A male common frog (Rana temporaria) floats amid a mass of millions of eggs, croaking loudly to attract a female. Oostvaardersplassen, the Netherlands.
After a long-stretched winter that brought on our coldest March since 1987, we’re finally seeing the arrival of spring here in the Netherlands. Ah! Can you smell it? It’s nature’s season of romance… Or is it?
A good three weeks behind schedule, but with temperatures shooting up into the twenties spring is now quickly getting into full swing. The rising night temperature is the cue many amphibians have been waiting for to start their yearly migration to the breeding waters. Dutch readers will know this mass synchronized event as the toad migration (“paddentrek”), but in reality many more species of amphibian are on the road at this time. As such I’ve spent the past few days filming the breeding behavior of the common frog (Rana temporaria) for De Nieuwe Wildernis.
Out in the open the frogs are vulnerable, tasty little snacks to herons and other predators. And so they migrate under the cover of darkness. Unfortunately that doesn’t protect them from us, humans, and our cars. In a densely populated area like the Netherlands, most frogs will need to cross one or more roads to reach their breeding ponds, and many don’t make it.
This female didn’t make it across the road, nor did her eggs.
The male in the photo at the top of this post, however, has just reached his breeding pond in the Oostvaardersplassen. It is probably the same pond where he once hatched as a tadpole. Now he floats in a mass of hundreds of thousands of eggs, croaking loudly in hopes of attracting a female to mate with. If he finds a potential mate, he wraps his arms around her in a mating embrace known as amplexus. He may need to hold her like this for two days before she is ready. When she finally lays her eggs in a small clump, up to 4,000 at a time, the male can fertilize them as they emerge from her belly. Isn’t that romantic?
In a mating embrace known as amplexus, a male common frog wraps his forelimbs around a female.
Not everything is quite as romantic as it seems, however. There is another reason for the male to hang around this mass of eggs. Scientists call it ‘clutch piracy’. Instead of attracting a female and waiting for her to spawn, unromantic ‘pirate’ males search for freshly laid eggs and attempt to fertilize them again. Hang on, weren’t those eggs already fertilized at the time they were laid? After all, an egg can be fertilized only once. Well, it turns out that a few eggs are usually left unfertilized the first time around.
That may not seem fair to the male that took the trouble of attracting a female, but actually it seems everyone benefits from this behavior. The ‘pirate’ male benefits as he will get more offspring. The female benefits as more of her eggs are fertilized, and she too will get more offspring. And the original male that mated with her can go on to become a clutch pirate himself.