Environmental Information from the Palynology of Bat Guano

Maher, L.J., 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.