In the hour of the wolf.

No one in the Maurienne range is quite sure what is going on – or, at least, few people admit to knowing much. Everyone is clear about one thing, however. The killings are related, in some way, to the long battle that has been waged between shepherds and wolves – and between sheep-lovers and wolf-lovers – since the European grey wolf, Canis lupus, decided to recolonise France from Italy 16 years ago.

The most recent attack, earlier this week, destroyed – in the space of a few minutes – a four-month-old Patou puppy called Dom Dom. Three other dogs, including Dom Dom's mother, Belle, were poisoned by balls of pork meat soaked in a chemical used in slug-repellent. They are recovering, after emergency intervention by the local vet, but may never be strong enough to work again.

"This can only have been the work of someone local, someone who knew the area," said Dom Dom's owner, René Grange, a shepherd in the hamlet of Les Villards, near Valloire. "It is two hours' hard walking, and 1,000 metres steeply up hill from the nearest road to the Pain du Sucre, the pasture where the sheep are grazing this week. You would have to know exactly where the night enclosures for the animals are. You would have to know how to avoid the hut where my two young shepherds were sleeping.  The pork meat balls were left, some time during the night, most likely just before dawn, in a place where the dogs would be sure to find them. This is the work of a maniac – a madman."

Philippe Martin is the local vet whose prompt action in finding an antidote saved M. Grange's other dogs this week. He said that the chemical placed in the meat balls is often found in slug poisons. It causes instant and catastrophic diarrhoea and lung failure in small mammals like dogs. "They finish up dying completely dehydrated but, before that, they drown in their own bronchial fluids," he said.

Why should anyone want to destroy such beautiful sheepdogs? In such a brutal way? Several theories have been put forward.

The most obvious possible explanation is that the dogs are being poisoned by accident by wolf-haters. There have been dozens of incidents of wolves being shot, or poisoned, illegally by shepherds in the French Alps in the past decade. M. Grange – and the Gendarmerie – have dismissed this possibility. "Anyone who knew anything about the mountains would not try to attack wolves by leaving poisoned pork near sheepdogs," M. Grange, 45, said. "This was a deliberate attempt to kill my dogs."

The second possibility is that this is some kind of revenge attack by militant pro-wolf activists, angry at the regular shooting and poisoning of wolves by French shepherds. Some local people are convinced that this may be the explanation but it also fails to add up.

The shepherds in the Maurienne area, including M. Grange, are among those who have most successfully adapted to living with wolves since the animals began to infiltrate across the French-Italian border in the early 1990s.

There are now thought to be about 100 wolves living in France. A few have already left the Alps and crossed motorways and railway lines to recolonise the Massif Central to the west, and the Jura and Vosges to the north.

Most Maurienne shepherds have accepted the sheep-protection methods put forward by the French government's Plan Loup (wolf plan) published four years ago. These include the permanent deployment of Patou and Beauceron dogs and young shepherds to watch the flocks day and night in the high summer pastures and the building of electrified enclosures to protect the sheep after dark.

There has been no wolf attack on sheep in the Maurienne massif for more than two years. There has been no recorded wolf-killing in the Maurienne area in the same period. Why would pro-wolf activists want to attack such a successful example of precisely the kind of man-sheep-wolf co-habitation that they have always insisted was possible?

The sheep-dog killings remain a complete mystery, then? Possibly not.

Daniel Vejux is a wolf expert and a member of the national committee of the main French wildlife study and lobby group, L'Association pour la Protection des Animaux Sauvages. He is convinced that he knows exactly what is going on.

"There are some shepherds, like those in the Maurienne, who have now accepted that the wolf is here in France to stay. But there are others who have sworn never to accept wolves," M. Vejux told The Independent.

"There is a kind of civil war between shepherds, between those who accept government advice, and have taken measures to protect their sheep, and those who only care about one thing, driving the wolves out.

"These attacks on sheepdogs are meant to destabilise the systems which have been adopted to protect sheep from wolves. They are poisoning sheepdogs to intimidate the shepherds who are playing by the new rules."

A local gendarmerie spokesman declined to comment on the suggestion that the dog poisonings were part of a pastoral civil-war. "All possible leads will be followed up," he said. "That's all we can say at present."