Combating Extreme tropical Seasonality: Use of rock Crevices by the Critically Endangered frog Nannophrys marmorata in Sri lanka

Abstract

Kirtisinghe’s Rock Frog (Nannophrys marmorata) is endemic to Sri Lanka and is found only on the northeastern slopes of the Knuckles Mountain Range, where a short wet season is followed by a prolonged dry season. The dry season is characterized by high air temperature, low relative humidity, and strong desiccating winds. These frogs take refuge in rock crevices during the daytime and emerge at night to forage. We monitored the body temperature of frogs while inside and outside crevices, and several environmental variables, during 4-d periods in the peak dry (August) and peak wet (December) seasons. In the peak dry season during daytime, ambient air temperature rose to 39.2° C (average) and relative humidity dropped to 27% (average). Under these conditions, the peak temperature of the rock crevices averaged 29.9° C, and the crevices served as a refuge for the frogs, whose body temperature did not exceed 27.4° C. Frogs used significantly smaller crevices (entrance height) in the dry season (10.2 mm + 1.1 SE) compared to those used in the wet season (17.9 + 1.6 mm). Frog activity outside of crevices at night was dramatically less during the dry season (98 frog-hours) than during the wet season (1,615 frog-hours). Our study shows that N. marmorata minimizes exposure to adverse environmental conditions in the dry season by taking refuge in rock crevices. The behavior by frogs of selecting smaller crevices and reducing activity outside of crevices during the dry season may also serve to minimize exposure to adverse conditions.