Roost characteristics and thermal competition in bats in a high-temperature cave

Background/Question/Methods

Bats of the genus Pteronotus (Chiroptera: Mormoopidae) are known to inhabit high-temperature roosts.  This study examines the thermal ecology of a colony of three different species of Pteronotus in a sea cave on the Osa Peninsula of Costa Rica.  Previous work found the cave to be inhabited by three species of the same genus: P. personatus, P. parnelli, and P. gymnonotus, and documented that temperatures in the cave at low tide with bats present can exceed 40° C.  This high ambient temperature makes this cave one of the few locations where bats have been documented to roost at temperatures warmer than their body temperature.  However, the cave fills with seawater during high tide, which potentially creates a cooling effect. We explored whether temperatures in the cave fluctuated significantly with tide levels and the presence or absence of roosting bats.  Hobo data loggers were placed in the cave for several days to continually record changes in ambient temperatures within the different parts of the cave at different combinations of tides and bats.  Upon entering the cave, temperatures of roosting individuals (during the day), and the cave ceiling (at night) were also recorded using an infrared thermometer.     

  

Results/Conclusions

Temperatures on the ceiling of the cave, which was entirely covered with roosting individuals, at low tide were measured at 40-41°C.  However, during low-tides at night (with bats absent), temperatures were only 37-39°C in the equivalent parts of the cave. Ambient air temperatures collected by the Hobo data loggers detected no significant effect of tide levels (low vs. high) on ambient temperature.  Results from sensors placed near the bottom of the cave showed significantly lower temperatures when bats were present, but when sensors were elevated ~8m, it was found that temperatures in the upper parts of the cave were significantly higher with bats present.  These patterns are likely related to the properties of the cave walls to absorb and radiate heat generated by the bats. In the upper areas of the cave, where bats roost, the presence of bats clearly elevates temperatures. The genus Pteronotus may use these high temperatures as a means of thermal competition to exclude species with lesser heat tolerance from using the otherwise highly-desirable roosting habitat.