An Analysis of Latitudinal Variation in the Larval Development of the Wood Frog (Rana sylvatica)

The larval life history characteristics of a population of wood frogs in Michigan (MI) is examined using complementary field and laboratory studies, and the results are compared to previously studied populations in Maryland (MD) and Virginia (VA). Dip-netted samples of tadpoles in populations from ponds in MI reveal that MI tadpoles grow faster, are larger at all stages including metamorphic climax, and have shorter larval periods than populations in either MD or VA. The developmental rates of MI, MD, and VA larvae did not differ in the laboratory. The faster developmental time of MI populations in the field must result from environmental factors rather than genetic differences between populations or from a complex gene x environment interaction. Michigan larvae had higher growth rates and were significantly larger at all developmental stages, including metamorphosis, than larvae from either MD or VA when raised under identical laboratory conditions. Thus a portion of the observed differences in growth rates and metamorphic size observed in the field must result from genetic differences. Michigan larvae also responded differently to the effects of population density than MD and VA larvae. Developmental time was negatively correlated with population density for MD and VA larvae, whereas population density had no effect on developmental time for MI larvae. Michigan larvae were, however, more sensitive to the effects of population density on metamorphic body size, particularly at the low density treatments. The observed geographic patterns in larval development are hypothesized to result from differences in the pattern of food production among ponds.