by Anna Alquitela
From 2006 to 2012, Michael Benard conducted research at a field station in southeastern Michigan where he used drift fences and pitfall traps to capture both adult and metamorphosing wood frogs (Rana sylvatica) at six wetlands (Benard 2014). Benard’s goal was to determine if a relationship exists between date of breeding and winter temperature and precipitation, and between the female reproductive rate (fecundity) and winter temperature and precipitation. Using these data, he was also able to discern if breeding dates affect changes in metamorphosis timing, length of the larval period, weight at metamorphosis, and larval survival.
Benard found that warmer winters led to reductions in female fecundity possibly due to the increased energy consumption caused by warmer winters. Because wood frogs survive through winter by producing glycogen, a cryoprotectant that minimizes freezing damage to their biological tissues, Benard states that there is a “metabolic cost” to thawing during warmer winters. During the time of his research, the difference in average maximum temperature between the coldest winter and warmest winter was 5° C. The number of eggs captured during this time was 15% of the average clutch size. Over time, this reduction in clutch size could negatively affect the population size of wood frogs. Bernard also found that when breeding occurred at an earlier date, the larval stage took longer, but the metamorphosis still took place earlier than when breeding occurred at a later date. He credits these effects to warmer weather causing tadpoles to develop more quickly, however the developmental stage is still much slower. Benard found no changes in larval survival due to shifts in breeding time, but did see an increase in average mass at metamorphosis during the course of his research.
Benard, M. F. 2014. Warmer winters reduce frog fecundity and shift breeding phenology, which consequently alters larval development and metamorphic timing. Global change biology. DOI: 10.1111/gcb.12720