The choir invisible

I've had an issue with invisible frogs as of late.

Ever since the snow's melted, I've heard an orchestra's worth of frogs singing away in the muck by the road on my way to work. But whenever I run up to the water's edge for a closer look, they all clam up and go still, becoming invisible in the dark waters.

The same thing happens when I then chase the sound of another group of frogs, and another. The siren song lures me deeper and deeper into the muck, promising hordes of tiny hoppers just metres away, but yielding disappointment. Eventually, the first group starts up again.

It's at this point that I suspect this is some sort of plot to get me drowned in the swamp.

The members of this choir invisible are boreal chorus frogs – the smallest and one of the noisiest frogs in Alberta. You can hear them around almost any water body in St. Albert right now – their finger-on-comb-like “prrreeeeEEEP…prrreeeEEEP!” can be heard from about a hundred metres away – but good luck spotting them.

Boreal chorus frogs are far easier to hear than to see, agrees Kris Kendell, former St. Albert resident and co-ordinator for the Alberta Volunteer Amphibian Monitoring Association.

“They're very well camouflaged,” he says, and extremely small – they top out at about four centimetres long.

“To top it all off, they're very wary,” he continues. If one frog senses danger and shuts up, its neighbours will soon follow suit. Even if you can hear them, it's very tough to home in on them, even if you're right next to one – it's almost as if they can throw their voices.

If you do spot one, expect to see a grey, brown or green frog with a yellow or green belly, granular skin and three dark, sometimes broken stripes down its back, Alberta Fish and Wildlife reports.

Unlike the wood frog, the boreal chorus frog's call is exceptionally loud, often audible through closed car windows, Kendell says. Wood frogs also have their vocal sacks behind their shoulder blades on their backs instead of up front of the throat like the chorus frog.

The secret to spotting them is patience, reports Arthur Whiting, an instructor at the Northern Alberta Institute of Technology who has made extensive study of boreal chorus frogs. Stand still long enough, and the frogs will usually start singing again. That's when you can look for their tiny yellow vocal sacks popping out of the water, their eyes just visible above the surface.

Love calling

Like wood frogs, boreal chorus frogs can survive freezing winters due to an antifreeze-like compound in their bodies, Whiting says. They're not as good at it as wood frogs, though, and die if the temperature dips below minus 5.

Once they thaw in the spring, these frogs immediately congregate around the nearest water body – often in the hundreds – and create a chorus of voices as they let loose their mating calls all at once.

Curiously, boreal chorus frogs prefer to mate in temporary ponds such as those in ditches rather than in permanent rivers or lakes.

“It does seem suicidal,” Whiting admits, as the frog eggs need water to survive, but it also reduces the chance that those eggs will get eaten by water beetles, dragonfly larvae and other predators in permanent water bodies.

Once a female arrives at the pond and picks a mate (presumably based on who's the best singer), the male gloms onto her back until they do the deed – a practice called amplexus.

The female usually lays a string of about 50 eggs (instead of a cloud of eggs like the wood frog), with the male fertilizing each egg as it emerges. Once the female hooks the string to a nearby stalk, the two break up and find new mates.

Each female lays up to 600 eggs a season, Whiting says. Roughly one out of 200 eggs will survive to adulthood.

About half of all male boreal chorus frogs die during the mating process, Whiting continues. Calling all day takes a lot of energy, and doing so doesn't leave frogs much time to eat.

Chorus frogs tend to be more discreet with their sex acts than wood frogs, Whiting says – they don't usually all dog-pile on a single female, for example. You will see treacherous males hide behind other singers, however, waiting to swoop in and steal another man's girl once she gets close enough.

Whiting says mating season typically runs from April to July for chorus frogs.

I suspect I'll spend much of the next few months listening to them as I prowl local ditches and sloughs, searching for any sign of these elusive creatures.