- Susan Thaw (née Haynes) - Department of Biology, University of York, UK
- Maarit Jaarola - Department of Genetics, Lund University, Sweden
- Jeremy B. Searle - Department of Biology, University of York, UK
- Keith M. Dobney - Department of Archaeology, University of Durham, UK .
Some of the major topics of archaeological research over the last 50 years have been related to establishing where people came from and the nature and extent of their contacts with other cultures. These are questions that are still hard for archaeologists to answer with any confidence. There are, however, ways we can now explore these issues. When people traveled, they often transported (deliberately and accidentally) various species of animal. Like people, these animals carried with them (in their teeth and bones) a variety of "signatures" of their place of origin. By studying these signatures, we can begin to establish whether there are any specific relationships/links between populations from different locations. Thus we can use evidence of animal dispersal as a proxy for studying ancient human migration and trade and exchange networks. The Orkney vole is one of the key species which can help archaeologists do just that.
The Orkney vole
The common vole Microtus arvalis (Pallas 1779) is the only vole that occurs on Orkney. Its presence is recorded on eight of the Orkney islands (Burray, Eday, Mainland, Rousay, Sanday, South Ronaldsay, Stronsay and Westray: Corbet & Harris 1991; Berry 2000) but is absent from the rest of the British Isles despite being present in most of Continental Europe. As reviewed by Berry & Rose (1975), some of the earliest descriptions of the voles on Orkney named them as the field vole, Microtus agrestis (Linnaeus 1761), which is the only Microtus species presently found on mainland Britain . In 1904, Millais described the Orkney vole as a separate species, M. orcadensis, notable for its large size (twice that of M. agrestis ) and various other characters. It was not until the 1950's, however, that chromosome studies (Matthey 1951) and breeding experiments (Zimmermann 1959) demonstrated that the Orkney vole was conspecific with M. arvalis and should merely be regarded as a subspecies, Microtus arvalis orcadensis Millais 1904.
There has been much debate concerning the origin of the isolated populations of M. arvalis on Orkney. The traditional explanation is the glacial relict hypothesis that was largely advocated by Hinton (1910) and was based on the assumption that M. arvalis was an early colonist of Britain after the last glaciation but failed to compete successfully with M. agrestis when the latter eventually colonised Britain . It was hypothesised that the voles on Orkney survived as a relict population because the land-bridge connecting Scotland and Orkney had disappeared by the time that M. agrestis reached Northern Scotland . However, there are numerous lines of evidence that argue against this hypothesis based on: ecology, palaeontology, biology and geology.
M. arvalis has a more southerly distribution than M. agrestis (Mitchell-Jones et al . 1999) and, if there were the possibility of natural colonisation, it is more likely the latter species would have reached Orkney first (Berry 2000). Second, as its vernacular name suggests, the common vole M. arvalis is abundant in mainland Europe and in many areas is more numerous than M. agrestis . The two species live sympatrically over much of their distribution range, so it is improbable that M. arvalis would have become extinct on the British mainland as a result of competition with M. agrestis (Berry 2000). Third, if the populations on Orkney really were relict then one would expect to find populations on other offshore islands around Britain . These are not present (Corbet 1961; Berry 2000).
Sutcliffe & Kowalski (1976) make the point that zoologists of the 19th and early 20th century did not appreciate the differences between M. arvalis and M. agrestis and as a consequence the two names should be regarded as synonymous in this early literature. As M. arvalis was believed to have been the first of these Microtus species to reach Britain, fossil remains were usually assumed to be this species, without foundation. These palaeontological data provide no basis for the glacial relict hypothesis.
Biological and geological evidence
Hinton's theory assumes that a land bridge existed between Scotland and Orkney, but both biological and geological evidence lend no support to this view. From a biological perspective, the present absence of ground predators, such as stoats, suggests that a land bridge was not in existence since they are unlikely to have lagged so far behind their prey (Corbet 1961). Geological data indicates that the sea levels in the vicinity of Orkney were much higher at the end of the Pleistocene than now because Scotland was depressed by the huge weight of ice upon it. In addition, the channel between Scotland and Orkney is considered to be too deep ever to have allowed a connection between the two in either late-glacial or post-glacial times (Corbet 1961; Yalden 1982).
Human introduction of voles to Orkney
The most plausible explanation for the presence of M. arvalis on Orkney is as the result of human introduction. Although human introduction is well documented in commensal species such as house mice ( Mus musculus ), voles are non-commensal and less-likely candidates for human introduction. However, there is clear evidence of human introduction for three other vole species in North-west Europe: the field vole ( Microtus agrestis ) to the Outer Hebrides (Corbet 1961), the bank vole ( Clethrionomys glareolus ) to Ireland (Smal & Fairley 1984) and the sibling vole ( Microtus rossiaemeridionalis previously called M. epiroticus and M. subarvalis ) to Svalbard (Fredga et al . 1990). Moreover, the preference of M. arvalis for agricultural habitats, including meadows and crops (Mitchell-Jones et al . 1999; Zejda & Nesvadbová 2000) means that they could be transported by humans in the materials collected for animal fodder (e.g. hay) which could also act as food for the voles.
Having established that there was human introduction of voles onto Orkney, the questions of by whom and when need to be considered. A number of sites of Neolithic date on Orkney (e.g. occupation sites such as Skara Brae [Mainland], Links of Noltland [Westray] and burial monuments such as Quanterness and Maes Howe [Mainland] and a stalled burial cairn on the Holm of Papa Westray) have yielded deposits that contained Microtus remains in varying quantities. While small mammal remains on archaeological sites are often assumed to be of an intrusive nature, radiocarbon dates of 3590 ± 80 BP and 4800 ± 120 BP from two bone samples of voles excavated from the Links of Noltland site (OxA-1080 and OxA-1081: Hedges et al . 1987) indicate that the bones are not intrusive and are probably thus Neolithic in date. It can, therefore, be inferred that M. arvalis arrived with early Neolithic farmers.
While human transportation maybe generally accepted as the means by which M. arvalis reached Orkney, the source area for this colonisation is still uncertain (e.g. Corbet 1961, 1986; Yalden 1982). Only Berry & Rose (1975) have specifically addressed this issue, using the frequency of non-metrical skull variants as phenotypic markers. They concluded that the Orkney samples were most similar to those from Yugoslavia, thus indicating an origin in the Eastern Mediterranean . However, this suggestion has been contested by Corbet (1986) who cites a number of reasons as to why this hypothesis is improbable: First, the measures of difference between Orkney and Yugoslavia are only slightly less than those between Orkney and other samples from continental Europe . Second, it is likely that other, unsampled populations would be more similar to M. arvalis orcadensis . Third, M. arvalis is not found in the Mediterranean coastal lowlands, and montane populations are unlikely sources for accidental transport by boat.
New molecular data
We have recently conducted a phylogeographic study of M. arvalis over its species range (Haynes et al. submitted). This entailed sequencing the whole mitochondrial cytochrome b gene (1140 base pairs) for 41 individuals from 30 localities. In the phylogeographic approach, sequences are used to infer colonisation history; the relationship of sequences is established with a phylogenetic analysis and the geographic distributions of the major phylogenetic groups are mapped (Avise 2000). In our phylogeographic study of M. arvalis we used both parsimony and distance methods to generate phylogenies (Hillis et al. 1996) and in both cases found that the sequences clustered into five distinct phylogenetic groups. These groups had strong statistical support, of greater than 80% by bootstrapping (Haynes et al. submitted). (It is generally taken that 70% bootstrap support is adequate to validate phylogenetic groups: Hillis & Bull 1993.) The groups occupied distinct geographic areas and were named accordingly. Thus, the 'Italian', 'Western', 'Central' and 'Eastern' phylogeographic groups occupied Italy, Western, Central and Eastern Europe, respectively. There was also an ' obscurus group' found in the extreme east and south-east of Europe, which appeared to correspond to the M. arvalis obscurus subspecies that occurs there (Haynes et al. submitted).
In the most part the geographic ranges of each of these five phylogeographic groups is thought to reflect the natural postglacial range expansion of M. arvalis from different glacial refugia (Taberlet et al. 1998; Hewitt 2000). However, the interest here is in the unnatural, human-mediated colonisation of Orkney. The sequences of 9 voles that we examined from 4 localities on Orkney (mainland, Sanday, Westray) belonged to the Western group (Haynes et al. submitted). Thus, it can be inferred that, within the natural range of M. arvalis in continental Europe, the source of the Orkney population must have been in France or Northern Spain, i.e. the areas that we have shown to harbour voles of the Western group. Our studies not only provided a broad source area for M. arvalis orcadensis ; we were also able to show that there was substantial sequence variation among the current population of voles on Orkney (Haynes et al. submitted). For example, among the 9 voles examined on Orkney, there were 20 variable nucleotides of the 1140 screened, comparable to the amount of variation that we detected over whole phylogeographic groups in continental Europe .
The high degree of genetic similarity between the populations of M. arvalis on Orkney and those in South-western Europe strongly supports France or Northern Spain as the source area for the colonisation. In this regard, it is interesting to note that Corbet, as early as 1979, remarked that the most likely source area for M. arvalis orcadensis was in the south, for example the coast of France or Iberia (Corbet 1979: 136). This statement appears to have been made on the basis that modern M. arvalis asturianus (the subspecies found in Spain) is closer in size to the even larger M. arvalis orcadensis - both being larger than M. arvalis from Central Europe. Corbet also showed that fossil remains of M. arvalis orcadensis from all strata at Quanterness were actually larger than their modern day counterparts, and that they appeared to become smaller through time towards the values of modern day specimens. In terms of habitat preference, however, it is more likely that the populations in France would have had greater opportunity for maritime dispersal, because they are more coastal, whereas the populations in Spain are mostly montane (Corbet 1966).
Although several lines of archaeological evidence imply that Neolithic Orkney may have had links with other parts of the Britain, Ireland and Continental Europe, the nature and scale of this contact is still very much debated. Perhaps the most contentious argument that has occurred over the last 40 years has been over the striking similarities in megalithic tomb art and architecture. These chambered tombs and passage graves are found from Shetland to Cornwall, and throughout Ireland; in Southern Scandinavia and Northern Germany; the Netherlands; throughout France; around the Iberian coast; in parts of Italy; and in all major islands of the Western Mediterranean (Fraser 1983; Henshall, 1972). "The chambered cairns of Orkney are thus representative of a type of building which is widespread in Europe." (Fraser opp. cit : 3).
Prior to the so-called "Radio-Carbon Revolution", these similarities were used by some to support 'diffusionist' models (e.g. Childe 1935; Hawkes, 1940). With the advent of "New Archaeology", fast developing and opposing schools of thought proposed the contrary concept of local independent development, and the wealth of Neolithic monuments of the Orkney archipelago were re-interpreted in this light (e.g. Renfrew 1979; 1985). Bradley and Chapman (1984: 348), in their synthesis on passage graves, stated that " ... the study of megalithic tombs is often reduced to a debate between those who believe that such monuments originated in the patterns of colonisation which characterise Neolithic Europe, and other writers who account for the appearance of these monuments in more local circumstances." They highlighted both the similarities and major differences in this megalithic tradition and suggested that early independent traditions of formal burial later gave rise to more complicated social formations, which, in turn, resulted in the flowering of the megalithic tradition.
More specific similarities do exist between the passage graves of the Boyne Valley (Ireland) and Orkney (i.e. similarities in plan, decorated stones, alignment of the orientation of entrance passages of Newgrange and Maeshowe, and special artefacts common to both areas - notably perforated mace heads). However, Renfrew (2000: 17), suggests that there is little evidence of groups of people from Ireland settling in Orkney (or vice versa), although some individuals from Orkney must have visited Ireland and been influential in Orkney on their return. Renfrew (1985: 255) also postulated that the spread of religious ideas (probably based on cosmology) relied on an agency of greater mobility of individuals. Thus, he promoted the idea that the megalithic monuments of Orkney acted as important ritual or religious centres, which (much like in medieval times) stimulated prehistoric pilgrimages from far afield.
More recently the debate has continued, with somewhat of a return to the original views of broader and direct contact involving the movement of ideas and people around the Western Atlantic seaboard. Recent work (Sheridan 2000 and 2003; Tresset 2003) has presented compelling evidence for direct cultural connections during the early Neolithic along the 'Atlantic façade' (e.g. between Iberia, North and Western France, Ireland and Scotland ) that seems likely to have involved the direct movements of people and even animals. According to Kinnes (1984: 370), the Mesolithic communities of the Atlantic zone were "stable economically and closely interconnected over long distances by coastal navigation and the exploitation of inshore and deep-sea resources." Sheridan (2000: 13) cites the existence of long-distance movements of ideas and prestige objects (e.g. jadeite axeheads, variscite beads perhaps from Catalan, pottery styles) around Europe already in the centuries preceding 4000BC and, on that basis, suggests that ".long distance seaborne movement northwards from France to Scotland around 4000BC should not be an unacceptable concept" (Sheridan opp. cit: 13).
The interpretation of the modern molecular data for M. arvalis summarised here, therefore, fits well within a broader framework of postulated cultural contacts and appears to support most recent views of long-distance movements of people along the Western Atlantic seaboard. These data may even eventually point to a specific origin in South-western Europe for the human mediated colonisation of Orkney. Of course, the voles did not necessarily come with the first settlers. They may have arrived (deliberately or accidentally) with goods or with people who had trade or other connections with the early or later Neolithic peoples of Orkney. Bradley and Chapman (1984: 355) suggested that it was only during the later stages of the development of megalithic architecture (i.e. when Maes Howe type passage tombs were in use) that long distance social relationships were formed. Cumulating evidence now indicates that, at this time, long-distance contacts between centres of power, linking Orkney with the Boyne Valley, and the Boyne Valley with Brittany and Spain did indeed exist (Sheridan 2003).
Whatever the exact dates for the arrival of M. arvalis in Orkney, the high modern molecular diversity implies that either large numbers arrived in one event, or that successive introductions occurred. If large numbers of M. arvalis were present in a 'cargo', it is likely that some individuals would have escaped at stop-off points and would have colonised suitable habitats in mainland Britain where they would surely still persist today. Their present day absence from the British mainland may, therefore, suggest that the journey from mainland Europe to the Orkney archipelago was made in a single crossing, indicating a direct long-distance cultural link between Orkney and mainland Europe . How feasible this was, is still very much open to debate, since little is known of the maritime technology of the Neolithic cultures of the Atlantic fringe. Traditionally it has been assumed that short trips were made across the Pentland Firth in skin-boats or on rafts, although dugouts are also known from this time. However, Sheridan (2003) states that "... a long-distance journey from France or Spain to Orkney at some point between 3,300-3000 BC is not inconceivable."
The early settlers of Orkney obviously succeeded in transporting (in boats) larger animals than voles to Orkney (i.e. sheep, pig, cattle, red deer and other humans) and their remains have also been recovered from a number of early sites. The preliminary evidence from the voles certainly seems to support the view that a more mobile (and perhaps more sophisticated than originally believed) maritime culture existed along the Western Atlantic fringe during the Neolithic, which had far reaching cultural and trading links.
One final, albeit controversial, explanation for the large numbers of M. arvalis individuals introduced to Orkney, could be one of deliberate transportation. They may have, for example, been taken as a potential food source, much like Polynesian rats were for long Pacific voyages (Matisoo-Smith et al . 1997), or they may even have had some kind of cultural significance to the people responsible for their introduction. Other small mammals have been associated with possible ritual practices, or have been imbued with specific cultural significance. For example, the hedgehog ( Erinaceus europaeus ) has been found in late Mesolithic and early Neolithic graves on the island of Gotland (Burenhult 2002.). Here there can be no doubt that not only human mediated introduction was responsible for their presence, but also that it was premeditated and for a cultural or symbolic reason.
M. arvalis remains have been found associated with burials (chambered tombs) and occupation sites and it has always been assumed that their presence was accidental. Little zooarchaeological research has been devoted into establishing whether, for example, the remains of Orkney voles show signs consistent with being ingested by predators, or whether there may be another explanation for their presence. For example, evidence of ritual activities has already been suggested by the apparent deliberate deposition of the bones of red deer, dog and sea eagle in some Orkney tombs and other special contexts associated with occupation sites (e.g. Clutton Brock 1979; Fraser 1983; Barber 1988; Davidson and Henshall 1989; Hingley 1996; Jones 1998; Sharples 2000).
A large phylogeographic and zooarchaeological study of M. arvalis now needs to be undertaken in order to enable us to address a range of important questions this initial study has raised .Our current research project (funded by the Wellcome Trust) aims to further explore new ways to help us understand the migration events of our distant prehistoric ancestors. The study focuses upon several species (voles, rats and pigs) in regions where questions about human dispersal events and cultural contacts can be readily explored (i.e. the Eastern Atlantic façade, Baltic and the Pacific). As previously mentioned, the Orkney vole provides an important case study for us and we are currently studying the remains of ancient and modern M. arvalis from Spain France and Orkney. It is hoped that more detailed research into the morphology of their teeth, linked with ancient DNA research, may eventually point to a specific origin for the human mediated colonisation of Orkney, perhaps somewhere in South-western Europe .
A phylogeographic study, involving sampling from several sites in France and Northern Spain, will help to determine more precisely the source area for the colonisation of Orkney by this species. It is also possible to retrieve DNA sequences from archaeological material and this has been demonstrated for another vole species, M. oeconomus (C. Brunhoff et al . unpublished data). DNA sequences from the fossil remains of M. arvalis orcadensis would be a valuable aid in pinpointing the source area for colonisation and the relationship of ancient and modern populations of these voles.
A systematic Accelerated Mass Spectrometry (AMS) dating programme is also required on a range of vole remains in order to address the question of 'when' M. arvalis was introduced to Orkney and whether there is any chronological pattern in the appearance of the species on the islands where it now occurs. Interpretation of the results of such a dating programme would, however, have to take into account the probable inundation of early-mid Holocene sites by rising sea level. It is important that dates are obtained from each of the islands where M. arvalis orcadensis occurs, as, in conjunction with the DNA sequence data, it may be possible to elucidate the sequence of colonisation within the islands.
The significance of M. arvalis orcadensis to the early settlers of Orkney could partially be addressed by performing a systematic survey of the zooarchaeological assemblages where vole remains have been recovered. It will be important to determine the nature of the sites and context types in which remains occur and then establish whether there are similarities or differences within and between different periods. Although it is clear that major differences do exist in terms of the type of sites where vertebrate remains have been recovered (e.g. the majority - but not all - of Orkney Neolithic sites are burial monuments of a kind that later generations did not build), detailed studies of a range of zooarchaeological and archaeological data will provide important clues as to the 'true' status of the Neolithic vole remains. For example, it would be important to ascertain what (if any) other species or particular cultural assemblages are associated with these remains, and whether the material exhibits evidence of cultural modifications or changes consistent with being ingested by birds of prey or other predators.
The Orkney vole is not only of interest in itself, but can, more importantly, tell us something about the humans that introduced it, providing a model for the concept that studies of animal colonisation history can help us explore human behaviour in the past. Davidson and Henshall (1989: 8) stated that "The wealth of evidence now available makes Orkney particularly attractive for statistical and spatial analysis... dealing with the structure of tombs, their relationship with the land, together with other aspects of the Neolithic colonisation of the islands." The Orkney vole ( M. arvalis orcadensis ) provides a novel addition to this 'wealth of evidence,' and a more detailed phylogeographic and zooarchaeological study of both the modern and fossil remains would make an important contribution to the debate about cultural connections, and perhaps even the origins, of the early Neolithic settlers of Orkney.
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Figure 1. Map showing distribution of the common vole ( Microtus arvalis ) (grey shaded area) and the field vole ( Microtus agrestis ) (solid black line) after Corbet & Harris (1991) and Mitchell-Jones et al . (1999). The arrow indicates the populations on the Orkney Islands . The circles represent the sampling localities in the present study, closed circles denote representatives of the Western phylogeographic group, open circles represent all other groups.