BELLHOLES, BELLBASINS AND BIOLOGICAL ENLARGEMENT OF NEOTROPICAL CAVES

Bellholes are bullet-shaped cavities in ceilings of limestone caves. They are vertically symmetrical and circular (commonly 60 cm height and 30 cm width, sometimes exceeding 1 m high) and are restricted to the tropics. They are solutional phenomena often paired with bellbasins, which are shallow floor depressions, located vertically beneath a bellhole in solid substrate such as rock or speleothem.

Neotropical caves contain tens of thousands of bellholes, found in association with bats, primarily the species Artibeus jamaicensis (Jamaican Fruit Bat). However, in caves where an obstacle bars the entry of bats to a section of passage, bellholes do not exist, even though they may be present in other areas of the same cave. An obstacle could be a flooded section of a cave passage or apertures too small for bat wings in flight.

Bellbasins directly beneath bellholes usually contain fresh or indurated bat guano, and walls of altered rock rinds of apatite/ hydroxyapatite. Bats of the species A. jamaicensis are frequently found in the bellholes above, whose walls are commonly stained with thin dark brown streaks. X-ray-diffraction analysis of the bellhole streaks in Puerto Rico and Barbados showed apatite minerals associated with bat guano-limestone reactions; unstained surfaces revealed only carbonate minerals.

A. jamaicensis roosts singly or in harem groups of 2-14 that commonly cluster in the bellholes, and it is hypothesized that the social habits of this species focus corrosion by the transfer of feces to rock (producing altered rock removed by claws). This creates discrete upward-growing cavities. Circular, progressively indented ceiling cavities with brown streaks illustrate a sequence of bellhole development.

The numbers of bellholes and bellbasins suggest they may be an important post-formational erosional process in tropical caves: e.g. the bellhole volume of the first 600 m of Cueva Represa (Puerto Rico) is equivalent to 4.8 cm depth of rock removed from the ceilings.