The Orkney Vole

Summary of evidence from peer reviewed science for the migration of the Orkney Vole from mainland Europe to Orkney. 

Quoted from:-

THE CHANGING PACE OF INSULAR LIFE: 5000 YEARS OF MICROEVOLUTION IN THE ORKNEY VOLE (MICROTUS ARVALIS ORCADENSIS) by Thomas Cucchi et al

"In this article, our objective is to examine morphological change through time in populations of the Orkney vole, Microtus arvalis orcadensis (Major 1905)—an endemic subspecies of the common vole Microtus arvalis (Pallas 1778)—introduced to the Orkney archipelago (Fig. 1) by Neolithic farmers around 5000 years ago, from a source outside the British Isles (Martínková et al. 2013). (See below)

Large samples of archaeological Orkney voles, as well as good palaeo-environmental records revealing anthropization of the Orkney archipelago (Bunting 1994, 1996), provided an important opportunity to investigate the pace of evolutionary change in this insular rodent over the last 5000 years, within the context of an island environment impacted by humans. Orkney voles have evolved their own particular dental phenotype, likely the result of human agency influencing its evolutionary trajectory in different ways over the last 5000 years. This human influence began with its Neolithic introduction to the Orkney Mainland at a time when there were no terrestrial predators and only one competing species (the wood mouse). The Orkney vole population rapidly diverged from continental European M. arvalis to reach a new ecological optimum, that included evolutionary changes in morphology of the molar teeth. Neolithic farmers then dispersed the species to other islands of the archipelago—from Mainland to Westray and during the Bronze Age to Sanday—generating several founding events contributing to idiosyncratic differences in dental characteristics. This initial divergence and diversification in Orkney voles was not followed by morphological stasis because the Orkney environment was subjected to continued human disturbance.

The case of the Orkney vole presented here demonstrates how, from Neolithic times, humans have played a major role in species evolution and suggests that anthropogenic modifications of the environment may have repeatedly disturbed the phenotypic evolutionary stasis of insular species. Given the continental-scale and increasing intensity of human-induced impact on ecosystems in the last centuries, such changes in the evolutionary trajectories of vertebrates are likely not restricted to insular systems."

Quoted from:-

Divergent evolutionary processes associated with colonization of offshore islands, Natalia Martinkova, et al

"Colonization history of Orkney voles

It is striking that there is substantial cytb variation in M. arvalis in Mainland Orkney and over the whole archipelago (Fig. 3, Table 3). Island populations often show low genetic diversity (Frankham 1997). This can relate to small population sizes and/or population bottlenecks associated with colonization of islands, particularly by sweepstake dispersal or human introduction. The high cytb diversity could indicate that the Orkney population of M. arvalis represents an island relict of a previously continuous mainland population, perhaps dating back to before the LGM (Beirne 1952). This would fit with the long tMRCA for the molecular variation on Orkney, potentially dating back to 25 400 BP (within the 95% CI). However, there are strong arguments against glacial survival of the Orkney vole population. First, the IMa analysis based on microsatellites and cytb and the ABC analysis based on microsatellites provide a date of arrival around 5000 BP, considerably more recent than the LGM. Second, all the other species of small mammals on Orkney are most reasonably viewed as human introductions (Yalden 1982), necessitating a special case for M. arvalis as a glacial survivor. Third, it is very difficult to make this special case given that M. arvalis is not a species currently associated with arctic or even moderately high latitude conditions. Its range extends eastward beyond Lake Baikal and yet barely traverses north of the 60th parallel (Fig. 2; Shenbrot & Krasnov2005). Orkney was under or near a glacial ice sheet at the LGM (Bowen et al. 2002) and M. arvalis is not part of the fossil fauna known from Britain from the last glacial period (Yalden 1999; Currant & Jacobi 2001). Fourth, M. arvalis is not currently found in Britain(Fig. 2). It is therefore contrary to think that M. arvalis should be a glacial relict on Orkney rather than M. agrestis, when the latter occurs further north in Eurasia (beyond the 70th parallel) and is distributed throughout Britain, including on many offshore islands, while M. arvalis only occurs on Orkney. There have been no land connections between mainland Britain and Orkney after conditions ameliorated following the LGM (Yalden 1982), and hence why M. agrestis (and other wide-ranging small mammals in Britain, such as common shrews Sorex araneus and bank voles Myodes glareolus) failed to colonize Orkney. If M. arvalis did no tsurvive on Orkney itself during the last glacial period, the absence of the species in Britain means there are no grounds to suggest sweepstake colonization from there. It is conceivable that there could have been sweepstake colonization of Orkney from Doggerland, the landmass connecting Britain, the Low Countries and Denmarkuntil about 8000 BP (Weninger et al. 2008) – but this would require the survival of small mammals on floating mats of vegetation over a substantial marine gap between Doggerland and Orkney. Thus, human introduction is by far the most likely explanation for the occurrence of M. arvalis on Orkney.

From the IMa and ABC dates, this introduction at about 5000 BP fits well with the earliest radiocarbon dates for archaeological M. arvalis from Neolithic contexts (5100 years old: Table 2) and the beginnings of the Neolithic culture on Orkney (5600 BP: Ritchie 2001; Schultinget al. 2010). Voles could have been brought to Orkney by Mesolithic hunter-gatherers, as early as c. 9000 BP, but no vole remains have been found in the one excavated Mesolithic site on Orkney (Lee & Woodward2009), in contrast to their abundance at Neolithic and later sites (Yalden 1999; Thaw et al. 2004). If, as appears most likely, the voles were introducedby Neolithic settlers about 5000 BP, various other implications flow from our molecular data, which are of considerable archaeological interest. First, the introduction implies long-distance maritime travel by Neolithic people between continental Europe and Orkney, extending on findings from elsewhere (e.g.Broodbank 2006). Our study highlights the Belgian coastline as the most reasonable source of the Orkney voles on the basis of available genetic data. This suggests Neolithic cultural linkages between Belgium and Orkney, of worthwhile focus for future archaeological investigation. Microtus arvalis were not introduced successfully into mainland Britain, which is consistent with relatively direct transport to Orkney from the continental source area. Second, if the introduction occurred about 5000 BP, then, because the tMRCA for the Orkney voles is so long (15 400 years), substantial numbers of femalevoles must have been introduced to explain the cytbvariation observed in modern and archaeological Orkney voles. High genetic diversity is already evident in the 16 aDNA sequences dating to 4200 BP or earlier, separated by up to 10 mutations (Fig. 3 and Table S3,Supporting information) and which produce an estimate for the tMRCA (14 780 years; 95% CI of 4681–36 379 years) similar to that of the full ancient and modern data set. To explain a substantial number of voles arriving accidentally on Orkney implies transport of plentiful grass livestock bedding/fodder in which the vole stowaways could have survived. This in turn may suggest the direct movement of livestock as part of the proposed Neolithic linkage between Belgium and Orkney. Alternatively, deliberate transport of voles onto Orkney could explain the large numbers introduced (Thaw et al. 2004). It is conceivable that voles were taken as food items, pets or for cultural/religious purposes – M. arvalis is docile in captivity (Berry 2000), so could theoretically have been ‘tamed’. This suggestion of deliberate transportation of small rodents has a precedent: it has been argued that Pacific rats (Rattusexulans), now present on islands throughout Oceania, were intentionally conveyed by Polynesians as a foodsource (Matisoo-Smith & Robins 2004). Evolutionary processes affecting voles on Orkney and the continental source area Compared with continental European M. arvalis, those on Orkney and another offshore island (Guernsey) are divergent in terms of tooth morphology, including increased tooth size. For Orkney, this divergence may have occurred over c. 5000 years, if introduced during the Neolithic. In addition to having larger teeth, the Orkney and Guernsey M. arvalis have a larger body size than continental voles (Gorman & Reynolds 2008). Quick-evolving rodent gigantism has been described previously on islands of the northeast Atlantic (Corbet1961; Angerbjorn 1986), but not within such a precisely defined time frame. A range of selective factors have been proposed to explain this gigantism, including an absence of small mammalian predators (Lomolino1985), and a genetic basis for gigantism has been identified in island house mice (Chan et al. 2012). Elsewhere,we further explore the dynamics of morphological evolution for the Orkney M. arvalis using archaeological specimens (T. Cucchi, R. Barnett, N. Martınkova, et al.submitted) extending substantially on previous studies (Berry & Rose 1975; Corbet 1986). Despite their similarity in large tooth and body size,Guernsey and Orkney voles exhibit distinctive mtDNA haplotypes (Fig. 3). It is therefore most reasonable to consider that the Guernsey and Orkney voles attained their large body size independently. It is not clear whetherGuernsey was colonized naturally before it became an island, as part of the continental European late glacial/postglacial species expansion (Haynes et al. 2003; Heckelet al. 2005; Tougard et al. 2008), or whether the voles were introduced by people after it became an island (Gorman& Reynolds 2008). However, given that Guernsey voles are likely to come from the same general (northernFrance/Belgium) source area as the Orkney voles and that they are also different in mtDNA from current northern France/Belgium populations, there would be much interest in further detailed comparison of Orkney, Guernsey and northern France/Belgium voles. In addition to the operation of selection in the evolution of Orkney voles suggested by morphology, stochastic processes appear to have been important based on microsatellites. The population on the largest island, Mainland Orkney, has retained much of the microsatellite variation found in continental Europe, while all theother Orkney Islands (which are considerably smaller:Fig. 1) show very low levels of microsatellite variation,consistent with founder events and genetic drift. Similar stochastic processes can also explain microsatellite variation among Scottish Island populations of common shrew (White & Searle 2007a). Our findings with regard to morphology and microsatellites in M. arvalis are unsurprising in comparison with previous studies on island small mammals, but the results from our mtDNA analyses are more unexpected. Although the cytb sequences from Orkney andthe proposed source area for the Orkney colonization both belong to the Western-North lineage of M. arvalis, the sequences are remarkably divergent given the timeframe for colonization. Also, it might have been expected that (as for the microsatellites) variability would have been lower on Orkney than in continental Europe. In fact, the opposite is the case. Taking either the principal island (Mainland Orkney) or the whole archipelago, mtDNA diversity is higher in Orkney than in coastal France/Belgium (Table 3). Our dating analysis also shows that the mtDNA sequences in coastalFrance/Belgium have a much more recent derivation than the Orkney sequences. So, here we are seeing another facet of evolution in association with the colonization of offshore islands, in this case occurring in the mainland population. Thepresence of derived sequences in coastal France/Belgium suggests a replacement event in M. arvalis, with one mtDNA type (the current type) replacing another(the Orkney type), similar to aDNA findings in other species (Barnes et al. 2002; Pergams et al. 2003; Hofreiteret al. 2007). The fact that there is an affiliation between coastal Belgium and Orkney on the basis of microsatellite genotypes argues against a complete population replacement (e.g. by extinction–recolonization) as an explanation for the mtDNA result. Instead, within-population processes of selective sweeps or genetic drift are implicated, and more likely expressed in the mtDNA data, as a single locus with small effective population size than in the microsatellite data. We cannot be surewhat environmental factors promoted the replacement. There could, for instance, have been a local, unrecorded disease outbreak. However, it is notable that the replacement occurred over a period when M. arvalis populations would have changed dramatically due to human land-use change, and this appears the most likely driver of the replacement. Over several thousand years, sustained forest clearance in continental Europe (Rackham 1998; Cyprien et al. 2004) created new agricultural habitats and associated selection pressures that essentially expanded the opportunities for M. arvalis as a species that particularly exploits managed grassland(Niethammer & Krapp 1982). In such a habitat, M. arvalis populations can undergo massive population expansions and crashes (Delattre et al. 1992) that reduce longterm effective population size, promoting genetic change through drift. On Orkney (which saw the rapid decline of low shrubs and tree species with the arrival of Neolithic farmers: Bunting 1996), M. arvalis utilizes a range of open habitats and does not show the same dramatic population fluctuations as seen in parts of continental Europe (Gorman & Reynolds 2008). 

Offshore islands as field laboratories 

There has been a tendency to view offshore island populations of small mammals (and other organisms with low density and low dispersal) as genetic deviants fromthe ‘norm’. This is because studies of various species have shown results similar to ours for morphology(substantial change) and microsatellites (loss of variation) (Lomolino 1985; Frankham 1997; Boessenkool et al.2007; Millien 2011). These have included detailed studies on small mammals such as wood mouse Apodemussylvaticus (Angerbjorn 1986; Michaux et al. 1996),masked shrew Sorex cinereus (Stewart & Baker 1992)and common shrew (White & Searle 2007a,b, 2008).However, as we have demonstrated with our M. arvalis mtDNA studies, island populations can also represent genetic ‘arks’, retaining the ancestral genetic variation,while evolutionary and other processes on the mainland may lead to a loss of that ancestral variation. Islands may have importance therefore in conservation of genetic variation. A further example involving human introduction of a small mammal onto an offshore island is provided by the Eurasian red squirrel Sciurus vulgaris. Thus, Irish red squirrels have genetic variants that apparently derive by introduction from Britain, but these are now absent in that source population (Finnegan et al. 2008; Searle 2008). For low density and low dispersal organisms such as small mammals, we suggest that genetic surveys of mainland areas should, where available, include populations from neighbouring offshore islands. It is very likely that those island populations will provide a new perspective on the temporal and spatial dynamics of the genetic variation in that region.The ‘ark’ concept that we discuss here is of course more general. Populations colonizing new areas will take the genetic and nongenetic characteristics of the source population, and some of those characteristics may subsequently be lost in the source population but retained in the population in the new area. In this way, for instance,the United States is a ‘linguistic ark’ for various English words that would have been common in the British Isles at the time of settlement of North America by the British, but which have subsequently fallen into disuse in the homeland (e.g. ‘fall’ meaning ‘autumn’). Returning to genetic characteristics of offshore islands, in addition to their potential as genetic ‘arks’, they also hold potential as field laboratories to study genetic change in the islands themselves. Compared with the classic evolutionary studies on oceanic islands, those based on offshore islands will tend to view events over shorter timescales and thus provide a different perspective on evolutionary processes. Offshore islands are particularly valuable for studying initial stages of diversification, with the opportunity (as in the current study together with T. Cucchi, R. Barnett, N. Martınkova,et al. submitted) to follow island populations from their foundation to the present day using advanced genetic and morphometric tools as applied to modern and ancient populations of different ages. Extremely accurate dating of ancient populations may be possible (e.g. in archaeological settings). With this short time duration and close proximity to the mainland, there is also a greater chance to find the precise source area for the island colonization, which allows interesting comparison of evolutionary processes on the mainland and island. This brings us back to the value of offshore island populations in interpreting mainland processes. Offshore islands are an underutilized resource for evolutionary analysis, with great potential. In some ways, they represent study systems intermediate between those in a continental setting and those on oceanic islands; they have the simplicity of the oceanic island system yet are clearly relevant to continental situations."

I offer here full quotes from two authors. Thomas Cucchi, and Natalia Martinkova.

I present them , as they are sources for persistent beliefs that people used boats before 2500BC 

"all the other species of small mammals on Orkney are most reasonably viewed as human introductions (Yalden 1982), necessitating a special case for M. arvalis as a glacial survivor."

I have not been able to access Yaldens work, but I think that the human introduction of Aurochs, Red Deer, Wild Pig, Wolf, and Pine Marten need careful explaining. Sheep were introduced by humans, helped by dogs.

Martinkova gives a clear range of natural possibilities for the appearance of the vole in Orkney, but discounts them in favour of human introduction. 

My work in "Orkney Riddle " gives an alternative route for the natural introduction of the Orkney Vole to Orkney, giving actual evidence that a route for their migration existed.

There is no evidence for seaworthy boat's before 2200BC,  the earliest vessel is a dugout canoe currently resting in Strangford Loch in Northern Ireland,  dated 3499BC. 

It can be distinctly unsettling crossing the Pentland Firth in a modern ro-ro ferry. Trying to make Neolithic people into mariners blurs their huge actual achievements. 

Orkney Riddle blog:-

http://orkneyriddler.blogspot.com/2025/04/the-orkney-riddle.html