Why Is Canada’s Wolf Population Splitting Into Two Groups?

Why Is Canada's Wolf Population Splitting Into Two Groups?

Chester Starr of the Heiltsuk First Nation knows that the wolves of British Columbia come in two varieties: timber wolves on the mainland and coastal wolves on the islands. Genetic research has finally confirmed what Starr’s tribe has always known.

It was Starr’s “traditional ecological knowledge” that initially inspired Polish Academy of Sciences researcher Astrid V. Stronen and University of Calgary scientist Erin Navid to take a closer look at British Columbia’s wolves. They wanted to see whether the Heiltsuk Nation’s folk knowledge was reflected in the wolves’ genes. Continue reading

Woolly mammoth DNA may lead to a resurrection of the ancient beast

Technical and ethical challenges abound after first hurdle of taking cells from millennia-old bodies is cleared

baby mammoth carcass

Even a well-preserved carcass like this baby woolly mammoth is unlikely to provide viable cells for cloning, as used to create Dolly. Photograph: Aaron Tam/Getty

By Ian Wilmut

It is unlikely that a mammoth could be cloned in the way we created Dolly the sheep, as has been proposed following the discovery of mammoth bones in northern Siberia. However, the idea prompts us to consider the feasibility of other avenues. Even if the Dolly method is not possible, there are other ways in which it would be biologically interesting to work with viable mammoth cells if they can be found.

In order for a Dolly-like clone to be born it is necessary to have females of a closely related species to provide unfertilised eggs, and, if cloned embryos are produced, to carry the pregnancies. Cloning depends on having two cells. One is an egg recovered from an animal around the time when usually she would be mated.

In reality there would be a need for not just one, but several hundred or even several thousand eggs to allow an opportunity to optimise the cloning techniques. The cloning procedure is very inefficient. After all, after several years of research with sheep eggs, Dolly was the only one to develop from 277 cloned embryos. In species in which research has continued, the typical success rate is still only around 5% at best.

Elephant eggs


Photo: Rubén Portas

In this case the suggestion is to use eggs from elephants. Because there is a danger of elephants becoming extinct it is clearly not appropriate to try to obtain 500 eggs from elephants. But there is an alternative.

There is a considerable similarity in the mechanisms that regulate function of the ovaries in different mammals. It has been shown that maturation of elephant eggs is stimulated if ovarian tissue from elephants is transplanted into mice.

In this way it might be possible to obtain a considerable number of elephant eggs over a period of time if ovarian tissue is obtained from elephants that die.

Cells from mammoths are required to provide the genetic information to control development. The suggestion is to recover cells from the marrow of bones emerging from the frozen north of Siberia. However, these cells will degenerate rapidly at the temperature of melting snow and ice. This means that cells in the bones may well become useless for this capacity as they thaw.

The chances of cells being viable would be increased if bones could be recovered from the lowest possible temperature rather than waiting until they emerge from snow. The cells can then be warmed rapidly. Alternatively, the nuclei could be transferred directly into eggs.

The very first stages of embryo development are controlled by proteins that are in the egg when it is shed by the ovary. One for example has a critical role in cell division. Together these proteins have an extraordinary ability to repair damaged nuclei so it may not be strictly necessary for the cells to be viable. It would be best if the mammoth nucleus could be introduced into an egg immediately, by injection of the contents of the damaged cell into the egg.

Research in 2008 found that when nuclei from freeze-dried sheep cells were transferred into eggs, some of the cloned embryos developed for a few days, but not to term. This was a very clear indication of the ability of the egg to repair damaged nuclei. However, freeze-dried cells are likely to be more stable than those that have been frozen with liquid still present. In the case of the mammoth, the cells would likely be killed by large ice crystals formed from the liquid.

Finally, if embryos that developed normally for a few days could be produced, they would have to be transferred to surrogate mothers to develop through pregnancy. Embryo transfer is only carried out routinely in fewer than a dozen species, and the elephant is not one of them. Success in embryo transfer depends upon introducing the embryo to a womb that is in a receptive state. The mechanisms that bring about this state in elephants are currently being defined by research in a number of zoos.

Taken together, it can be seen that there is biological uncertainty about the availability of viable cells, and that several complex techniques would have to be developed for cloning of mammoths to be successful. There is no guarantee that these techniques are even biologically possible. There may be unknown differences between species that would prevent the procedures that we developed in sheep being successful in mammoths.

Copyright: Royal BC Museum

Mammoth stem cells
An alternative ambition would be to try to use mammoth cells to produce stem cells. In several different species it is possible simply by the introduction of four selected proteins to give adult cells the characteristics of embryo stem cells. The four factors give embryo stem cells their unique characteristics and were found to be able to impose these characteristics on skin cells. This type of stem cell can be grown for very long periods in the laboratory while retaining the ability to form all of the tissues of the body.

They would provide extraordinary opportunities to compare mammoth cells with those of elephants. This knowledge would be of fundamental biological interest. It would enable us to begin to answer groundbreaking questions. What are the differences between the cells and tissues of these species? What are the similarities? The mammoth lived in a different climate, so was the metabolism of their cells different? Does this information cast any light on the cause of extinction of mammoths?

Stem cells of this type can also be induced to form gametes. If the cells were from a female, this might provide an alternative source of eggs for use in research, and perhaps in breeding, including the cloning of mammoths.

From a male, they would be sperm, and they might be able to fertilise eggs to produce a new mammoth embryo. It would be interesting to know if mammoth sperm could fertilise eggs of the elephant. If so, would the embryos develop to term to produce a hybrid animal?

Only a small proportion of mixed matings between species produces viable offspring, but the mule is one example and has been used by humans for centuries.

In all of these discussions it is necessary to consider the welfare of the animals. Mammoths lived in cold climates, whereas their current relatives including potential surrogate mothers live in warmer regions.

It would be essential to provide mother and clone with the appropriate environment of temperature, moisture and diet. It would almost certainly be necessary to keep the animals in captivity, so it would be essential to provide as interesting an environment as possible. Ideally this should include other elephants, mammoths or hybrids to provide social interaction for the animal.

So while unlikely at present, the development of some form of mammoth creature or hybrid might be possible in the longer term, the research of which could lead to major biological discoveries and advances.

This is another area of biology where studies of stem cells would be very rewarding. In stem cell research rather than cloning there would also be fewer concerns over animal welfare, or the effect of the animal on the environment. All in all, research to produce mammoth stem cells would be the appropriate choice, and extraordinarily scientifically rewarding, should it be possible to find viable mammoth cells.




Rare Maned Lionesses Explained

Posted by Christine Dell’Amore

If it looks like a male lion and is perceived as a male lion—well, sometimes it isn’t. That’s the case of Africa’s unusual maned lionesses, which sport a male’s luxurious locks and may even fool competitors.

Though uncommon, maned lionesses have been regularly sighted in the Mombo area of Botswana‘s Okavango Delta (including the individual pictured below), where the lion population may carry a genetic disposition toward the phenomenon, according to Luke Hunter, president of the big-cat conservation group Panthera, which collaborates with National Geographic’s Big Cats Initiative. (The Society owns National Geographic News.)

Hunter said it’s possible that maned lionesses in Mombo are related—including a safari favorite named Martina, which disappeared in 2002. (Learn more about how you can see the maned lionesses at Mombo Camp.)

A maned lioness in Botswana’s Okavango Delta. Photograph courtesy Deon De Villiers.

Such masculine females likely occur when the embryo is disrupted, either at conception or while in the womb, he said by email.

“If the former case, the genetic contribution of the sperm—which determines the sex of the fetus in most mammals—was probably aberrant, giving rise to a female with some male characteristics.

“Alternatively and perhaps more likely, the problem may have occurred during gestation if the fetus was exposed to increased levels of androgens— male hormones such as testosterone.”

If a lion mother had abnormally high androgens during pregnancy, her female offspring may end up “masculinized”—a situation that occurs occasionally in people but which is rarely observed in wild animals.

A maned lioness with a fellow female. Photograph courtesy Ryan Green.

Whatever the case, such lionesses would likely be infertile but otherwise “perfectly capable” of surviving, Hunter noted. (See more lion pictures.)

In fact, their manes may actually be a boon to the pride—for instance, if the female is perceived as a male, she may better defend kills from hyenas or the pride from attacks by foreign males. In the case of the pictured female, Hunter said, it seems like she’s treated as a lioness by the rest of the pride.

“It would be interesting to know if she behaved like a male,” he added. “Two similarly aberrant Serengeti lionesses were outwardly female—they did not have manes, but were almost male-sized, and they challenged and fought unfamiliar males for territories as though they were males!”

Source: National Geographic

See comments on: http://newswatch.nationalgeographic.com/2012/10/09/weird-wild-rare-maned-lionesses-explained/

Is the Dingo Special Enough to Save?


A startling discovery: Commonly believed to be a breed of wild dog, scientists now consider the dingo to be a species in its own right. Photo: Neil Newitt

When you look at the picture above, what do you see? A wild dog? A strangely colored wolf? Or something entirely different: a dingo? For centuries, scientists have debated whether Australia’s native canine is its own species or merely a type of wolf or dog. Now, based on physical and genetic evidence, a team of scientists is making the case that the dingo is a unique species that deserves protection under Australia’s federal conservation laws. If they can’t convince governments and landholders, the dingo may be doomed.

Wild dingoes live across Australia, in grasslands, deserts, and even wetlands and forests. Archaeological evidence suggests that the animal arrived on the continent at least 3500 years ago as people sailed back and forth from Asia, where it first appeared, then continued to evolve in isolation until the arrival of Europeans and their dogs in the late 18th century. The European naturalists who first heard descriptions of the dingo believed it represented a new species of canine and gave it a species name to match: Canis dingo. Domesticated dogs, on the other hand, are known as Canis lupus familiaris, indicating that they are a subspecies of wolf (Canis lupus).

But over the next 300 years, scientists began to argue about what to call the dingo, given the lack of early physical specimens and the fact that the original classifications were based on nothing more than a painting and description given by Australia’s first governor, Arthur Phillip. Dingoes have continued to change as they bred with settler dogs. Today, Australia’s native canine is most often referred to by scientists as C. lupus dingo, relegating it to a subspecies of wolf based on a notion that dingoes evolved from wolves in Asia. Recent studies suggest that dingoes, dogs, and wolves are cousins, all descended from a distant ancestor.

That classification left University of Sydney wildlife biologist Mathew Crowther unsatisfied, given his experience studying wild dingoes. He believed that conservation and management decisions were not being based on firm evidence. “A dingo is a distinctive thing in Australia,” identifiable by its erect ears, bushy tail, and neck that can arc backward into prime howling position, he says. Still, it’s difficult to tell a dingo from a dingo-dog hybrid, or even a feral dog, because natural variation within dingoes is poorly understood and mating with wild dogs may have altered the genome of living dingoes, Crowther explains.

Being able to define what a dingo is—and isn’t—is increasingly important in Australia. While some scientists argue that dingoes with no dog DNA fill the important niche of apex predator in Australia’s ecosystem by eating feral cats and foxes, ranchers lump dingoes, feral dogs, and dingo-dog hybrids into the category of pests that attack and kill valuable livestock. Current policies in some jurisdictions of Australia aim to exterminate dingo-dog hybrids while conserving dingoes. But without a clear definition of what distinguishes a dingo, it’s hard to manage wild dingoes, dingo-dog hybrids, and free-roaming domestic dogs, says Damian Morrant, an ecologist with James Cook University, Cairns, who was not involved in the new study. He says the work is a “baseline” for developing clear guidelines for identifying dingoes in the wild.

To begin sorting dingo from nondingo, Crowther and his colleagues at the universities of Sydney, New South Wales, and Western Sydney reviewed genetic work conducted by other researchers and began tracking down pre-1900 dingo specimens, which would allow them to study the species before it encountered—and mated with—domestic dogs brought by European settlers. “One of our colleagues went to all the European [natural history] museums: London, Paris, Germany, Oslo,” Crowther explains. The team discovered a range of coat colors on dingoes preserved in the museums: yellow, brown, ginger/red, black, and white. That indicated that these colors are not the product of recent mating with dogs, and that animals boasting them today can be considered pure dingoes.

The researchers also compared the skulls of the dingo specimens with those of wolves and similar-looking domestic dogs such as Australian cattle dogs and collies. While there were overlaps, the dingoes had wider and shorter skulls and no hind leg dewclaws, vestigial toes that don’t touch the ground, which are common among dogs and wolves, the team reported online this week in the Journal of Zoology. Based on these physical and genetic differences, the researchers propose changing the dingo’s scientific name back to Canis dingo, once again classifying it as its own species.

Given the contentious attitudes about dingoes as either top predators or pests, as well as the uncertainty among scientists about their evolutionary past, the paper will “inflame passions across the board,” says Christopher Dickman, a conservation ecologist at the University of Sydney who is not part of the team. And so it has. Although J. William Ballard, an evolutionary geneticist at the University of New South Wales in Sydney, says the study provides a “road map for the debate,” he believes the methodology is weak and the data unconvincing. Still, dingo specialists such as Christopher Johnson, a conservation biologist and ecologist at the University of Tasmania, Sandy Bay Campus, welcome the work. “It places the dingo on firmer biological ground as a distinct [group],” he says.

Crowther and his colleagues acknowledge that they’ve not yet identified “consistent and clear diagnostic features” that characterize all members of C. dingo, but they claim they’ve set limits on physical traits of the species. And for conservation and land managers that’s a start, says Euan Ritchie, an ecologist at Deakin University, Melbourne Burwood. “If it looks like a dingo, smells like a dingo, and acts like a dingo, is that enough” to count it as a dingo?




Access to the paper: http://onlinelibrary.wiley.com/doi/10.1111/jzo.12134/abstract

Iconic island study on its last legs: genetic rescue needed for Isle Royale’s inbred wolves

by Emma Marris

Rolf Peterson/Minneapolis Star Tribune/MCT/Newscom

Since 1958, ecologists have watched wolf and moose populations on Isle Royale in Lake Superior wax and wane in response to each other, disease and the weather. But for the longest predator–prey study in the world, the wolf is now at the door. Devastated by inbreeding, the wolf population has dropped from 30 individuals a decade ago to just 10 spotted in field counts so far this year, leading the US National Park Service to consider importing new animals for a ‘genetic rescue’.

Now, nature is intervening — and could either save the landmark project without the need for tranquillizer darts and wolf crates, or sound its death knell. As temperatures plummeted last month, Lake Superior froze for the first time in six years. The 24-kilometre ice bridge could let wolves from the Canadian mainland cross to the US island, bringing an influx of genes (see ‘Wolf island’). But project scientists say that the opposite is more likely: free to roam, the last wolves could leave the island in search of mates. 

That would put an end to a study that has provided textbook ecology lessons for generations. It has shown how predation can structure populations of prey: when wolf numbers plummet, moose populations tend to soar (see ‘Ecosystem in flux’). And it has offered insights into wolf behaviour, moose physiology, the life cycle of moose ticks and how wolves might be driven to form packs to ward off scavengers such as ravens, rather than for any hunting advantage.

Through the decades, the search for cause and effect in the ecosystem has been rendered much easier by isolation from the mainland’s human and animal populations. Occasionally, however, Lake Superior freezes. The very first wolves came to Isle Royale over an ice bridge in the early 1940s, some 30 years after the first moose. The lake froze nearly every year at the beginning of the study, but that has changed. The most recent ice bridge was in 2008; before that, the last one was in 1997, when a wolf that biologists called ‘the old grey guy’ came to the island. He sired 34 pups and provided a rare boost of genes that doubled the population by the mid-2000s.

Whether any wolves have crossed this year’s ice bridge will not be clear immediately. The scientists are conducting their annual population survey, and are flying along the island shore in their Super Cub plane two or three times a week, but snow fills wolf tracks very quickly. If new wolves do arrive, their presence will probably be confirmed in the coming months, when DNA is extracted from faeces samples.

Source: John Vucetich/Rolf Peterson

John Vucetich, co-leader of the project and an ecologist at Michigan Technological University in Houghton, says that the need for an influx of genes is becoming urgent. In the past two decades, wolf skeletons have displayed spinal deformities that can painfully pinch nerves and affect gait and generally reduce fitness. According to work led by Vucetich and Rolf Peterson, also an ecologist at Michigan Technological University, this might explain why the number of moose needed to support a given number of wolves has increased: the predators’ attacking efficiency may be compromised (J. Räikkönen et alBiol. Conserv. 142, 1025–1031; 2009).

For Vucetich, genetic rescue is required not so much to maintain the continuity of the study as to preserve the ecosystem. Moose eat balsam fir trees. When the moose population expands, unchecked by predation, fewer fir seedlings can grow large enough to ‘escape’ into the canopy above the reach of moose and reproduce. There is already a missing generation of trees from between about 1910, when the moose arrived on the island, and 1940, when the wolves came. Most of Isle Royale’s balsam firs are thus either older than 100 years and near the end of their lives, or young and short enough to be browsed to death. If the trees do not achieve escape in the next decade or so, says Vucetich, “large portions of Isle Royale are not going to generate balsam fir, which is a really basic component of a boreal forest ecosystem”.

Many scientists familiar with Isle Royale support genetic rescue, especially because human activity has contributed to the current population crash. Climate change has led to the decreasing frequency of ice bridges. Canine parvovirus, probably caught from a domestic dog, caused the wolf population to fall from around 50 to 14 in the early 1980s. And in 2012, three wolves were found dead in an abandoned mining pit. Given this history of human influence, the argument that leaving the wolves alone would be allowing nature to take its course does not sway most ecologists.

David Mech, a US Geological Survey wolf biologist based in St Paul, Minnesota, argues in favour of “watchful waiting”. He says that much can be learned from studying how inbreeding affects population persistence, and that the knowledge would be useful for conservation biologists, who often need to nurture small, inbred populations of endangered species. He is not convinced that the wolves will die out; they have hit low numbers before and bounced back, he notes. And even if they do disappear, new wolves can be brought in quickly.

But Vucetich says that it could be five years before the last wolf dies and scientists confirm its demise, and another five before federal bureaucracies approve a genetic rescue and a pack develops into a predation force. He fears that a decade without significant moose predation would leave the fir trees devastated.

Phyllis Green, superintendent of the Isle Royale National Park, is considering three alternatives: doing nothing; watchful waiting followed by reintroduction if the population hits zero; or genetic rescue. She has not initiated a formal decision-making process, and will not commit to a timeline, but says that she wants to make a decision in consultation with her regional and national directors “before we run out of options”.

She is proceeding cautiously, she says, in part because of the implications of her decision. The mandate of the National Parks Service, as enshrined in a 1916 Act of Congress, is to “conserve the scenery and the natural and historic objects and the wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations”. Generally, this has meant a hands-off approach, but a genetic rescue could set a precedent for interventions to counteract the effects of climate change in other parks.

Green knows that many scientists are in favour of genetic rescue, but she also hears from “wilderness-oriented” advocates who urge her not to intervene. “It is one of the wicked problems,” she says.

Nature 506, 140–141 (13 February 2014) doi:10.1038/506140a

Source: http://www.nature.com/news/iconic-island-study-on-its-last-legs-1.14697

How do polar bears stay warm? Research finds an answer in their genes

polar bear looking straight at camera

New study is part of a broader genomic research program aimed at understanding what makes a polar bear a polar bear

A polar bear looking straight at the camera
A male polar bear. Credit: U.S. Geological Survey, Steven C. Amstrup
polar bear walking on an icy terrain
A male polar bear walks on pack ice. Credit: U.S. Fish and Wildlife Service, Eric Regehr
polar bear walking by water
A polar bear in Alaska. Credit: U.S. Fish and Wildlife Service, Steve Hillebrand
a polar bear lying down and facing camera
A polar bear at rest. Credit: U.S. Fish and Wildlife Service, Susanne Miller
Charlotte Lindqvist in front of a background showing a polar bear walking in a cold climate
Charlotte Lindqvist, assistant professor of biological sciences University at Buffalo led the study, which is part of a larger research program devoted to understanding how the polar bear has adapted to the harsh Arctic environment.
 In the winter, brown and black bears go into hibernation to conserve energy and keep warm.

But things are different for their Arctic relative, the polar bear. Within this high-latitude species, only pregnant females den up for the colder months.

So how do the rest survive the extreme Arctic winters?

New research points to one potential answer: genetic adaptations related to the production of nitric oxide, a compound that cells use to help convert nutrients from food into energy or heat.

In a new study, a team led by the University at Buffalo reports that genes controlling nitric oxide production in the polar bear genome contain genetic differences from comparable genes in brown and black bears.

“With all the changes in the global climate, it becomes more relevant to look into what sorts of adaptations exist in organisms that live in these high-latitude environments,” said lead researcher Charlotte Lindqvist, PhD, UB assistant professor of biological sciences.

“This study provides one little window into some of these adaptations,” she said. “Gene functions that had to do with nitric oxide production seemed to be more enriched in the polar bear than in the brown bears and black bears. There were more unique variants in polar bear genes than in those of the other species.”

The paper, titled “Polar Bears Exhibit Genome-Wide Signatures of Bioenergetic Adaptation to Life in the Arctic Environment,” appeared Feb. 6 in the journal Genome Biology and Evolution.

Co-authors include scientists from UB, Penn State University, the U.S. Geological Survey Alaska Science Center, Durham University and the University of California, Santa Cruz.

The genetic adaptations the research team saw are important because of the crucial role that nitric oxide plays in energy metabolism.

Typically, cells transform nutrients into energy. However, there is a phenomenon called adaptive or non-shivering thermogenesis, where the cells will produce heat instead of energy in response to a particular diet or environmental conditions.

Levels of nitric oxide production may be a key switch triggering how much heat or energy is produced as cells metabolize nutrients, or how much of the nutrients is stored as fat, Lindqvist said.

“At high levels, nitric oxide may inhibit energy production,” said Durham University’s Andreanna Welch, PhD, first author and a former postdoctoral researcher at UB with Lindqvist. “At more moderate levels, however, it may be more of a tinkering, where nitric oxide is involved in determining whether — and when — energy or heat is produced.”

The research is part of a larger research program devoted to understanding how the polar bear has adapted to the harsh Arctic environment, Lindqvist said.

In 2012, she and colleagues reported sequencing the genomes of multiple brown bears, black bears and polar bears.

In a paper in the Proceedings of the National Academy of Sciences, the team said comparative studies between the DNA of the three species uncovered some distinctive polar bear traits, such as genetic differences that may affect the function of proteins involved in the metabolism of fat — a process that’s very important for insulation.

In the new study, the scientists looked at the mitochondrial and nuclear genomes of 23 polar bears, three brown bears and a black bear.

The research was funded by the University at Buffalo and the National Fish and Wildlife Foundation.

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The origin of recently established red fox populations in the contiguous United States: translocations or natural range expansions?

The origin of recently established red fox populations in the contiguous United States: translocations or natural range expansions?

European Red Fox (Slovenia)


Red foxes (Vulpes vulpes) are native to boreal and western montane portions of North America but their origins are unknown in many lowland areas of the United States. Red foxes were historically absent from much of the East Coast at the time of European settlement and did not become common until the mid-1800s. Some early naturalists described an apparent southward expansion of native foxes that coincided with anthropogenic habitat changes in the region.

Alternatively, red foxes introduced from Europe during Colonial times may have become established in the east and subsequently expanded their range westward. The red fox also was absent historically from most lowland areas of the western United States. Extant populations of red foxes in those areas are considered to have arisen from intentional introductions from the east (and by extension are putatively European), escapes or releases from fur farms, or range expansions by native populations.

To test these hypotheses we compared mitochondrial DNA sequences (cytochrome band D-loop) from 110 individuals from 6 recently established populations to 327 native (primarily historical) individuals from Eurasia, Alaska, Canada, the northeastern United States, and montane areas in the western contiguous United States, and to 38 individuals from fur farms
red fox.

We found no Eurasian haplotypes in North America, but found native haplotypes in recently established populations in the southeastern United States and in parts of the western United States. Red foxes from the southeastern United States were closely related to native populations in eastern Canada and the northeastern United States, suggesting that they originated from natural range expansions, not from translocation of European lineages, as was widely believed prior to this study.

Similarly, recently established populations in the Great Basin and in western Oregon originated primarily from native populations in western montane regions, but also contained a few nonnative North American haplotypes. In contrast, populations in western Washington and southern California contained nonnative, highly admixed stock that clearly resulted from intracontinental translocations. Several common haplotypes in these populations originated in regions where fur-farm stocks originated.

Although European red foxes translocated to the eastern United States during Colonial times may have contributed genetically to extant populations in that region, our findings suggest that most of the matrilineal ancestry of eastern red foxes originated in North America.

Statham, M. J., B. N. Sacks, K. B. Aubry, J. D. Perrine, and S. M. Wisely. 2012. The origin of recently established red fox populations in the United States: translocations or natural range expansions?. Journal of Mammalogy 93(1):52-65.
Access to the original paper in pdf: http://www.fs.fed.us/pnw/pubs/journals/pnw_2012_statham001.pdf