Tue April 16, 2013
The Chorus of the Leopard Frogs
On thealmanac.org last week, Marilyn Stone noted that she hasn’t heard the chorus of leopard frogs she usually hears by this time of year, and wondered about the effect of a nearby wetlands that dried up last fall. KVNF’s Marty Durlin has some answers from a scientist who studies leopard frogs.
Max Joseph is a graduate student in ecology and evolutionary biology at the University of Colorado. Last year he took part in a study of leopard frogs in and around Boulder, going so far as to equip 13 of the four-and-a-half-inch, 33-gram frogs with radio transmitters. Following their movements, Joseph and his colleagues were able to glean new information about the once-plentiful amphibians.
“Historically it was one of the most abundant frogs in wetlands across the U.S.,” says Joseph. “But it’s gone through declines in Colorado in the last century. So there still are some populations but they’re much less abundant and much less common than they used to be.”
Joseph says there are multiple reasons for the leopard frog decline, including habitat loss and fragmentation, urban development, environmental contaminants, introduced fish, invasive species, and a fungus with a name that’s hard to pronounce.
“It’s called Batrachochytrium dendrobatidis,” says Joseph, “and this chytrid fungus has been responsible for amphibian declines worldwide and we know it does occur locally, although the exact effects of this fungal disease still remain unclear.”
But drought also plays a large role in the frogs’ ability to survive. “Generally drought can reduce breeding habitat by causing fewer temporary ponds, or faster drying of temporary ponds. Also drought can reduce overwintering survival if there’s a permanent wetlands that’s important for overwintering and that wetland becomes more shallow, the likelihood of that wetland freezing completely is greater during winter.
The study Joseph completed last year showed that frogs will go a long way to find a good place to overwinter. “We observed adult frogs moving up to 500 meters over the course of just a few days,” he says. “And they were moving all at the same time away from the breeding site. And we were able to figure out what the movement corridor was. So it turned out the frogs were moving along depressions in open fields in order to get to a ditch system, and once they reached the ditch, they followed that ditch to the overwintering site.”
Joseph adds, “Typically leopard frogs will try to find a permanent water body that won’t freeze completely. And what we think they do, is they sort of burrow into the substraits, in other words the bottom of the pond in order to try to avoid freezing. And then they go into a dormant state throughout the winter until temperatures warm and they can begin feeding and breeding.”
But as far as Marilyn’s question about the frogs next door, Joseph says it’s really too early to tell. “So certainly if this pond was an important overwintering site, and it dried up, that could be problematic,” he says. “But we also know that leopard frogs often breed in temporary ponds. So it could be that they haven’t started breeding yet. I think that comment was prior to the snow event. From the amphibians’ perspective, it probably wouldn’t have been a great idea to breed prior to the big snow. I mean the eggs don’t really deal too well with freezing and extreme cold. So I think it’s still a little bit too early to know how leopard frogs are going to do this year.”
Marilyn called the leopard frogs’ call a "creaky door" chirp. Max Joseph had another description of it. “Have you ever had a balloon and maybe your hands are a little bit wet or a little bit sweaty and you kind of squeeze the balloon and move your hand so you have a squeaking sound? It’s kind of like that.”
When asked if he can imitate the sound, Joseph says, “No, unfortunately I can’t. I wish I could though. That would be a really effective tool in the field if you were trying to find them.”