‘Food, reproductive potential and a place to hibernate. Those are the three requirements of a grass snake. In that respect, we are in the perfect location here.” Edo Goverse, biologist at the Ravon foundation, points out the Diemerzeedijk, between Amsterdam and Muiden. “In the crevices of the basalt stones, on both sides of the dike, they can hide for their winter rest. The pools and ditches nearby are full of frogs and other amphibians: prey is plentiful. And in the hothouse” – he points to a brown bump, more than a meter high, to the left of the dike – “they can lay their eggs.” Geert Timmermans, urban ecologist and grass snake coordinator for the Greater Amsterdam region, nods enthusiastically. “You can hardly have it better as a grass snake.”
And that’s good news, because the grass snake, the largest of the three endemic snake species in the Netherlands, has been listed as ‘vulnerable’ on the Red List for endangered species for many years. Above the large rivers, the species is fairly widespread near water, and grass snakes have recently been observed again in South Limburg. Timmermans: “There are an estimated 10,000 grass snakes around Amsterdam, things are going reasonably well here.” But the numbers don’t want to go really thunderous in most places.
If you come stamping, they’ll shoot away before you know it
Edo Goverse biologist
The role of man in this is ambiguous. On the one hand, the grass snake is a real follower of culture, which benefits from human proximity. “The species we have in the Netherlands – the mottled grass snake, Natrix helvetica, which can be recognized by the spots along its body – came here thousands of years ago from southern Europe, following the farmers,” says Goverse. “The females used manure and compost heaps to deposit their eggs.” But in the past century, conditions have deteriorated: due to agricultural intensification, suitable breeding waters have disappeared, and the water quality has declined. Suitable wintering places also became scarcer. Timmermans: “In railway embankments, for example, you increasingly see that rabbit holes are closed and the vegetation is mowed short. It’s a shame, because important hiding places will be lost as a result.”
Moreover, the dung heaps changed character: larger, more isolated. “An adult grass snake can easily travel ten kilometers in a breeding season, so a pile does not necessarily have to be right next to the water for them,” says Timmermans. “But it is useful for young grass snakes if there is shelter and food nearby.”
Skirt day
The Diemerzeedijk is one of the fixed routes along which Goverse and Timmermans regularly monitor grass snakes, together with ecological employees Marina den Ouden and Ruud Lutterlof from the municipality of Amsterdam. Also today the four of them search the ground, but so far without success. Den Ouden: “You would think that given their length it would be difficult to overlook them. Male grass snakes are usually about 80 centimeters long, females can grow up to 1.20 meters long. But they are very shy, so especially in the summer they dive into the vegetation.” “They don’t have ears, but they can feel vibrations,” says Goverse. “So when you come stamping, they’ll bolt before you know it.”
One female often mates with several males, so there is multiple paternity
Geert Timmermans urban ecologist
A good time to spot grass snakes is early spring, when it is not too hot yet and the vegetation is not too high. Because grass snakes are cold-blooded animals, they need the sun to warm up. In the middle of summer this can also be done between the greenery, but in the spring the open spaces are most efficient for this. Timmermans: “Around Rokjesdag, the first day that we go outside with bare legs, the mating season for the grass snakes begins. At the end of March, the male snakes come out of hibernation first, and they are already ready to mate. But only a few weeks later the women arrive, and they then get the men after them in no time. Sometimes such a woman still has the mud from her winter residence on her head and then she has to get to work.”
The tallest women are the most in demand, he adds. “Then you sometimes see them entwined in the sun with several at the same time. One female often mates with several males, so there is multiple paternity: not all eggs are fertilized by the same individual.” Male grass snakes have a penis that consists of two halves. This hemipenis makes it easier for them to reach the cloaca of the female and, depending on the position, the left or right side can be used.
Heavy work
At the end of June, the beginning of July, the grass snake eggs have developed and are deposited in a warm, humid place, for example in a hothouse. They come out in August and September.
Den Ouden walks down the dike to one of the hothouses that were made just last week by local grass snake volunteers – if you take a deep breath you can still smell a slight dung smell. “You take horse manure, wood chips and fresh clippings and mix them together in equal parts,” she explains. “If you mix that together, you have the ideal hothouse. A few more branches in it, so that openings are created for the air supply, and you’re done.”
Lutterlof shows a video of how two wheelbarrows are pushed up. “They are pushing from below, and on the dike they are pulling with ropes. It’s very hard work. This hothouse alone took them an hour and a half. It warms you up naturally.” During the construction of the heap, a bucket of water is occasionally added to start the scalding process.
Empty egg shells
This heating is a chemical process in which nutrients are converted into carbon dioxide and water and heat are released. “For the eggs to hatch, temperatures between 25 and 30 degrees Celsius are needed. That is why we build the greenhouse quite early in the season, so that it can cool down a bit. Initially it is still much too hot in there, then you cannot even put your hand in it. The eggs would be cooked immediately.” In principle, heating can also occur naturally, for example in a rotting piece of wood. “But especially in northern countries, the temperatures are often far from optimal for eggs to incubate. Then the manure heap, compost heap or greenhouse really plays a crucial role.”
In the dozens of breeding grounds around Amsterdam, the female snakes can deposit their egg packages in the summer. In the autumn, the same heaps are dug up – insofar as they have not yet decayed – in search of empty egg shells, in order to get an impression of the number of snakes. “One female lays about thirty eggs,” says Den Ouden. “In a successful breeding heap, which attracts several females, you can easily find hundreds. And often you also encounter all kinds of extra guests. Look, here’s the rhinoceros beetles. They come to the horse manure.”
Grass snakes are creatures of habit and basically return to the same place year after year. For that reason, the researchers always leave the remains of the old hothouse behind. Timmermans: “The smell of the females is still there. Then they know: this is a good place.”
Look, the woman still looks quite pale and dull. It still needs to get bored
Geert Timmermans urban ecologist
Young grass snakes look like licorice laces shortly after hatching. “When rolled up, they are no bigger than a euro coin,” says Goverse. “It’s only after two or three years that they are fully grown.” The snakes remain in the pile for the first few days, then they move out. In that phase they are easy prey for hedgehogs, for example, and even for the frogs that grass snakes themselves later often target.
Larger grass snakes can fall prey to mustelids and birds of prey. “There are hawks and white-tailed eagles in the area, among other things,” says Timmermans. Grass snakes rely on their speed precisely because they are non-venomous. “If you do manage to catch them, they sometimes blow themselves up a bit, like a cobra. Pure appearance. Or they play dead: curled up, tongue sticking out of wide-open mouth. And from their cloaca, the poop hole where the eggs come out of the females, they secrete an enormous smelly substance. A smell of death and destruction, which you carry with you for hours. Some people detect a hint of weed in it, or garlic.”
Not much later he finds, hidden under a tile, two grass snakes: a male and a female. With one lightning-quick grab, he has them both. “Look, the woman still looks quite pale and dull. It still needs to get bored. The man already has his new skin.” Goverse: “That little tongue that sticks out so searchingly, we call that tongues. This allows them to smell their surroundings.” Moments later, the male indeed pretends to be dead. A whitish substance emerges from the cloaca, a striped opening on the ventral side, near the tail. Timmermans, laughing: “Just watch out. If you get that on you, no one will want to sit next to you on the train.”