It's 40 years since the Chernobyl disaster. This is what it has meant for wildlife living around the devastated nuclear power plant.

"Pa-pa-pa-pa-pa!" In the middle of the night, a noise from the darkness in the abandoned, irradiated landscape of Chernobyl. Pablo Burraco, a scientist, stepped quietly between the trees, not far from the ruins of the power plant at the centre of the world's worst nuclear disaster. In the aftermath of the catastrophic reactor explosion in 1986, the surrounding area was evacuated for many miles, so few people trod where Burraco now did.

With only his head torch illuminating the ground before him, Burraco closed in on the source of the night-time racket – a tiny male tree frog, urgently calling for its mate. A swoop of his hand and he had plucked the 5cm-long (two inch) amphibian from its perch on a small tree.

It was 2016. Burraco, an evolutionary biologist at Doñana Biological Station, a public research institute belonging to the Spanish National Research Council, was making his first field trip to this troubled part of the world.

Peering at the creature now safely confined within the curl of his fingers, Burraco immediately noticed the frog was slightly dark in colour, unlike other frogs of the same species that lived further away. "It was super exciting," he says, recalling the moment. This frog raised a question that many have asked ever since the explosion at Chernobyl: had radiation from the stricken power station changed the creatures living near it? That's what Burraco wanted to find out.