Orkney Riddle
The Orkney Vole
"Orkney vole
Microtus arvalis orcadensis
The Orkney vole is a sub-species of the common vole and unique to the Orkney Islands. It is larger and heavier than the common vole. It is bigger than a mouse, with a rounded head and a relatively short tail.
Behaviour
Orkney voles do not hibernate. The vole is the islands’ only rodent to be active during the day and is an important food source for hen harriers and short-eared owls. The diet of the Orkney vole consists largely of plant materials, seeds and cereal crops. Females can have several litters a year, each with 3-6 young. The young are fed by the mother for about 20 days, when they then switch from milk to normal feeding.
Size
Length: 9 – 12cm (tail length approximately one third of body length)
Average weight: 30g; up to 90g
Lifespan: 18 months
Status
Although once common, it is thought that the Orkney vole population has declined substantially over the past 50 years. This is linked to changes in agriculture, with the drainage of rough grassland and conversion to improved grasslands for livestock. More recently, their lives are threatened by stoats, which were first sighted in Orkney in 2010.
Distribution
Orkney voles are present on eight of the Orkney Islands. Formerly widespread, even in agricultural crops, Orkney voles are today confined to semi-natural habitats with maximum densities occurring in old peat cuttings and rough grassland.
When to see
January to December. It is active throughout the year, but is not so apparent in winter when it spends more time underground.
Facts
The Orkney vole has an enigmatic history. Its closest relative is the common vole which is found throughout Europe, but intriguingly not on the UK mainland or Ireland. Following numerous studies and theories, it required cutting edge DNA techniques to throw a definitive light on its origin. The most recent analysis of nuclear DNA suggests the closest match is with an archaeological specimen in Belgium.
The vole’s mode of arrival to Orkney is hotly debated. Genetic research has revealed that there is a large genetic diversity in both modern and ancient Orkney voles. This suggests that they arrived with humans, (deliberately or as unintended stowaways) around the Neolithic period when the first farmers settled on Orkney. Copious vole remains have been found in excavations of tombs and houses from that time.
The oldest known radiocarbon-dated fossil of the species in Orkney is 4,600 years old.
Locally, Orkney voles are also known as volo, cutticks, or cutoo." (Scottish Wildlife Trust)
"Much debate surrounds the origins of the Orkney vole Microtus arvalis orcadensis (Yalden 1999; Corbet 1961). It is the only vole on Orkney and is found on eight islands, while in mainland Britain the field vole (M. agrestis) is the only Microtus species. M. arvalis and M. agrestis occur widely across continental Europe with overlapping distributions (Mitchell-Jones et al 2003). It is therefore clear that M. arvalis did not colonise Orkney naturally (Haynes et al 2003; Haynes et al 2004), and although modern consensus supports a human introduction during the Neolithic (Hedges et al 1987), their geographic origin and mechanisms of introduction still remain uncertain."(Thomas Cucchi et al)
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."
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 not survive 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; Schulting et 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."
Recent research suggests that s population of European Vole origin survived in south-west england during the final deglaciation of Britain, about 11,000BP,. The report on this research is entitled "Diverse responses of common vole (Microtus arvalis) populations to Late Glacial and Early Holocene climate changes e Evidence from ancient DNA" by Mateusz Baca et al
"Two radiocarbon dated specimens from Trou Al’Wesse (Belgium) yielded a pre-LGM age, 30,000BP to 28000BP. They belonged to the Western/Northern lineage although they diverged earlier than the coalescence of the extant populations. This suggests that population continuity existed in the region throughout the last 30 ka rather than there being a turnover, even if the population decreased in numbers as a result of the LGM. Although the genetic evidence is still very limited this is concordant with the cryptic northern refugium hypothesis (Stewart and Lister, 2001). The other interesting case is the single specimen from Bridged Pot cave dated to the Early Holocene. At present there are no common voles in the British Isles with the exception of the Orkney Isles, where they have been introduced by humans ca. 5000 years ago (Martínkova et al., 2013 ). Microtus arvalis has not been considered to be part of the British fauna of the Last Glacial and only the field vole has been identified in the fossil record (Coard and Chamberlain, 1999; Currant and Jacobi, 2001) although there are problems distinguishing these two species based on dental characters (Navarro et al., 2018).
Recently, a number of common vole remains were identified using collagen fingerprinting from Pin Hole cave (Creswell Crags, UK) (Buckley et al., 2018). Although the deposits Pin Hole have been shown to be mixed (Stewart and Jacobi, 2015), this suggests that the common vole was present in the British Isles during the Late Pleistocene and/or early Holocene. The phylogenetic position of the individual from Bridged Pot cave suggests that it was a part of the continuous population of mainland Europe. Given the age of the specimen, 11,000BP, possible scenario explaining its presence in the British Isles may be that the common vole may have expanded to Britain during the Late Glacial warming and their local extinction was then caused by either YD cooling or the Holocene reforestation. The disappearance of the connection between the British Isles and continental Europe during the Early Holocene may have prevented the species’ subsequent recolonization. In other small mammals inhabiting Britain today such as the field vole, bank vole (Clethrionomys glareolus), water vole (Arvicola amphibius) and pygmy shrew (Sorex minutus) the populations of these taxa, which colonized Britain during Late Glacial, retreated to fringes of the island possibly as a result of YD cooling (Brace et al., 2016; Searle et al., 2009). Subsequent populations of all these species were able to recolonize Britain at the onset of the Holocene. This was not the case for the common vole. Recently Martínkova et al. (2013) showed that recolonization of the northern areas of France and Belgium by the common vole was relatively late and started not earlier than 2000 years ago, long after the disappearance of land connecting the British Isles and mainland Europe"
Radiocarbon dating of vole at Bridget Pot Cave , UGAMS 43,314 MI1371 11,125BP
Fig. 2. Distribution of mtDNA diversity of common vole in Europe.
A modern,
B Holocene,
C post-LGM.
Colour correspond to mtDNA lineages
pink, Eastern;
yellow, Central;
orange, Italian;
green, Western-North;
violet, Western-South;
navy blue, Balkan.
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All views and opinions expressed are my own, but it remains a work-in-progress for which positive criticism and comment is welcomed.
Jeffery Nicholls
South Ronaldsay
Orkney
The
Orkney Riddle (index here) is a series of blogs promoting the notion that Neolithic people walked from Scotland to Orkney to build the Ring of Brodgar.
#Orkney #archaeology #Neolithic #prehistory #British #SkaraBrae #NessofBrodgar #BIIS ##Britice-Chrono
