A Skeletochronological Study of the Age Structure, Growth, and Longevity of the Mountain Yellow-legged Frog, Rana muscosa, in the Sierra Nevada, California

We used skeletochronology to determine the ages of 149 (74 females, 44 males, and 31 juveniles) Mountain Yellow-legged Frogs (Rana muscosa) from 13 locations (elevation 1509–3501 m) throughout their current range in the Sierra Nevada mountains of California. Lines of arrested growth (LAGs) from excised toe bones were distinct in these high elevation frogs, and each LAG was assumed to represent one year of age. Females ranged in age from 0–10 years (mean = 4.1 years) and males from 0–8 years (mean = 4.0 years). The skeletochronological age was that of the post-metamorphic frog and did not include the tadpole stage. Mountain Yellow-legged Frogs spend 3–4 years as tadpoles, but no age markers are found in their cartilaginous skeletons; thus, their total age, if both tadpole and post-metamorphic stages were included, would range up to 14 years. Females were significantly longer (snout–vent length: SVL) than males and had greater mean mass, but there was no difference in the mean ages. Juvenile frogs of unknown sex ranged in age from 0–3. The von Bertalanffy growth curve demonstrated that female SVLs were larger than males for all ages. Using a semi-parametric growth model, we also found that elevation within the Sierra Nevada range was an important variable in the relationship between SVL and age; frogs from lower elevation sites were consistently larger at a given age when compared to higher elevation sites. For each increase of 1000 m in elevation, the estimated length (on average) decreases by 8.7 mm. This is the first age determination study of a Sierra Nevada amphibian, and compared to other anuran species, Mountain Yellow-legged Frogs were found to be relatively long-lived, which will have implications for restoration and recovery plans.

Figure 1

Map of sites throughout the Sierra Nevada where Rana muscosa toes were sampled.

Figure 2

Photograph of toe cross section showing eight lines of arrested growth (LAGs) for Rana muscosa. Scale bar = 25 µm.

Figure 3

Fitted growth curves using model 1 (von Bertalanffy growth model) with length (SVL) and age data for male and female frogs. Ages do not include tadpole phase. We offset t₀ = 0.5 years to account for not sampling true age 0.

Figure 4

Observed versus fitted lengths (SVL) using the semi-parametric regression with elevation (model 2). Median absolute deviation is 5.0 mm. Adjusted R² for model 2 is 0.78.

Figure 5

Elevation versus observed SVL (mm) for male (+) and female (▪) frogs. Lines through the points were made with a scatterplot smoother of estimated lengths using model 2.

Contributor Notes

Section editor: T. W. Reeder.

Kathleen R Matthews, (KRM) USDA Pacific Southwest Research Station, P.O. Box 245, Berkeley, California 94701, e-mail: (KRM) kmatthews@fs.fed.us(CM) CNRS 5553, Laboratory Biology of Alpine Populations, University of Savoie, Le Bourget Du Lac, France 73376. E-mail: (KRM) kmatthews@fs.fed.us. Send reprint requests to KRM.

Received: 23 Feb 2006

Accepted: 30 Apr 2007