESULTS
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The Chi-square test on the directions of
the leaves revealed a marginally significant difference. The
leaves pointing south were used more often for tents than
leaves pointing in other directions (.05 < P < .1, Fig.
1).
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The paired
t-test revealed a significant difference between the height
of the leaves with tents and those without tents (t = 5.9, df
= 31, P < .005). The leaves with tents were significantly
taller than those without tents (Fig. 2). The paired t-tests
for length and width of the leaves and petiole diameters revealed
no significant differences between leaves with tents and the
paired control leaf. |
DISCUSSION
Our first hypothesis, which stated that tent-making bats would construct
their tents in one direction more frequently than in other directions,
proved to be marginally significant (.05 < P < .1). The tent-making
bats in Golfito tended to select a southerly direction for their
tents. This may be due to climatic factors at the site. Perhaps
the prevailing winds or the usual storm patterns in the area bias
the bats into making the tents in a southerly direction to better
protect themselves from the elements. Also, it is possible that
the site studied contains several microclimates, which might affect
bat leaf choice within much smaller areas. Extensive study of the
area’s wind, rain, and sun regimes would help direct future
investigations into the effects of the local climate on bat tent
placement.
Our second hypothesis, which stated that leaves
containing tents would be higher than leaves of the same age without
tents, was also supported. The reasons that bats tend to select
higher leaves might be related to predation. Among bats’ potential
predators are several terrestrial animals which may have an easier
time reaching lower tents.
Finally, our third hypothesis, which states the
length, width and diameter of the petiole of the leaves with bat
tents would be greater than those without, was not supported. One
reason for this might be the uniformity of leaves within each tree.
A study that might better evaluate this hypothesis would involve
sampling bat preferences for tent building between trees, rather
than simply between leaves within a tree.
Further studies which could be done on tent-making
bats would be to diversify the study to other sites. Another way
to diversify the study would be to study different species of palms
in which bat tents also appear. Finally, a more in-depth study would
also incorporate the different species of bats which make such tents
and determine the differences in their preferences of sites and/or
palm species.
ACKNOWLEDGMENTS
I would like to thank Kathy Stoner for her continuous help on my
project, Andrea Sweigart for her artistic contributions and encouragement
and Thomas Jesse Fox for always being there for me.
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