Amphibians worldwide are breeding earlier due to climate change, scientists have found.
After warmer winters, wood frogs breed earlier and produce fewer eggs, a Case Western Reserve University researcher has found.
Michael F. Benard, the George B. Mayer Chair in Urban and Environmental Studies and assistant professor of biology, also found that frogs produce more eggs during winters with more rain and snow.
Benard's study, published today in the journal Global Change Biology
, is among the first in a natural habitat to measure the consequences of one of the major effects of climate change: warmer temperatures that lead to earlier breeding in amphibians and other animals.
Benard also found that when wood frogs breed early in the year, their offspring have delayed development but still metamorphose earlier in the year. He identified the broad patterns by examining and tracking important life events of more than 50,000 juvenile and hundreds of adult wood frogs over seven years and comparing the data to winter weather records.
"There have been lab studies on the effects of warming on frog breeding, but what we see in the lab is not exactly what we're seeing in the field," Benard said.
Wood frogs, found from Alabama to Alaska, literally must thaw out from winter. Biologists believe they breed after temperatures and precipitation reach a certain threshold over a number of days. But the exact formula is unknown. During breeding, each female lays all of her eggs in one mass, called a clutch, which makes egg-counting possible.
After analyzing data collected from 2006 to 2012, Benard found that for every degree Celsius increase in average daily maximum winter temperature:
Wood frogs bred 4.2 days earlier.
The number of eggs per clutch collected in the ponds decreased 3.3 percent, or 24 eggs.
The number of eggs per clutch collected from frogs that had been marked and placed in buckets of water to breed decreased 4.3 percent, or 31 eggs.