Wolves with an extra toe on their hind legs are the products of cross-mating between wolves and domestic dogs, scientists in Italy have confirmed.
By Paul Rincon
BBC News Online science staff
Recovery of threatened canine groups can be hampered by cross-mating, so the data has important conservation implications.
The finding could help efforts to monitor the recovery of threatened wild dogs and wolves around the world.
These "dewclaws" are the underdeveloped first toes common in domestic dogs but thought absent from wolves.
They are a good "rule of thumb" for spotting hybrids in the wild, the researchers say, and therefore a tool for assessing the scale of cross-breeding.
Recovery programmes for dwindling populations of wolves and wild dogs can be hampered if small populations get swamped by an influx of domestic dog genes.
Researchers from the Universita di Roma and the Instituto Nazionale per la Fauna Selvatica looked at 18 so-called microsatellite markers in the DNA of three dewclawed wolves from Tuscany, central Italy.
They found grey wolves (Canis lupus) with dewclaws had unique dog alleles - variant forms of a gene - suggesting the extra toes did not arise through spontaneous mutations but through hybridisation with dogs.
"I've done wolf tracking in North America, Italy and elsewhere and until now I never found an animal with a dewclaw. We are taught in school that wolves have only four claws on the back leg," co-author Dr Paolo Ciucci, of the Universita di Roma "La Spaienza", told BBC News Online.
Prof Rolf Peterson, of Michigan Technological University, Houghton, US, told BBC News Online: "Anything like this is potentially useful because it can be real hard to diagnose hybrids. I'm not aware of another field indicator that clear."
Prof Peterson said hybridisation with dogs had caused problems at early stages of wolf recovery programmes in North America.
Recently, attempts to reintroduce wolves to eastern Germany were complicated when dog hybrids were discovered amongst offspring.
Dewclaws might also be a useful indicator of domestic breed hybridisation in other wild canid - or wild dog - populations.
"When coyotes first came into New England states 30 or 40 years ago, they hybridised with dogs and we were taught that one of the distinguishing features of hybrids was dewclaws on the hind feet," said Prof L David Mech, of the University of Minnesota, St Paul, US.
Dewclaws (circled) are a sure sign of hybridisation say the scientists
However, it is unlikely that all hybrids have dewclaws. There may be other signs of hybridisation not currently known.
The team has identified possibilities amongst other rare features seen in some of the wolves they studied.
The three Italian dewclawed wolves came from the south-central Tuscan province of Siena. Dr Ciucci said conditions for hybridisation might be more likely in this region, which is on the margins of the wolf's range in Italy.
"There is a high density of guard dogs there, a highly fragmented landscape and the wolf density is lower. There is persecution because of high human density," Dr Ciucci explained.
"In these conditions, we hypothesise that the social structure of the pack would be disrupted quite often and create more likely conditions for hybridisation. To be sure we should monitor all situations like this across the range."
There are an estimated 500-600 wolves in the whole of Italy but these numbers can fluctuate significantly, said Dr Ciucci. The animals have full legal protection.