Gray wolves survived Ice Age extinction event by changing their diets – which may help them cope with ongoing climate change

Zoe Landry @zoey_landry

Master’s student in Biology at Carleton University, soon to be PhD student at the University of Ottawa and member of the SAiVE Lab

Climate change is arguably one of the biggest issues that the Earth is currently facing. Ecosystems all over the world are being affected in different ways, with northern ecosystems being some of the most severely impacted. The Canadian Arctic is currently warming at nearly double the rate of the global average, which is drastically altering the Arctic environment. The effects of climate change will be felt long past our lifetimes, but how exactly climate change will impact northern species, some of which are not found anywhere else in the world, is unknown.

            An unparalleled tool for investigating the long-term responses of species to climate change is, fortunately, available in the form of the fossil record. In the Yukon Territory of Canada, there is a diverse array of fossils from the last Ice Age that provide an excellent opportunity to study the effects of climate change on ancient animals. The end of the Ice Age is characterized by a global shift in climate, with warming temperatures and melting glaciers, culminating in an extinction event where about 70% of large mammal genera died out in North America – similar to what we are seeing in the Canadian Arctic today. One of the most abundant fossil animals found in the Yukon is the gray wolf (Canis lupus), which to this day still calls the Yukon home. Ancient wolves managed to survive the extinction event at the end of the Ice Age, but how did they do so, when so many other species did not? Could wolves have changed aspects of their ecology, such as their diets, over time to cope with environmental changes?

Zoe Landry, lead author of the study, holds a 40,000-year-old gray wolf skull from the Yukon. Credit: Danielle Fraser, © Canadian Museum of Nature.

My colleagues and I set out to investigate these questions, using a combination of two different methods: dental microwear and stable isotope analyses in a recent paper. We studied both Ice Age and recent (circa 1960s) gray wolf specimens from the Canadian Museum of Nature, in Ottawa, Ontario, and the Beringia Interpretive Center (Yukon Government), in Whitehorse, Yukon. By comparing the dietary ecology of modern and ancient wolves, we can better understand how wolves responded to past climatic disturbances, and how they may respond at present and in the future.

Zoe Landry and Danielle Fraser prepare for sampling.

Dental microwear analysis

            Dental microwear analysis involves observing microscopic wear patterns on the surface of animal teeth. Through observations of these patterns, we can make inferences about the types of food an animal ate during the last few days to weeks prior to its life. There are two main types of dental wear: pits and scratches. Pits are small, circular features that are caused by an animal consuming hard food – in the case of carnivores, pits indicate that the animal was eating bones. Scratches are long, linear features that are generated when the teeth of carnivores slide against each other to slice meat from prey.

Our dental microwear analysis revealed very similar wear patterns between both the Ice Age wolves and the recent wolves, which indicated that both groups primarily consumed flesh. This is especially interesting, as previous studies had suggested that during the Ice Age, wolves were scavengers that fed on the remains of animals that other carnivores had killed. We can conclude that gray wolves have maintained their status as competitive hunters over thousands of years, and that actively taking down prey is an important aspect of wolf ecology that is unlikely to change even as their habitat does.

Top: Taking moulds of teeth; Bottom: SEM of tooth surface. Credit: Danielle Fraser

Stable isotope analysis

Stable isotope analyses were performed at the University of California, Merced, by Dr. Sora Kim and Dr. Robin B. Trayler. We analyzed the δ13C and δ15N values from both bone carbonates and collagen. There were minimal changes in the δ13C values between the two groups, which suggest that wolves have remained generalist predators that hunted a variety of large herbivores. There was a very clear difference in δ15N values between the Ice Age and recent wolves, which we determined to be driven by a change in the main prey species that the wolves were hunting.

Zoe sampling material for stable isotope analysis. Credit: Danielle Fraser

Based on diet modelling, we discovered that the primary prey of wolves during the Ice Age were horses (Equus sp.), with moderate contributions from other herbivores, such as caribou (Rangifer tarandus) and muskox (Ovibos moschatus). Obviously, there are not huge herds of wild horses in the Yukon today, as they were one of the species that went extinct in North America at the end of the Ice Age. Instead, gray wolves that inhabit the modern Yukon rely mainly on hunting caribou and moose (Alces alces) to fulfill their dietary needs. This shift from a diet composed largely of horse to one of caribou and moose indicate that the survival of wolves is not fixed on a single prey species. Rather, gray wolves are able to shift their diets from one species to another, and are capable of surviving significant climatic and environmental changes, provided that they have access to a sufficiently large population of large-bodied herbivores.

Implications for wolf conservation

            Our study revealed that gray wolves from the Yukon were able to survive the threat of extinction at the end of the Ice Age by changing their diets to target different prey species, and are very adaptable predators. The key to modern wolf survival is the survival of their current prey species, caribou and moose. Conservation efforts should focus on preserving these large herbivores, and ensuring that their populations remain healthy and at stable sizes as the Arctic environment changes. The continued survival of grey wolves in the Yukon is directly linked to these animals, which further demonstrates the important role of caribou and moose in the Canadian Arctic.

We acknowledge that the institutions at which this research took place are located on the traditional and unceded territories of the Algonquin, Anishnaabeg People, the Yokuts and Miwuk People, the Taa’an Kwächän and Kwanlin Dün People. We are grateful to the placer gold mining community and the Tr’ond¨ek Hw¨ech’in First Nation for their continued support and partnership with our research in the Klondike goldfields region; and the Vuntut Gwitchin First Nation for their collaboration with research in the Old Crow region. This research was funded by: Canadian Museum of Nature Research Activity Grant, and Natural Sciences and Engineering Research Council of Canada Discovery Grant (RGPIN-2018-05305), awarded to my supervisor, Dr. Danielle Fraser; and by funds of the Yukon Palaeontology Program of the Yukon Department of Tourism and Culture, contributed by Dr. Grant Zazula.

Seaweed-eating sheep and crop fertilisation trials on the Orkney Islands, Scotland

Magdalena Blanz, Vienna Institute for Archaeological Science (VIAS)

Seaweed consumption

In modern Europe, for most the idea of eating seaweed may seem strange. Outside of sushi, and memories of beach holidays where rotting algae spoiled the fun, most people today have no particular connection to seaweed. It may therefore come as a surprise that a fellow mammal, namely sheep (Ovis aries), can subsist nearly exclusively on seaweed. The most well-known breed of seaweed-eating sheep today are located on North Ronaldsay, the northernmost island of the Scottish Orkney archipelago. Elevated sheep δ13C values attributable to seaweed consumption were common in Neolithic Orkney as well, as the last two decades of archaeological research have shown (cf. Balasse et al. 2019, 2005 for a review).

Seaweed-eating sheep on North Ronaldsay. Picture by Jasmijn Sybenga.

Since the interpretation of archaeological data relies on modern comparative data to gain an understanding of what individual values mean in the archaeological context, analyses of modern material are required. Our study, recently published in the Journal of Archaeological Science, was conceived to collect further modern baseline δ13C and δ15N data for seaweed, terrestrial vegetation, and sheep bone collagen on the Orkney Islands. The results showed extremes of −10.7‰ for δ13C and +10.4‰ δ15N for modern seaweed-eating sheep bone collagen, and a δ13C average of −17.4 ± 1.4‰ for the analysed seaweeds (n = 20; mostly kelps). This modern dataset will help with future identification of seaweed-consumption, and allows an improved assessment in archaeological contexts of how much seaweed was likely consumed when the analysed bone formed.

Seaweed fertilisation

In addition to the consumption of seaweed, the use of seaweed as a fertiliser for terrestrial crops has also been suggested to influence consumer δ13C and δ15N values. To test this, we performed a field trial on Orkney where we fertilised bere barley (a six-row, hulled barley landrace) with seaweed. We followed traditional, historically attested practices with respect to the time and amount of fertilisation, but used a tractor for power-harrowing, sowing and rolling to reduce the still considerable amount of manual labour.

Fertilisation of 3 m × 3 m plots with seaweed (top left) and growth and harvest of the seaweed-fertilised bere barley. Pictures by Peter Martin, John Wishart and Magdalena Blanz.

Isotope ratio analysis showed significant elevation in δ15N values for seaweed-fertilised barley compared to barley from unfertilised control plots, but no significant difference in δ13C. This can be explained by the differing sources of carbon and nitrogen: Since barley takes up nitrogen from the soil (as nitrate, NO3, and ammonium, NH4+), the change in δ15N values upon fertilisation is due to the elevated δ15N values in the seaweed compared to the bioaccessible nitrogen in the unfertilised soil. In contrast, the lack of any major effects of seaweed fertilisation on δ13C is due to barley taking up most carbon by photosynthesis from the air (as carbon dioxide, CO2): Carbon stemming from the fertilisation at the roots has little likelihood of being taken up by the plant.

These results show that fertilisation of terrestrial crops with seaweed does not lead to these crops having more “marine” δ13C values, but that, similar to fertilisation with animal dung, δ15N values are affected by seaweed fertilisation. Further detail on the seaweed fertilisation field trial is available in Blanz et al., 2019 and Brown et al., 2020.

This research was undertaken as part of my PhD (2016-2020) at the Archaeology Institute, Orkney, supervised by Ingrid Mainland, Philippa Ascough, Mark Taggart and Jörg Feldmann. It was funded by the European Social Fund and Scottish Funding Council as part of Developing Scotland’s Workforce in the Scotland 2014–2020 European Structural and Investment Fund Programme.

Lab Experience during a Pandemic

Adam Andrews, Department of Biological, Geological, and Environmental Sciences, University of Bologna, Italy

Rachel Winter, Groningen Institute of Archaeology, University of Groningen, Netherlands

Willemien de Kock, Groningen Institute of Archaeology and Groningen Institute of Evolutionary Life Sciences, University of Groningen, Netherlands

SeaChanges project

SeaChanges (2019-2023) is a MSCA-multidisciplinary international training network of 15 PhD projects exploring long-term perspectives on the exploitation of marine vertebrates by integrating archaeology and marine biology. As a part of the training provided by the network, students undertake research secondments. Here we provide an account of our experiences at the University of York (UK) to complete collagen extraction and stable isotope analysis to contribute to our projects. 

To briefly acquaint you with our projects, small summaries are provided below.

ESR 12 – Adam Andrews, University of Bologna, Italy. I am investigating adaptation in Atlantic bluefin tuna (Thunnus thynnus) throughout the last 6000 years in the Mediterranean and Black Sea. I combine genomics, isotopes, and morphological analyses to understand how adaptable this species was/is and how resilient it remains now which may allow us to predict how it may respond to future change. 

ESR 13 – Rachel Winter, I am based at the University of Groningen in the Netherlands and am combining traditional zooarchaeology (NISP, osteometrics) with biomolecular archaeology (proteomics, stable isotope analysis) and marine ecology to understand the exploitation of groupers (Epinephelus spp.) in the Levant from the Bronze Age through the Hellenistic period. 

ESR 14 – Willemien de Kock, also based at the University of Groningen I study the foraging ecology and genetic connectivity of ancient green turtles (Chelonia mydas) along the Levant. Sea turtle bones are found at various archaeological sites, but my research focuses on remains from Kinet Höyük (Turkey), Tell Fadous-Kfarabida, and Tell el-Burak (both in Lebanon).

Lab Experience during COVID

Conducting lab work during a pandemic is certainly a peculiar experience.

It was a trip that was stressful before it began; for the Groningen delegation it meant traveling despite the government advice. We discussed the most responsible route and decided to cross the North Sea by ferry from Rotterdam to Hull. Sea turtles are always complicated to send across borders as they are endangered and listed on CITES, even old bones require proof that they existed before the animal was added to the CITES convention (a condition which we satisfy by several thousand years!). Willemien managed to courier them to York just before Brexit officially kicked in on the 1st of January 2021 saving her from more paperwork, phew! Rachel however crossed the sea with extra luggage consisting of ancient Mediterranean fish (none are on the CITES list!), it’s a glamorous life.

Due to the pandemic, acquiring samples has been predictably challenging. Closures to museums and institutions responsible for granting permission to destructively sample these materials, has resulted in delays to this project where only a subset of samples have been analysed to date. Fortunately, bluefin tuna are not listed on CITES which would have complicated matters further. 

Upon arrival into the UK, there were mandatory quarantine periods accompanied by the stress of hoping the courier finds the house and picks up our at home COVID tests, waiting for those results (negative, thankfully!), and then being permitted to arrange starting lab work. Once lab work commenced with regularity there are new considerations to going into the lab versus in non-covid times. For example, having to complete all of your training with colleagues at least 2 metres away and only being permitted a finite number of people in a space at any given time. To orchestrate all of this and ensure necessary precautions were taken, this further involved signing up on booking sheets for time slots in lab spaces, getting tested for covid twice a week, and when space was limited, working in slightly atypical spaces (casting sidelong looks to the zooarchaeology laboratory).

Of course, for one of us (the Englishman), quarantine was a relaxing (and merry) stint at home over the Christmas break.

Whilst we could access lab space and prepare samples for isotopic analysis, the experience was (not shockingly) simply incomparable with pre-pandemic times. Despite this, we still shared jokes in the lab that lightened the mood and made us feel very welcome and positive about the research. Even with so few colleagues around, we were very well supported when teething problems in new protocols/equipment arose and are grateful to everyone in BioArch at the University of York for enabling us to complete our lab work as planned.

Adam, Rachel and Willemein in York BioArch lab

The three of us managed to extract collagen from an impressive number of samples (nearly 500 in total) and we were lucky that the York BioArch crew were so patient with us especially when we were using ALL of the test tubes. There was not much else to do except work, but eventually that played to our advantage when we saw the collagen come out of the freeze-drier looking exactly like cotton-candy – admittedly not as sweet but just as satisfying.

Stay tuned for the results coming to a journal near you!

Contribution of food residues preserved in archaeological ceramics to the understanding of animal husbandry.

Emmanuelle Casanova

Organic Geochemistry Unit, University of Bristol and soon to be a MSCA IF fellow at APPSPE, MNHN, Paris.

Food residues in ancient ceramics

Pottery vessels appeared during the Neolithic and were used extensively to store, transport, cook and process food. During cooking, grease from the food migrates into the vessel walls and survive there through the ages, either complete or in a degraded form. These lipidic residues can be recovered and analysed at the molecular level. Based on the presence of specific molecules, called biomarkers, and their distribution, the source of food that was processed in the vessels is identifiable. Lipid residues from animal products are by far the most common residues recovered in the ceramics but it is also possible to find evidence of beeswax, plants, vegetables and even cereals.

Animal fats survive in the archaeological context in a hydrolysed from with the palmitic and stearic acids as the two major compounds. Measuring the stable carbon isotope values (δ13C) of these individual fatty acids allows to identify whether the fats came from non-ruminant, ruminant animals, and aquatic sources. This method is particularly interesting to highlight the exploitation of the dairy products of domesticated animals. It is, nonetheless, not species-specific and correlation of lipid data with the faunal records allows for further understanding about the source of animal fats cooked and processed in ceramics.

Example of Neolithic pottery from the Linearbankeramik of the Upper-Alsace analysed for its absorbed lipid residues.

The example of Alsatian Neolithic

I studied the region of Alsace for my PhD project, a part of the NeoMilk project. During the Neolithic, two sub-groups of the Linearbandkeramik culture originating from two separate migration waves settled following the modern border of Lower and Upper Alsace. Faunal remains were dominated by domesticated animals (>96%) particularly cattle, then pig and caprines (sheep/goat). The number of identified specimens in an assemblage can be weighted for the meat available per animal (i.e. if a caprine is 1 unit, a pig is 3.3 units and a cow 7.3 units for Neolithic animal) for the data to be comparable to the abundance of lipid sources in the ceramics. A noticeable difference from the faunal remains in the two groups is the availability of pig meat being ~30% in Lower Alsace but ~5% in the Upper Alsace.

A. Map of the Alsace region showing the boundary between the Lower and Upper Alsace and the sites selected for lipid residue analyses. B. Pottery with decorative motifs representative of successive LBK phases of the regional groups in Alsace and relative chronology (Lefranc, 2007).

Ruminant meat and milk were extensively processed in pottery vessels of the Upper Alsace, while non-ruminant products were scarce. On the contrary, in the Lower-Alsace both non-ruminant and ruminant meat dominate the residues in the ceramics, while dairy products were scarce. The results support that the two contemporaneous sub-groups of the Linearbandkeramik exploiting their herds differently with a particular focus on ruminant products, including dairy in the Upper Alsace.

Typical meat weight of the faunal remains for Linearbandkeramik groups of the Lower Alsace and Upper Alsace and results of compound-specific stable carbon isotope analyses on fatty acids extracted from pottery vessels

Direct radiocarbon dating of food residues: what can it tell us?

Radiocarbon dating is a key technique used to resolve the chronology of archaeological sites. I developed during my PhD and postdoc a method for directly dating ceramics from their preserved animal fats as a compound-specific approach. Among other applications, it permits to resolve the antiquity specific food commodities such as ruminant dairy by directly dating the commodity. I have applied this method to the dating of ruminant dairy residues in several regions of Europe and North Africa but also to the dating of equine products in Central Asia.

The method is particularly interesting at sites where evidence for the exploitation of a specific food commodity is scarce or unsupported by other archaeological and archaeozoological evidence. For instance, I applied the method to the radiocarbon dating of dairy residues from ceramics associated with hunter-gatherers of the Lesotho groups of 1st Millennium AD. The presence of dairy residues and their age compatible with the hunter-gatherer groups provided evidence of their access to domesticated animals, a parameter which was debated based on the morphological and ancient DNA analyses of the faunal assemblage.

These studies demonstrate the potential for organic residue analysis to enhance our understanding of past human-animal interactions. My new project, VARGAH, funded as part of the MSCA-IF will start in June hosted by Dr Marjan Maskhour at the AAPSPE, MNHN (Paris, France), uncovering the economy and chronology of early sendentary and mobile pastoralists in Iran.

My PhD was funded by the NeoMilk project (ERC-advanced grant awarded to Prof. Richard Evershed), subsquently followed by a post-doc funded by a ERC proof of concept project (LipDat awarded to Prof. Richard Evershed).

Intra-individual Sequential Carbon and Oxygen Isotope Analyses of Neolithic Livestock for Assessing the Early Husbandry Strategies in the Southern Caucasus

Masato Hirose

Department of Earth and Environmental Sciences at Nagoya University

By sequential sampling and analyzing tooth enamel, we can read the seasonal variations in stable isotope ratios recorded in teeth. Seasonal variations in the stable carbon (δ13C) and oxygen (δ18O) isotopes can be used to infer what the individual consumed, where, and in what season. In other words, this method is suitable for studying the husbandry strategies from livestock remains. However, in the analysis of an isolated tooth, the term of the obtained variation would be short, and difficult to capture the feature of the variation. Analyzing two teeth per individual can provide long-term variation, although this requires the use of well-preserved archaeological materials.

Excavations at Göytepe (Photo: Dr. Seiji Kadowaki)

In our new study, we used this intra-individual sequential isotope analysis with M2 and M3 teeth per individual as far as possible to investigate early husbandry strategies in the southern Caucasus. We analyzed the mandibular tooth enamel of goats, sheep, and cattle (only isolated teeth) from the Neolithic settlements, Göytepe (ca. 5650–5460 cal BC) and Hacı Elamxanlı Tepe (ca. 5950–5800 cal BC) located in western Azerbaijan. As reference samples, we also analyzed modern goat and sheep individuals that are known to have grazed in the vicinity of the sites.

Map of the southern Caucasus, showing the location of Göytepe, Hacı Elamxanlı Tepe, and other main Neolithic sites belonging to the Shomutepe-Shulaveri culture. The locations of the obsidian sources are only those where the use of the obsidian was recognized in Göytepe materials (Nishiaki et al., 2019).

In this region, the first full-fledged Neolithic agro-pastoral economy called the Shomutepe-Shulaveri culture, emerged suddenly around 6000 cal BC. Despite their geographical closeness to each other, this is about 2000 to 3000 years later than the emergence of agro-pastoral practices in the Fertile Crescent. It is important to study the delay in this phenomenon in detail in order to understand the process of diffusion of agro-pastoral economy in human society. Therefore, in this study, we focused on how early agro-pastoral economy was practiced in this region under the peculiar environmental conditions colder in winter, adjacent to mountainous regions, that are uncommon in the Fertile Crescent. Livestock management regarding mobility and migration is a key aspect in understanding agro-pastoral societies. Therefore, we attempted to provide isotopic evidence indicative of early husbandry practices in the southern Caucasus.

Sequential stable carbon and oxygen isotope values of the Neolithic livestock from Göytepe.

The obtained data showed several different patterns that may be explained by different modes of husbandry practices:

  1. Some of the goats and sheep exhibit large amplitudes in δ13C and δ18O variations (see figure above: part a). This was interpreted as a lowland pasturing pattern because the modern goat and sheep showed the same pattern.
  2. A Neolithic goat and three Neolithic cattle samples exhibited relatively small amplitudes and/or inverse cyclical variation patterns (figure b and d). While these patterns may have been caused by multiple factors, such as drinking water and food/fodder, vertical transhumance has also been proposed to result in the similar isotope patterns (e.g., Henton et al., 2010; Tornero et al., 2016). If animals experience seasonal vertical transhumance between lowlands in winter and highlands in summer, it is expected that the δ13C fluctuation range reduces. In addition, the δ13C value of C4 plant feeders is presumed to decrease if they spend summer in highlands where C4 plants are less.
  3. Some individuals illustrated a pattern with a larger amplitude of δ18O seasonal variations but a smaller amplitude of δ13C variations (c). It is likely that some factor reduced the variation of δ13C. We proposed a possibility of the use of fodder (C3 plants) collected in a short term and given to livestock thus dampening the seasonal variation in δ13C.

The factors contributing to the patterns of isotopic variation presented in our study may not be limited to transhumance or the use of fodder but are consistent with such possibilities. To verify these hypotheses, other analytical methods, such as strontium isotope analysis, are required to specify pasturing places in different seasons on the basis of a local and regional isoscape. In any case, at least, these various sequential isotopic patterns suggest that a variety of livestock breeding strategies were already adopted by Neolithic inhabitants in the southern Caucasus. Thus, long-term sequential isotope data from plural teeth per individual would provide us more specific seasonal variation patterns.

This research derives from the Azerbaijani-Japanese Archaeological Mission directed by Prof. Yoshihiro Nishiaki (The University of Tokyo) and Dr. Farhad Guliyev (The National Academy of Science, Azerbaijan). The financial support for this study was provided by the JSPS KAKENHI (No. 17H04534), the MEXT KAKENHI (No. 16H06408 and 20H00026), and The Takanashi Foundation for Historical Science.

A consistent desire for animal proteins in the Bronze Age Xinjiang, China

Minghao Lin

Department of History, Shanghai Jiao Tong University, 200240 Shanghai, China

Location of Xinjiang (China)

The Xinjiang region is located in far northwestern China neighboring central Asia and Mongolian Steppe. Throughout antiquity it has been a nexus for exchange in ideas, technologies, livestock, or even populations. Although most of its land is covered by severe environments such as the Taklamakan Desert, diverse cultures have been recognised since the early Bronze Age. Near Eastern cultivars such as wheat and barley have been excavated and dated to by the beginning of the second millennium BC in this region. In spite of the appearance of wheat and barley at some sites, did the subsistence of ancient Xinjiang populations really rely on these novel C3 cultivars? The degree to which these Near Eastern cultivars contributed to human dietary intake and how these new food types shaped local subsistence strategies remains poorly understood.

The northern slope of the East Tian Shan Mountains in Balikun, Xinjiang (China). (Credit: Minghao Lin)

In this research, part of my Humboldt fellowship project hosted by Prof Cheryl Makarewicz at Kiel University, we conducted a regional scale meta-analyses of carbon and nitrogen isotope values measured from humans and animals including cattle, sheep/goats, and deer from sites dating from c. 2000 BC to AD 1000 within the Xinjiang region to examine the degree to which novel cultivars and animal proteins were incorporated into local subsistence system from the perspective of isotopic expression. We notice a narrow gap (1.7-2.7‰) in δ13C values but a wide space (4.5-6.5‰) in δ15N between humans and bovid samples. Meanwhile, we also record a consistent expression of high nitrogen isotope values visible in humans across time, indicating a persistent preference of the Xinjiang people for animal resources (e.g. meat, dairy proteins). This suggests novel cultivars of wheat and barley were not rapidly adopted in Xinjiang highlighting the food dispersal and acceptance across Eurasia during prehistory was a complex process.

Results of meta-analysis of stable carbon and nitrogen isotopic results from Xinjiang (China)
(Credit: Minghao Lin).

Current Methods and Best Practices in Strontium Isotope Mapping

Emily Holt, Jane A. Evans, and Richard Madgwick

Drawing on variations in bioavailable strontium in different environments to provenance biological materials has become increasingly common since its first application in archaeology almost four decades ago, and it is frequently applied to zooarchaeological materials. Provenancing biological materials, including faunal remains, using strontium isotope ratios generally requires a map of bioavailable strontium, commonly known as an isoscape, to compare results with. However, both producing the isoscape and using it to interpret results present methodological challenges that researchers must carefully consider.

To help researchers understand the complexities of strontium analysis, we reviewed current research to produce a critical synthesis and recommend best practices. We addressed sampling the archives needed to build an isoscape, applying the different mapping and modeling methods currently available, and interpreting the results of archaeological analyses against isoscapes. Our critical review is freely available to read and download from Earth-Science Reviews until May 8, 2021


Current research indicates that, while many archives can be analyzed to produce isoscapes, modern plant materials usually provide the best approximation of bioavailable strontium. Modern plants can be used alone or combined with other archives if applying a machine learning approach to mapping. In areas where erosion has significantly shifted the location of surface sediments, modern plants may not provide an appropriate archive for building an isoscape that is applicable to archaeological materials. In these areas, alternative archives such as archaeological rodent remains may be preferable. Archaeologists should also be aware that contemporary soil treatments like fertilizers may affect the bioavailable strontium in soils. Areas used for agriculture should be avoided when sampling.

Strontium pathways (adapted from Bataille et al. 2020, Fig. 1)

It is essential to collect appropriate metadata when sampling. Collecting metadata improves the legacy benefits of strontium data by making them more broadly applicable. These metadata and the results of the analyses should be archived in one of several online databases to maximize their usefulness.

Mapping and modeling

We found that domain mapping currently produces the most accurate, most interpretable isoscapes for most research questions. However, machine learning approaches are also powerful and promise to provide more accurate and geographically wide-ranging isoscapes over time. Machine learning is computationally intensive and may not currently be an option for many researchers, but will be more widely available in the future.

Strontium isoscapes that are both appropriate and sufficiently high resolution to answer specific research questions do not exist for most parts of the world. Researchers intending to incorporate strontium analysis into their research designs should expect to conduct primary sampling and analysis to create appropriate isoscapes or refine existing ones, which should themselves not be utilized uncritically.


Strontium isotope analysis is currently the best developed and understood provenance tool. However, because isotope data can only exclude options of possible origin, the development of multi-isotope methods provides the route to a more powerful future approach. Using strontium isotope analysis for provenancing is most successful when combined with other isotopes and/or trace elements as part of a likelihood approach.

Future directions

In the future, we expect that increasing amounts of primary data and the increasing application of machine learning approaches to mapping will mean that strontium analysis continues to improve as a method of provenancing.

This research was apart of the ZANBA – Zooarchaeology of the Nuragic Bronze Age, a Marie Sklodowska Curie fellowship awarded to Emily Holt.

Forest pasturing and foddering: stable isotope prespectives

The use of forests for pasturing and fodder resources remains globally an important, preventing over-grazing and providing complementary fodder in times of poor pasture. Traditionally, forests in Europe were a rich source of collected forage (leafy hay) in the form of branches and leaves. At the same time, herd animals can have a negative impact on forests, where grazing is unchecked. The first herders of Central and Northern Europe experienced a landscape ‘Bristling with forests and foul swamps’ as described by Tacitus in 98AD [1]. The impact of animals on forests has been a focus of archaeologists for over 40 years. For example, Iversen [2] highlighted the importance of cattle in the expansion of the initial Neolithic settlements in his landam theory. The elm decline was initial contributed to increased use of leafy hay as animal forage.

How can we study forest pasture and foddering via stable carbon isotopes?

The canopy effect is where plants growing under dense forest canopies will exhibit depleted δ13C values. This is due to a combination of carbon -13 depletion of atmospheric CO2 under the canopy caused by CO2 respired by decaying organic matter and low light intensity at the forest floor decreasing photosynthesis efficiency. Consequently, animals browsing and grazing under heavy forest canopies or being fed leafy hay from these environments will exhibit low carbon isotope values in their tissues. Using this principal, researchers such as Dorothee Drucker and Rhiannon Stevens have explored the use of forests by wild ruminants past and present.

Schematic of the Canopy effect on δ13C values of plants growing under different canopy densities and its relationship to different ruminant tissues (cone collagen/enamel bioapatite)

A cautionary note

The issue of equifinality can arise with the interpretation of stable isotopic values because different growing environments can produce similar effects on the stable isotopic ratios of plants. These continue up the food chain. For example, waterlogged environments can have a similar impact on δ13C values of plant communities as a dense forest canopy. Lynch and colleagues interpreted depleted δ13C values observed in British aurochs as a reflection of animals feeding on plants from waterlogged environments. Whereas, a similar study by Noe-Nygaard and colleagues of Scandinavian aurochs suggested these animals were forest dwellers. This is why it is key prior to the interpretation of stable isotopic results that robust interpretative frameworks using paleoenvironmental are created for testing hypotheses.

Independent methods for determining forest foddering

Compound-specific stable nitrogen isotope analysis of collagen amino acids provides an independent means for identifying consumption of woody plants, such as leafy hay. Developed by researchers at University of Bristol, direct evidence of the plant composition of animal fodder (woody/herbaceous) can be uncovered using the dietary β values based on δ15N CSIA of amino acids from incremental samples of dentine from cattle molars. These values represent the Δ15NGlx-Phe values of the plants at the base of the food web, using a known trophic offset of −4.0‰ between cattle and their diet. The dietary β values are then be compared with established ranges of Δ15NGlx-Phe values expected for herbaceous (−5.4±2.1‰) and woody plants (−9.3±1.6‰), based on modern references. Combining incremental analysis of enamel bioapatite and CSIA-AA of dentine of the same tooth provides a powerful method to identify forest pasturing and seasonal use of leafy-hay.

Look out for upcoming papers by myself and colleagues from University of Bristol, and European institutions from Hungary, Poland, France and Germany, discussing the role of forests in LBK cattle husbandry uncovered during the NeoMilk project (ERC-advance awarded to Prof. Richard Evershed).


1. Bogucki, P., 1988. Forest farmers and stockholders. Early agriculture and its consequences in North-Central Europe. Cambridge: Cambridge press.

2. Iversen, J., 1969. The influence of prehistoric man on vegetation, in The Neolithisation of Denmark: 150 years of debate., A. Fischer and K. Kristiansen, Editors. Sheffield Archaeological Monographs: Sheffield. p. 105-16.