Red Deer Genetic Diversity & Hybridization

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The British Isles is home to the largest population of wild Red deer to be found anywhere in Europe. The majority of these 300,000 or so animals live on the Scottish mainland, where the species has been a constant feature of the landscape since the end of the last glaciation, 11,000 years ago.

Stock Improvements

A Red deer (Cervus elaphus) bolving at Woburn Abbey in Bedfordshire. English park stags were a source of deer introduced into Highland areas of Scotland in a bid to "improve the quality" (i.e. antler size) of the resident population. - Credit: Marc Baldwin

The Red deer has long been viewed as a “beast of chase” and hunted by commoners, aristocracy and royalty alike. Consequently, there have been introductions to wild populations, both to increase numbers in the face of substantial poaching and in often misguided attempts to increase antler (trophy) size; much during the 19th Century. The sources of these introduced animals have varied, but includes animals from continental Europe and previously wild animals trapped in deer parks as well as North American wapiti (Cervus elaphus canadensis) introduced to the central Highlands. Certainly, the Victorians were well known to have boxed up park stags from southern England before sending them, by train, to Scotland to improve the quality of Highland stags.

There don’t appear to be any reliable accounts of direct introductions of Red deer from Europe into the wild, but many did end up in deer parks, stock from which were subsequently translocated elsewhere. Hence, it is not difficult to see how animals of European descent could be found in our wild populations and most British stocks are now expected to be allocthonous. In other words, they’re not native to the area but reintroduced from elsewhere. The Red deer on Rum are a case in point.

Genetic data presented in a 2006 paper to the journal Heredity by biologists at the University of Edinburgh indicate that the population on Rum is descended from at least two geographically separate ancestral stocks – one that’s closely related to Mediterranean and North African animals, and another from mainland European stocks.

The introductions and mixing of subspecies is, of course, not necessarily a problem because it can add much needed genetic diversity to a population and bolstering numbers can help secure the future of heavily hunted or stressed populations. That said, it can also have some less desirable consequences. Recently concern was raised regarding the introduction of Fallow deer hybrids to Richmond Park, for example. These deer, which were apparently bred in Germany, have larger antler sets than the existing deer and this leads to fighting of mismatched individuals, resulting in a higher injury rate among the deer.

Hybridisation with Sika deer

While wapiti are poorly adapted to Britain, with lower reproductive success when Reds are present (the latter being more aggressive during the rut), and their introduction seems to have had a negligable impact on Scotland's Red deer genome, other imports have thrived. Since the 1960s, evolutionary biologists have been investigating an idea that mutations (i.e. changes to genes) on strands of DNA can build up at a reliable rate. This concept is known as a molecular clock and means that, by comparing the same DNA segments or proteins of two different species, you can get an idea of how recently they diverged (split) from each other. The idea is controversial, but has a large following. Several studies published during the 2000s took fossil data and calibrated it using molecular clock estimates, concluding that Red and Sika (Cervus nippon) deer diverged between 5.2 and 7 million years ago.

Sika deer originate from Japan, but a small group were introduced to Powerscourt in County Wicklow, southeast Ireland during 1860. According to Derek Yalden in his The History of British Mammals, this introduction was so successful that Sika stags were transferred to many sites in Britain. Yalden lists several introductions of Sika to Britain, including 11 animals to the Carradale Estate on the Kintyre Peninsula on the west coast of Scotland during 1893, and several in Inverness (Scotland) during 1900. There were also several cases where park Sika escaped into the wild (e.g. from Beaulieu in the New Forest during 1904), where populations were introduced to islands but swam to the mainland (e.g. from Brownsea Island in Poole Basin during 1896), and where the deer were deliberately released to provide hunts with quarry. These releases, intentional or otherwise, were initially fairly contained, with populations remaining local until the early 1970s, when the maturing conifer forests and plantations allowed a dramatic range expansion. We also know now that Sika stags range considerable distances away from the hinds, increasing their colonisation potential.

Japanese Sika deer (Cervus nippon) in the New Forest, Hampshire. Introduced to Britain around 1893, this species is considered invasive and is known to interbreed with Red deer in parts of its range, producing Red-Sika hybrids. - Credit: Marc Baldwin

Despite being separate species, and this is one of the issues that muddies the waters when trying to define what a species actually is, Red and Sika deer are sufficiently closely related that they can inter-breed and produce fertile calves. In other words, they can hybridize to produce offspring that are genetically part Sika, part Red deer.

In his 1897 British Deer and their Horns, John Guille Millais presented a drawing of a skull suggested to be a Red-Sika hybrid from County Wicklow in Ireland dated 1894, although it's unclear whether this was actually a hybrid. Kenneth Whitehead published a photo of a hybrid stag killed in the Lune Valley in Lancashire during 1964 in The Deer of Great Britain and Ireland and, in a brief paper to Deer in 1967, Peter Delap described an apparent “Manchurian/Red half-breed” deer during the ruts of 1965 and 1966 in the Lake District. In his 2002 book, Yalden notes that Red-Sika hybrids were reported as early as 1940, in the southern Lake District, and that it has been known for some time that the deer living in the Wicklow Mountains of Ireland are neither entirely Red, nor entirely Sika; they’re a completely hybridized population. Several fascinating studies have been conducted by Edinburgh University molecular biologists Helen Senn and Josephine Pemberton looking at the phenomenon of Red-Sika hybridization in Britain.

In general, the biologists have found that rates of hybridization are low, but there were local exceptions. A study of 735 deer from 20 sites across Scotland, published in the journal Molecular Ecology in 2009, found that only about 7% were hybrids. Nonetheless, in one particular population at West Loch Awe in central Scotland, 43% of the deer were hybrids, as were just over half those on South Kintyre. Over the UK as a whole, the biologists suggest that, where the two species meet, hybrids probably only account for 0.01% to 0.02%. Elsewhere, reasonably high levels of introgression have been found in north and west England, while Bournemouth University ecologist Anita Diaz and colleagues found very low levels of Sika introgression into New Forest Red deer, although interestingly from Purbeck rather than New Forest Sika. Perhaps more interesting was that the majority of the hybrids couldn’t be identified as such by the rangers employed to collect the tissue samples.

A Japanese Sika (Cervus nippon) with a possible Red-Sika hybrid (behind) in the Scottish Highlands. It can be difficult to identify hybrids, particularly without seeing the hock glands and rump patch, but this individual is facially similar to a Sika. - Credit: Ken McInnes

In many cases, including Red-Sika hybrids in captivity, hybrids display characters intermediate of the two parent species; that is, they look like a cross between a Red deer and a Sika and may have a rutting call intermediate between the two species. Nonetheless, there is considerable variation within the Red population that can manifest as white leg patches, prolonged spotting to the coat and a brow, while the animals remain Red. Characteristics may also be much more subtle in first generation hybrids, possibly varing in only a single feature. One Scottish stag shot in mid-October 2020, for example, showed all the characteristics of Red deer except for a shorter nose; just over 3cm (1.25 in.) shorter than the others taken from the same location. A fascinating study by Zhipeng Li and colleagues, published in Environmental Microbiology Reports during 2016, suggests hybrids even have a rumen microbiota and amino acid metabolism intermediate between Red and Sika. In this case, however, it seems that there may be a domination of one particular phenotype (appearance), which would make it very difficult—if not impossible—to selectively cull the hybrids out of the population.

The reason why deer hybridise is unknown, particularly given the difference in size, appearance and rutting call. It is curious, too, that few deer parks experience problems with Red-Sika hybridisation, despite establishments such as Woburn Abbey having some 200 of each species free-range in the park. Some stalkers have suggested to me that hybridisation is most likely to be found on the perimeter of each species’ range, particularly in areas where the population is stressed. Senn and Pemberton suggest that hybridization is probably a sporadic event, mostly between Red deer hinds and Sika stags, implying events may be based around individual wide-ranging Sika stags.

Interestingly, a series of studies by University of Sussex biologist Megan Wyman and colleagues have shown that Red deer hinds tend to prefer the calls of Red stags over Sika stags during the rut, although this is not the case for every hind. Sika hinds, by contrast, weren’t fussy and were equally attracted to the rutting calls of Red and Sika stags. In their most recent playback experiment, the results of which were published in Evolutionary Biology in 2016, Wyman and her co-workers found that neither Red nor Sika hinds showed a preference for the pure-bred over hybrid calls. This suggests that the females may not be able to easily distinguish between the calls of a pure-bred stag and a hybrid, increasing the potential for further introgression.

Two questions often asked about the Red-Sika hybridization problem are along the lines of: “Does it really matter if the two species mix?” and “What can stop it happening?”’ These are complicated topics, but I shall touch on them here.

So what?

A mature Red deer (Cervus elaphus) stag in the New Forest after the annual rut. - Credit: Marc Baldwin

There are proven biological, ecological and even psychological arguments for preserving biodiversity; literally the “diversity of life”. Red and Sika deer fill slightly different ecological niches and we have no idea what impact losing Red might have on our woodland ecology. As the old adage goes, however, “money makes the world go around” and there are also financial incentives for preserving the Red deer as a species.

From a purely economic perspective, many communities depend on Red deer as a source of tourism. Photographers flock to places such as the New Forest, Exmoor, Richmond Park and Woburn every year to watch and photograph the rut, bringing business to local hotels and restaurants. There is also a stalking industry in England, with people paying to be taken stalking for Red deer. The stalking industry is, however, perhaps more important in Scotland and, on their website, Scottish Natural Heritage point out that “Red deer are managed as a sporting resource on many Scottish estates”.

A big draw for hunters are so-called “trophy stags”; Red stags over about five years old who sport impressive antlers. One of the features we see in hybrids is animals of Red deer proportions, but with Sika-style antlers. So, will hunters pay £350 (plus travel, accommodation and meal costs) to shoot a smaller deer with a less impressive set of antlers? Invariably some will, but demand may well be lower. Furthermore, given how introgression (genetic mixing) between the two species appears to lead to a blending of traits, we may end up with a hybrid that’s less capable of surviving in the harsh landscape of the Scottish mountains, which is akin to what we’re seeing with the Scottish wildcat (Felis silvestris grampia). We could end up with a “mongrel of the glen” as Josephine Pemberton put it in an interview with the BBC News website in December 2009. As a worse case, this could mean we lose Red, Sika and hybrids from the Highlands altogether.

Prevention better than cure?

Generally-speaking, the best way to prevent the hybridization of two species is to keep them apart. In the natural world, this separation can take many forms but can be roughly divided into physical barriers or biological barriers.

Physical barriers include a variety of features, including mountain ranges, roads, railway lines, lakes and rivers. Biological barriers, by contrast, include obstructions. Different numbers of chromosomes, which usually leaves hybrids sterile and prevents them from reproducing, is one such biological barrier. Additionally, physiological adaptations that make mating difficult or impossible, differences in behaviour that mean the two avoid one another, and differences in habitat use that prevents them from coming into contact all serve to obstruct hybridisation.

We know there don’t appear to be many biological barriers to hybridisation between these species, although there are also cases, such as in parts of Argyll in Scotland, where the two species use the same habitat in different ways and thus rarely encounter each other. Unfortunately, introductions have placed Sika where they probably couldn’t have spread to on their own, thereby eliminating some of the physical barriers that may naturally prevented them hybridising. Ultimately, without human intervention, Red-Sika hybridization would’ve been impossible in the UK. Arguably, it may be impossible to prevent hybridisation going forward without heavily reducing or even extirpating them altogether.

The Beaulieu Railway line that cuts through the New Forest in Hampshire also serves as a management tool for Red and Sika deer on Crown Lands. Beat keepers try and keep Reds on the west side of the railway line (on the right in this photo) and Sika to the east. Although the deer can cross the tracks, any found in the other's area are culled. - Credit: Marc Baldwin

There are cases where barriers or intensive management appear to be effective at keeping the species apart. In the New Forest, for example, the beat keepers aim to keep the Reds on the Forest to the north/west of the railway line and Sika in the Forest to the south/east. (Whether it's north/south or east/west depends from where in the Forest the line is viewed.) Either species found in the range of the other is shot. It is not fool proof, and there are reports of Sika stags spending the summer on grazing pastures with Reds, but generally it seems to have been fairly successful at preventing interbreeding, with very few hybrids reported by the keepers. Nonetheless, many conservationists are concerned about the potential ‘merger’ and, in his 2002 book, Derek Yalden laments:

The saddest change seems likely to be the total loss of the red deer through introgression with sika. … Conserving at least some native genotypes on the Scottish islands, safe from sika, seems essential.”

Yalden is not alone in his concerns and, in a bid to protect the ‘Red deer genotype’ (i.e. to conserve pure-blood Red deer), Schedule 9 (Part 1) of the Wildlife and Countryside Act (1981) makes it illegal to release any Cervus species onto the Scottish islands of Rum, Islay, Jura, Arran and the Outer Hebrides. Hopefully it is not too late to halt the loss of what Archibald Thorburn called “unquestionably the grandest wild animal we now possess in the British Islands”.