A Model for Estimating Final Clutch Size from Follicle Counts in Plethodontid Salamanders
Estimates of clutch sizes are essential for modeling population dynamics, yet for many species of amphibian, clutches can be difficult to observe or methodologically problematic to measure. Clutch sizes for direct-developing Plethodontidae are regularly estimated from counts of ovarian follicles. Because many more follicles begin to develop in an ovary than will ultimately reach full size and be deposited, follicle counts change dramatically over an individual female's follicle development cycle, and a high level of subjectivity is inherent in the process of estimating clutch sizes from follicle counts. Many published studies are not transparent in how they determine clutch sizes from follicle counts. Some investigators address this bias using threshold sizes or other characteristics to separate those follicles that will ultimately mature and be deposited from those that will not, but our experience indicates that such approaches still likely overestimate clutch sizes. To move beyond the subjectivity inherent in estimation of final clutch size from follicle counts, we modeled large Plethodon clutch size as a function of female body size (snout–vent length, SVL) and follicle diameter, then used that model to predict the likely number of mature eggs deposited. We propose that this approach provides reasonable estimates of clutch sizes and variances for use in demographic models.Abstract
Average follicle diameter of 127 adult, female P. shermani–P. teyahalee salamanders (with follicles 0.5 mm in diameter or greater), based on the day within the year when collected. Females caught between day 100 (early April) and approximately day 225 (mid-August) fall into two categories: those females with small follicles unlikely to produce a clutch until the following year at the earliest (open circles) and those females with large follicles likely to produce a clutch in late spring or summer (filled circles), suggesting a 2-yr cycle of follicle development.
Correlations between SVL and the observed total number of follicles (left panel) and the model predicted final clutch size (right panel) of 121 adult, female P. shermani–P. teyahalee salamanders. Lines represent average linear prediction and bands are the 95% confidence intervals.
Estimated relationships between average follicle diameter, SVL, and residual mass on total follicle count among 127 adult, female P. shermani–P. teyahalee salamanders. Lines represent estimated mean ± 95% confidence bands based on the top model (Table 1).
Distributions of the observed total number of follicles and the model predicted final clutch sizes of adult, female P. shermani–P. teyahalee salamanders.
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