Natrix natrix is oviparous, and its eggs need sustained warmth of roughly 21 to 28 degrees Celsius for six to ten weeks. Across much of the species' northern range, ground temperatures do not deliver that reliably.
The solution is borrowed heat. Females seek out piles of decomposing plant material, where bacterial and fungal activity generates warmth independent of the weather: dung heaps, hay piles, reed litter, sawdust and, increasingly, garden compost. Several females often lay in the same heap, producing accumulations of dozens of eggs. The habit makes the species one of the clearest cases of a British and European reptile depending on human-made structures for reproduction.
It also makes it easy to harm by accident. Turning or removing a heap between roughly June and September destroys the clutch, and in Britain grass snakes and their eggs have legal protection against deliberate killing and sale. The snakes themselves defend only by bluff: hissing, feigning death, and releasing a strongly foul-smelling fluid from the cloaca.