ENVIRONMENTAL INFORMATION
FROM THE PALYNOLOGY OF BAT GUANO
L.J. MAHER, JR
Department
of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton Street,
Madison, WI 53706 USA (maher@geology.wisc.edu)
Bat droppings accumulate in
caves, and the resultant guano contains a stratigraphic record of the
environment analogous to the record from lake sediment and peat. The bats
forage at night for insects over a particular area, and they return to the cave
during the day to sleep and care for their young. They attach themselves to
suitable perch areas of the cave ceiling, and their excrement accumulates on
the floor below. Flying requires a lot
of energy, and this requires the bats of temperate regions to consume large
numbers of night-flying insects. In some
situations the guano can reach a depth of meters in hundreds to thousands of
years, and it has a valuable chronostratigraphy. The bat scats occur as small
pellets that individually represent the non-digestible portion of the animal's
diet in the preceding day; hence the diet provides information about the time
of the year the feeding occurred. Bat
guano contains, among other things, insect fragments, hair, pollen, and some
mineral matter. Night-flying insects do
not normally visit flowers for the pollen; many species do not eat during the
flying phase of their life cycle, and those that do generally are nectar
feeders. Although the insects are not
after the pollen, they do fly through a pollen-laden environment, and the
pollen and dust adheres to their bodies.
The insects essentially act as living traps for airborne debris. The bats also are furry pollen traps; during
grooming they ingest pollen and dust enmeshed in their fur, and this also is
excreted. Study of the pollen in an
individual scat contains a record of the atmospheric pollen during a single day
in the past. Pollen from multiple scats
can provide a record of a season. This
kind of detail is hardly ever available from lake sediment.
Chemical
analysis of individual bat scats in a time series can provide a record of the
region's changing environment caused by agriculture, industry, volcanic dust,
and a host of other details that depend only on the cleverness of the
researcher. Careful carbon-14 analysis
can isolate times when bats did not use the cave, and that negative evidence
can also be used to interpret past conditions.
If the types of insect in the guano change over time, that may also
provide evidence of changing climate.
Tom Aley, owner of the Ozark Underground Laboratory,
Protem, Missouri, USA, allowed me to collect guano samples for an exploratory
study. Tumbling Creek Cave contains a
maternal colony of the Grey Bat (Myotis grisescens) that occupies
the cave for a short time each year.
Scats collected from the base of a 75-cm thick cone of guano yielded an
AMS 14C date of 2810 ± 40 (CAMS 85667). The fecal material has a
crumbly structure below the surface; it was of mahogany color (7.5 YR 2/1 to
3/2) and had no noticeable odor. The
feces contained many insect fragments, hair, and a wide assortment of pollen
and spores. Guano can be processed
like normal sediment, but simple washing in a weak detergent solution followed
by acetolysis appears adequate. Normal sediment coring devices compress the
guano; this can be avoided by using a modified "Russian" peat sampler
or by freezing the guano to a dry-ice-cooled probe.
PROGRAMME & ABSTRACTS
JOINT MEETING OF AASP-TMS-NAMS 11th –13th SEPTEMBER 2002,
UNIVERSITY COLLEGE LONDON