Phenological Differences of Two Sympatric Ranid Frogs in the Southeastern United States
The Southeastern Coastal Plain of the continental United States is a hotspot of amphibian diversification. Whilst most species have since dispersed and extended their geographic distributions beyond the coastal plain, the Florida Bog Frog, Rana okaloosae, remains restricted to three counties of northern Florida. Across its range, R. okaloosae co-occurs with its sister species, the Bronze Frog (R. clamitans clamitans). Hybridization between R. okaloosae and R. c. clamitans has been documented, raising concerns about the microendemic’s future. To date, however, the phenology of R. okaloosae, as well as the mechanisms by which reproductive isolation from R. c. clamitans is achieved, is poorly understood. Using 13 years of survey data from Eglin Air Force Base, Florida, we evaluated the environmental correlates of R. okaloosae and R. c. clamitans across 80 sites where they occur in sympatry. We found that although the two species had similar breeding seasons and were active at similar times of night, calling occurred under different environmental conditions. Specifically, R. c. clamitans were more likely to call on calm, dark, humid nights, whereas bog frogs were more likely to call across a range of conditions. Our results suggest that R. okaloosae may maintain reproductive isolation in part by timing breeding activity to nights when R. c. clamitans is less likely to be active. However, there remains a large degree of overlap in the phenology of the two species, highlighting the precarious nature of the Florida Bog Frog’s existence.
Detectability of Bronze Frog (R. c. clamitans) and Florida Bog Frog (R. okaloosae) as a function of (A) date and (B) time of night. Shaded regions show the 95% confidence intervals.
Detectability of Bronze Frog (R. c. clamitans) and Florida Bog Frog (R. okaloosae) as a function of (A) relative humidity, (B) lunar illumination, and (C) cloud cover. Lunar illumination (0 = new moon, 1 = quarter moon, 2 = intermediate phase moon, 3 = full moon), cloud cover (0 = no clouds, 1 = partly cloudy, 2 = overcast), and wind speed (Beaufort wind scale) are categorical variables (light condition, sky code, and wind scale, respectively) that have been converted to continuous predictors for analysis. Shaded regions show the 95% confidence intervals.
Calling index of Bronze Frog (R. c. clamitans) and Florida Bog Frog (R. okaloosae) as a function of (A) date and (B) time of night. Calling index is measured on a scale from 0 to 3, indicating whether calls were (0) absent, (1) audibly separated, (2) overlapping but non-continuous, or (3) continuous chorus.
Calling index of Bronze Frog (R. c. clamitans) and Florida Bog Frog (R. okaloosae) as a function of (A) relative humidity, (B) lunar illumination, (C) cloud cover, (D) temperature, and (E) wind speed. Lunar illumination (0 = new moon, 1 = quarter moon, 2 = intermediate phase moon, 3 = full moon), cloud cover (0 = no clouds, 1 = partly cloudy, 2 = overcast), and wind speed (Beaufort wind scale) are categorical variables (light condition, sky code, and wind scale, respectively) that have been converted to continuous predictors for analysis. Calling index is measured on a scale from 0 to 3, indicating whether calls were (0) absent, (1) audibly separated, (2) overlapping but non-continuous, or (3) continuous chorus.
Contributor Notes
Associate Editor: J. M. Davenport.