Skeletochronology and geographic variation in age structure in the wood frog, Rana sylvatica

Age structure of a breeding population of Rana sylvatica from southwestern Quebec, Canada, is described based on the enumeration and analysis of Lines of Arrested Growth (LAGs) in phalanges Analyzed using skeletochronology, a ges ranged from 1-4 yr among 98 males and from 2-4 yr among 33 females. Fem ales were on average slightly older than males, but the difference was not significant Mean snout-vent length was 43.6 mm +/- 2.0 (SD) among 179 males and 48.8 mm +/- 27 among 33 females; the difference between the sexes was significant Endosteal resorption completely destroyed LAG 1 in 6.1% of frog s, about twice as frequently in males as in females. A minimum of three LAG enumeration readings by two observers differed for 24.4% of preparations a nd independent readings by the same observer differed for 15.3% of preparat ions, highlighting the importance of reducing subjectivity in skeletochrono logical analyses. This was accomplished, in part, by plotting LAG diameters on a histogram whereby outliers from an expected normal distribution indic ated loss of inner LAGs via endosteal resorption. Consistent with predictio ns regarding environmental influences on anuran populations, southern Quebe c R. sylvatica matured later and were larger than more southerly conspecifi cs from a low-elevation area, but matured earlier and were smaller than sou therly frogs from a high-elevation area.