Why Did the Amphibian Cross the Road?

Wednesday, March 6, 2024. It was around 9:30 PM and I was headed home after a long day at work. Night had fallen three hours earlier, and a gentle rain had been falling all day.

As my route brought me just south of Earlville, a largish frog hopped across the road. It’s early March! In my mind, that means it is still winter - we shouldn’t be seeing such signs of spring for, oh, almost another month. But then I remembered - it’s different now. Thanks to climate change, winter this year was nearly non-existent, and the birds, the trees, and even the amphibians are following the temperatures - spring has arrived.

I was so stunned by the frog, that I pulled over in the village to post it on my Facebook page (mostly so I’d remember the next day and write it on my phenological chart, where I track such annual occurrences). But I needn’t have worried about forgetting, because as I continued my commute back home, hundreds of frogs were on the road - many permanently.

I tried dodging the ones that were still alive, obliviously striving to make their way to the other side, but I was not the only car on the road. Soon my curiosity took over, however, and I had to know which species of frog had chosen this night for migration. Making sure I was the only car in sight, I pulled over, and using the glow of my taillights, I stood in the rain until I located a live frog and took its picture. It was a wood frog, Lithobates sylvatica, one of the first frogs of spring.

Usually our first awareness of the springtime frogs is hearing their vocalizations from ponds and wetlands: the persistent high-pitched peeps of the spring peeper, the thumb-down-the-teeth-of-a-comb rasp of a chorus frog, and the funny quacking of the wood frog. The frogs are singing to attract a mate so the next generation is assured. But never, in all my years as a naturalist, have I encountered a mass frog migration like this.

One of the great naturalists/environmental educators of New York State, George Steele, has been posting on his Facebook page this week about his nightly forays down his rain-soaked road in search of salamanders on the move. This phenomenon is a regular springtime ritual: the adults head to vernal, or spring, pools in the woods to mate and lay their eggs. Rainy nights in the spring when the temperature is just right, they all head out en masse. In many locales where these migrations are known, volunteers line the most heavily-traversed roads to help the migrants get across to safety, sometimes by picking them up and carrying them across, and other times by slowing down traffic and alerting drivers to take care.

But this was the first time I’ve been aware of a similar situation for frogs - minus the volunteer assistance. This doesn’t mean it’s not also a regular occurrence - it just means I’ve never encountered it, in person or print. Perhaps salamanders just have better press.

I’ve had an ear peeled the last couple of weeks for frog calls - the temps have certainly been mild enough - but so far the woods and wetlands around here have been silent. This is probably why I was so surprised to see the frogs out in such great numbers last night: they hadn’t given any indication yet that they were up and about from their long winter naps.

But what does such an early migration mean? Can we logically expect winter to really be over? We’ve hardly had any winter at all this year, but I wouldn’t be surprised if more snow and frost were in the forecast this month. It is March, after all. Will additional snow and ice affect the newly fertilized spawn, or do they have safety measures in place for just such occasions?

As it turns out, the hardy wood frog is prepared! Wood frogs are notorious for being able to freeze solid in the winter and then defrost to live a full and robust life come spring. Their bodies produce an “antifreeze” that keeps their cells from bursting with ice crystals, while at the same time they stop breathing and their hearts stop beating. Herpetological cryogenesis. When spring arrives, they thaw out and that is that - no muss, no fuss.

Their eggs, apparently, aren’t much different. Like most amphibian eggs, wood frog eggs are laid in a large gelatinous mass. The “jelly” is a mucoprotein that acts as a protective barrier. It allows water, oxygen, and nutrients in to the developing eggs, but keeps out bacteria and pathogens. It also freezes at a different temperature than water, so as the pond freezes, the jelly pulls water from the eggs, resulting in dehydrated eggs. This is a good thing, for when there is little to no water in the eggs, ice crystals cannot form (or fewer crystals form), thus protecting the developing embryos.

It is also believed that when eggs are laid in large masses (individual egg masses rafting together), their collective size affords them additional protection from freezing.

So it seems that other than my dismay at the deaths of so many frogs last night, the ones that made it to their breeding pools have a pretty good chance of securing the next generation’s survival, barring predators and evaporation of the breeding pool.

If you are out driving on a rainy spring night the next month or two, please slow down and watch out for frogs and salamanders crossing the roads (and later on, turtles). Do you best to avoid hitting them. And if your road has an annual crossing “event,” consider going out on rainy nights to monitor for these amphibians and help them get to their breeding pools.