81 
American Toad in Northern Virginia 
but without knowledge of HK’s counts and measurements. These data 
serve to estimate the age of the individual toads based on the standard 
skeletochronological assumption that each MSG (= one growth cycle) 
represents one year in the life of a toad. 
RESULTS 
Age Estimates and Population Structure 
The initial goal of the mark and recapture study was to repeat 
Hemelaar/van Gelder’s confirmation of the addition of one new MSG 
each year. Only one marked toad (a male) was caught in a subsequent 
year. This toad showed two MSGs in 1984 and three MSGs in 1985. 
Because of the poor recapture rate, marking was discontinued after 
1985. 
Comparison of MSG counts. Independent counts permit the 
evaluation of the precision of data collection and the “legibility” of the 
MSGs in this sample. Of the total sample (N = 69), 61 phalangeal 
sections could be read by both observers; 46 counts (75%) were identical, 
13 differed by ±1 and two by ±2. The differences were examined by the 
Wilcoxon matched-pairs signed-rank test (z = 3.26, P - 0.0006). This 
nonparametric test evaluates the null hypothesis that the differences 
between the two observers’ counts are random. The z value rejects that 
hypotheses, because when different, GZ’s counts are less than HK’s 
counts with only one exception. 
Age estimates. All subsequent discussions of age are based on HK’s 
counts and measurements. Examination of the phalangeal histology 
suggested that the first year’s periosteal growth layer (MSG) had been 
resorbed in many of the Eakin Park toads. To correct our age estimates 
for the lost MSGs, we needed to determine the number of MSGs lost 
from each toad’s phalanx. Hemelaar’s graphic technique (1985: figures 
2,3) offered a mechanism for such a determination. We selected a 
maximum diameter of 220 jum for the first LAG/ MSG (= first year’s 
periosteal growth), because 12 toads showed minimum LAG diameters 
<220 and toads with resorption/endosteal core diameters <220 had 
minimum MSG diameters >230 jtim. By establishing a minimum first- 
year MSG diameter, the minimum MSG diameter data for each toad 
could be assigned to a year class (Fig. 1). The assignment of each toad’s 
minimum MSG to a year class locks the larger MSG diameters for that 
toad into subsequent year classes. We then summed the diameters for 
each age class and calculated MSG diameter means and standard 
deviations for each class (Fig. 1). The estimated age for each toad is, 
thus, the number of MSG diameters measured plus the estimated 
