Wild Ideas: Warmth and rain trigger ‘explosive’ frog breeding   

With all the warm spells this winter, I’ve been checking in the forest above my house for the sound of wood frogs calling, with no luck . . . until last week.

By Pam Owen
Wood frog males wait for mates in a woodland pond.

With a kind of antifreeze in its blood, the wood frog (Rana sylvatica) can become active throughout the winter during brief warm periods that start thaws, making it Virginia’s earliest anuran to breed. This species spends most of its adult life in damp woodland, moving upland to shelter as winter approaches and returning to its natal pool to breed.

Wood frogs will only breed in pools that are devoid of fish, their biggest predator. Generally the frogs use ephemeral vernal pools that dry up before fish can get established. An old concrete trout tank and small pond up the mountain behind my house fit the frogs’ criteria. But it’s also been dry this winter, so the water in both “pools” has been much lower than usual.

One day last week (Feb. 20), as I was out hanging laundry in my backyard, I heard a faint chattering coming from up the mountain. I went inside to grab my camera and headed toward the sound.

Reaching the tank, I found only a few wood frogs calling there, which is typical for that spot. Slowly peering over the side, I saw two dive for cover. Hearing more clamoring at the pond, a few yards away, I headed up over the dam. Most of the frogs stopped calling as I came into view — those in the water diving under the surface of the water, and others on the banks joining them, roiling the water.

I hunkered down near the edge of the pond, watching quietly as a couple of frogs reappeared. One swam toward me, stopping just a few inches from the pond’s edge and staring at me. Perhaps he thought I was a rival, but he was soon distracted by a newcomer swimming past him and left in pursuit.

Wood frog breeding is often described as “explosive,” and generally occurs after the first warm rains in late winter or early spring. With such a dry winter, perhaps just the light sprinkling we’d received a couple of days before, coupled with the warm weather, had been enough to finally bring the frogs to the pond.

Dozens of little heads started popping up all over the pond’s surface, and the frogs’ clacking calls rose again to a din. So far all I could see is one egg cluster, attached to a submerged limb just a few feet from where I was sitting. When possible, females usually deposit their eggs next to others’ clusters, forming “rafts” of up to thousands of eggs.

Most of the frogs were about two inches long and dark brown, almost black, so probably males. The males also have bold stripes on their legs, although when they are dark (which can happen when they are breeding or cold), it’s hard to see these usually distinguishing markings. Females are typically larger, paler and redder. Both genders have pale undersides and brown eye masks. From the evidence at hand, it looked like few females had yet to respond to the males’ calls.

Making a quick estimate, I concluded that there were more than 100 frogs in the pond. As they floated, each male would fill up with air, then make a single clacking call. It was the sheer number of frogs that was creating the incessant din.

Most of the frogs were clustered on the side of the pond opposite of where I was sitting, about 30 feet away. That’s where I’ve found most of the egg clusters in previous years, probably due to a combination of more detritus in the water for attaching egg clusters to and more cover up the slope for taking shelter. That side is also sunnier and the slope is steeper, so gets less traffic from wildlife coming to drink than the side I was on. The dominant males had their pick of location, forcing the less-dominant ones to my side of the pond.

As I sat quietly, I could hear leaves on the ground being stirred — mostly from the slight breeze blowing. But when I heard a rustling only a couple of feet away, I turned to see a lovely caramel-colored wood frog making its way to the water. Despite its light coloring, the bold stripes on its legs and its size indicated it was another male. Reaching the water, it dived in quickly and disappeared, under the radar of the few frogs on that side.

Soft-focusing my eyes, I could see the movements of the frogs crawling around in the forest litter at the on the other side of the pond, seeming to look for shelter or food. A couple plunged their heads to the ground, probably after an invertebrate prey. A few more frogs hopped from further up the slopes surrounding the pond while I was there, jumping into the more densely packed areas of the water. Each was met with a frenzied response those already in the pond, churning up the water.

I had hoped to photograph a male and female engaged in amplexus, in which the male grasps the female behind her forearms, hooking his thumbs together around her. The male will hang on for up to three days, hoping to fertilize the female’s eggs when she releases them. Sometimes more than one male attaches to the same female, creating a “mating ball,” which can be lethal for the female. But, if females were in the pond, I couldn’t see them interacting with the males.

The day was sunny, with temps in the 70s, so sitting next to the pond was pleasant, but soon I reluctantly headed home to work. As I passed the tank, I looked in, seeing a couple of frogs diving for cover. But two were so still I didn’t not them right away — a female, fat with eggs, was basking in the sun on a concrete slab that had fallen from the wall of the crumbling tank, and a male was sitting on top of her, grasping her in amplexus! Perhaps the combined weight of the eggs and the male had immobilized the female. I took the opportunity to snap a few photos of the hapless couple.

The wood frogs’ calling grew louder over the next few days, only going silent when the temps dipped down to around 50 or below. On a chilly day the next week, I checked the pools again, and obviously the females had arrived. Both pools had single clusters and rafters of eggs — at the pond, mostly on the preferred side.

I saw no frogs, so either they’re done or they are just taking cover in the forest litter until the next warm spell, which is soon. Rain is coming with it, so watch out for more frogs on roads, heading for their breeding locations.

© 2017 Pam Owen

More signs of an early spring

While wood frogs often breed throughout the winter during warm spells, Other animals and plants that normally go dormant in winter are already active this year, including some that are rarely out this early, which signals a very early spring. The following are among my first sightings of animal and plant species so far this year:

  • Feb. 17 — hepatica blooming
  • Feb. 20 — skunk cabbage blooming, water striders mating on the ponds
  • Feb. 23 — Virginia bluebells (which usually start blooming in April), with foliage up to three inches and a few about to bloom; spring peepers calling; eastern phoebe calling in the yard; eastern comma butterfly flying from tree to tree, looking for sap to eat
  • Feb. 24 —eastern box turtle (like most reptiles, rarely out this early) sunning in the driveway
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