Carbon and nitrogen isotope analyses of human and dog diet in the Okhotsk culture: perspectives from the Moyoro site, Japan

Abstract

The Okhotsk people were sedentary hunter-gatherer-fishers who lived and prospered in Sakhalin, Hokkaido, and the Kurile Islands during the fifth–thirteenth centuries AD. They expanded rapidly along the north-eastern coast of Hokkaido where archaeological evidence suggests an increasing dependence on hunting marine mammals. In this study, we present the results of carbon and nitrogen isotope analyses of 18 faunal (including two domesticated dogs) and 58 adult human skeletons excavated from the Moyoro site of the Okhotsk culture in eastern Hokkaido. Although the mean human isotope ratios did not differ between sexes, the variances of carbon isotope ratios were significantly greater in males. Carbon and nitrogen isotope ratios suggest that the Moyoro humans and dogs heavily depended on marine foods for their dietary protein intake. The Bayesian mixing model suggests that humans obtained a maximum of 80–90% of their dietary protein from marine mammals, whereas domesticated dogs obtained 2–33%, 3–40%, and 5–45% of dietary protein from brackish-water fish, marine fish, and marine mammals, respectively. This suggests an avoidance of significant dietary overlap between the sympatrically living humans and dogs at the Moyoro site. Significant maritime adaptation would have enabled the subsistence of the Okhotsk people in the harsh northern environment of Hokkaido.

Introduction

The relationship between diet and population dynamics in prehistoric periods is of interest in anthropology and archaeology. Human populations sometimes increased in size and expanded their habitats by adopting novel food sources using appropriate acquisition and processing techniques (Stiner et al., 1999; Hu et al., 2009); the Okhotsk people are one such population. They adapted to marine food sources and expanded their habitat along the coast of the Sea of Okhotsk over a period of several hundred years (Amano, 2003; Hudson, 2004). This study aims at reconstructing the feeding ecology of the Okhotsk people based on stable carbon and nitrogen isotopes of human and faunal skeletal remains excavated at the Moyoro site, Hokkaido, Japan.

Carbon and nitrogen stable isotope analysis

Analysis of stable carbon and nitrogen isotopes in bone collagen is the principal method used to reconstruct past dietary habits of individuals, because body tissues record the isotopic composition of dietary items consumed (Lee-Thorp, 2008). In particular, the carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N values, respectively) of bone collagen mainly reflect those of the dietary protein (Kellner and Schoeninger, 2007; Froehle et al., 2012). In Japan, δ¹³C values have been used to determine the consumption of C₄ plant and marine protein sources (Chisholm et al., 1992; Yoneda et al., 2004a). Nitrogen isotope ratios are primarily used to estimate the amount of terrestrial vs. marine proteins in the diet and compare them among individuals of the same or different populations (Minagawa and Akazawa, 1992; Kusaka et al., 2008, 2010).

Okhotsk culture

The Okhotsk culture flourished in Sakhalin, Hokkaido, and the Kurile Islands of Northeast Asia during the fifth– thirteenth centuries AD (Amano, 2003). The Okhotsk people were sedentary hunter-gatherer-fishers with a characteristic significant dependence on marine resources, and lived in a harsh environment with subzero winter temperatures (Hudson, 2004). Biometrics (Ishida, 1996; Komesu et al., 2008; Kudaka et al., 2013) and ancient DNA (Sato et al., 2009) analyses have revealed strong similarities among Okhotsk people and populations from the lower Amur River region and the Neolithic Baikal. They spread along the coastline from southern Sakhalin to the Okhotsk coast of eastern Hokkaido and the Kuriles during 600–700 AD (Yamaura and Ushiro, 1999). At that time, the Epi-Jomon and Satsumon people occupied the interior regions of Hokkaido, and there was some cultural interaction between the Okhotsk and the indigenous populations (Okada, 1998; Yamaura, 1998; Yamaura and Ushiro, 1999; Masuda et al., 2001). After approximately 1000 AD, the Okhotsk culture seems to have been absorbed or replaced by the Satsumon culture, which developed into the Ainu culture several hundred years later (Yamaura and Ushiro, 1999).

The Okhotsk culture has unique social and economic characteristics. Archaeological data suggest clear seasonality in subsistence activities, such as fishing from fall to spring in northern Hokkaido, and marine mammal hunting during the winter in eastern Hokkaido (Amano, 2003). Large pit dwellings, on average three times larger than those in the Satsumon culture, have been found in Okhotsk archaeological sites (Amano, 2003); these are believed to represent a way to organize labor activities and respond to increased social production and ecological changes (Hudson, 2004). Compared with the contemporaneous Satsumon culture in Hokkaido, the trade with mainland Japan and the Eurasian continent was less intensive (Hudson, 2004).

Archaeological evidence has revealed that the Okhotsk people mainly subsisted on marine mammal hunting, shallow- and deep-water fishing, and terrestrial animal hunting (Yamaura, 1998; Yamaura and Ushiro, 1999; Amano, 2003). It is assumed that cereals such as barley, millet, and foxtail millet were not routinely consumed but were utilized for rituals (Yamada, 1996). Furthermore, the Okhotsk people domesticated pigs and dogs in the incipient period of the Okhotsk culture, although pigs are absent from the sites in eastern Hokkaido, possibly because of the harsh winter environment (Amano, 2003; Hudson, 2004). Coastal sea ice prevented the Okhotsk from fishing during the winter in eastern Hokkaido; this low productivity in winter would have further prevented them from feeding pigs (Amano, 2003) and thus strengthened their dependence on marine mammal hunting (Ono, 1996a, b). Due to the discovery of cut marks on excavated dog bones it is assumed that, besides working and hunting, dogs were also eaten by the Okhotsk people (Amano, 2003).

Previous isotopic studies on human skeletal remains support the above-mentioned archaeological evidence. Chisholm et al. (1992) compared the carbon isotope ratios of adult humans from various prehistoric sites in Japan, ranging from the Jomon (c. 16500–3000 BP) to the Edo (1657–1683 AD) period. The reported mean δ¹³C value of adult human bone collagen from two Okhotsk sites in northern Hokkaido (Omisaki and Hamanaka) was −17.5‰. Yoneda (2002, unpublished data) analyzed adult human skeletons excavated from two Okhotsk sites, Hamanaka 2 and Utoro in northern and eastern Hokkaido, respectively. The mean δ¹³C and δ¹⁵N values were −13.1 ± 0.2‰ and 19.2 ± 0.6‰ for Hamanaka 2, and −13.8 ± 0.7‰ and 19.4 ± 0.5‰ for Utoro, respectively. These studies indicate that the Okhotsk people depended on marine mammals as their protein sources more strongly than any other prehistoric population in Japan, such as the Epi-Jomon and Ainu (Chisholm et al., 1992; Yoneda, 2002). Unfortunately, however, these studies did not conduct isotope analyses of faunal remains from the same sites in the same period, and thus their conclusions require further scrutiny. Naito et al. (2010a) conducted amino acid-specific nitrogen isotope analyses of adult human and faunal skeletons excavated from three different Okhotsk sites (i.e. Hamanaka 2, Kafukai 1, and Moyoro) and revealed dietary differences between northern and eastern Hokkaido. Although the sample size was small (i.e. n = 2 or 3 per site), they concluded that fur seal contribution was higher in Moyoro of eastern Hokkaido (78–80%) than that in Hamanaka 2 and Kafukai 1 of northern Hokkaido (<24%) populations.

Isotopic data from the various fauna and domesticated animals are required to fully investigate the feeding habits of the Okhotsk people. In this study, we conducted isotope analyses on archaeological faunal remains excavated from the Moyoro site as well as human remains to estimate the relative contributions to dietary proteins of different food sources such as marine mammals and fish. Compared with previous studies, we covered a relatively large number of adult human individuals (i.e. n = 58), which enabled us to investigate the intra-population variation in diet. Furthermore, we reconstructed the diet of domesticated dogs: what humans feed their domesticated animals is an important aspect of feeding ecology (Barton et al., 2009).

Materials and Methods

Moyoro site

The Moyoro site is located in eastern Hokkaido, Japan, on an estuarine sand area of the Abashiri River (Figure 1). This site is one of the representative shell mounds of the Okhotsk culture in eastern Hokkaido, which contains human burials and pit dwelling remains. The site has been excavated several times since the beginning of the twentieth century (Yonemura, 1935; Kodama, 1948; Ito, 1965; Kiyono, 1969). Radiocarbon and palaeomagnetic dating of recent excavations indicate that the site was used by as early as the fifth century (Abashiri City Board of Education, 2009). A number of biometric (Ishida, 1996), ancient DNA (Sato et al., 2007, 2009), and palaeodemographic (Nagaoka et al., 2012) analyses of human skeletons excavated from the Moyoro site have been performed. Although there is little quantitative assessment, the excavated shell mounds and faunal assemblages contained remains of marine mammals, fish, shellfish, and terrestrial animals (Abashiri City Board of Education, 2009). Only two adult individuals were investigated using carbon and nitrogen isotopes (Naito et al., 2010a). No other isotope analyses have been reported for the Moyoro skeletal population.

Figure 1

Location of the Moyoro and other Okhotsk archaeological sites mentioned in this study.

Skeletal samples

We obtained elemental and isotopic data from 18 faunal remains (including two domesticated dogs, Table A1) from the excavated Moyoro faunal collections at the Abashiri City Historical Museum, and 58 adult human individuals (22 females, 34 males, and two sex-unknowns, Table A2) from the skeletal collection at the Hokkaido University Museum. Elemental and isotopic data of two faunal (M14 and M32) and two human adult (1006 and 1011) individuals reported in Naito et al. (2010a) were also included in the dataset. The sex of the adults was determined by the morphology of the pelvis: the composite arch, the greater sciatic notch, the inferior pelvis, the ischiopubic proportion, and the preauricular sulcus (Bruzek, 2002), the medial aspect of the ischiopubic ramus, the subpubic concavity, and the ventral arc (Phenice, 1969).

To eliminate skeletons from periods other than that of the Okhotsk culture, radiocarbon dating of the Moyoro human skeletons was performed at NIES-TERRA, the accelerator facility of the Institute of Environmental Studies, Ibaraki, Japan (Yoneda et al., 2004b; Komesu et al., 2008). Corrections of +382 ± 16 years for the local reservoir effect (Yoneda et al., 2001) and 90% contribution from marine proteins were applied considering the marine reservoir effect on the ¹⁴C age. The ¹⁴C ages were calibrated by the OxCal program (Ramsey, 1995) based on atmospheric and marine data sets (IntCal13 and Marine13, respectively: Reimer et al., 2013). Skeletons with calibrated ages apparently later than the Okhotsk period were excluded.

Isotope analysis

All samples for isotope analysis were prepared at The University of Tokyo, Japan, following the modified gelatinization method (Longin, 1971; Yoneda et al., 2004a). Approximately 0.1–0.5 g of bone samples were cleaned first by sand-blasting and then ultrasonically. Samples were soaked in 0.2 M NaOH to remove exogenous organic matter. After washing with pure water, the samples were freeze-dried and crushed into powder or small chunks. Crushed samples were demineralized in 1.0 M HCl solution at 4°C. The remaining portion was isolated and gelatinized at 90°C for 12 h, filtered with a glassfiber filter (Wattmann GF/F), and freeze-dried.

Most of the isotope analyses were performed at the National Institute for Environmental Studies, Ibaraki, Japan. Approximately 0.25 mg of the resulting gelatin was measured by elemental analysis–isotope ratio mass spectrometry (Carlo Erba NA1500 elemental analyzer, Finnigan MAT ConFlo II interface, and Finnigan MAT 252 mass spectrometer). The analytical standard deviation (SD) was approximately 0.1‰ and 0.2‰ for carbon and nitrogen, respectively. Elemental concentrations and isotope ratios were calibrated against a laboratory alanine standard.

Statistics

All statistical analyses were performed with the software R, version 3.0.2 (R Core Team, 2013). Dietary contributions of various food sources was estimated by using a Bayesian mixing model SIAR, version 4.2 (Parnell et al., 2010; see also Phillips, 2012) with a “very long” run configuration of MCMC (i.e. 1000000 iterations and 400000 burn-in). Food sources were grouped into several categories with distinct isotope ratios. This categorization is meaningful only in isotope ecology and may not necessarily have archaeological significance. The discrimination δ¹³C and δ¹⁵N values from prey to consumer bone collagen were set as 1.5 ± 0.5‰ (5.0 ± 0.5‰ in the case of C₃ plants) and 4.0 ± 1.0‰, respectively (Lee-Thorp, 2008). Isotope ratios of modern Hokkaido C₃ plants and modern Japanese marine shellfish with correction for the Suess effect (Friedli et al., 1986) were those reported in Tsutaya et al. (2013) and Yoneda et al. (2004a), respectively. Two applications were independently performed: applications into two dogs and all adult humans.

Results

Preservation of bone collagen

The results of isotopic and elemental analyses of faunal and human bone collagen are shown in Table A1 and Table A2, respectively. The preservation of collagen was evaluated from the atomic C/N ratios and the yield of collagen. On the basis of acceptable C/N ratios (2.9–3.6; DeNiro, 1985), six human samples were excluded from the dataset. Following the criteria proposed by van Klinken (1999), all samples were considered to have acceptable yields (>1%). The ¹⁴C and calibrated ages of human skeletons are also shown in Table A2.

Fauna

The δ¹³C and δ¹⁵N values of faunal bone collagen are summarized in Table 1 and shown in Figure 2. Carbon and nitrogen isotope ratios of terrestrial herbivores range from −23.3‰ to −22.3‰ and 2.1‰ to 7.5‰, respectively. The herbivorous mammals fell within the range of C₃ consumers, which indicates that the mammals consumed little or no C₄ plants. Isotope ratios from wild terrestrial carnivores range from −21.0‰ to −17.6‰ for carbon and from 5.7‰ to 10.2‰ for nitrogen. The δ¹³C and δ¹⁵N values of some terrestrial mammals (e.g. M14, MF18, and MF27) are unexpectedly higher than the natural isotopic abundances in Japanese terrestrial resources (Yoneda et al., 2004a). Such higher isotope ratios could be due to the sea spray effect (Virginia and Delwiche, 1982; Heaton, 1987).

Table 1 Carbon and nitrogen isotope ratios (‰) of faunal and human bone collagen from the Moyoro site.

		δ13C		δ15N		n
		Mean	SD	Mean	SD
Fauna	Brackish-water fish	−14.8	1.8	11.4	0.2	2
	Dog	−13.7	0.1	16.7	0.4	2
	Marine fish	−14.3	0.8	13.9	1.5	5
	Marine mammal	−14.2	0.3	15.8	0.9	7
	Terrestrial carnivore	−19.2	1.8	8.0	2.1	4
	Terrestrial herbivore	−22.7	0.4	4.1	2.3	4
Adult human	Female	−13.5	0.5	19.3	0.9	21
	Male	−13.7	0.8	19.5	0.8	29
	All	−13.6	0.7	19.4	0.8	52

Figure 2

δ¹³C and δ¹⁵N values for faunal bone collagen from the Moyoro site. The mean δ¹³C and δ¹⁵N values for adult humans are also indicated. The isotope ratios of modern C₃ plants and marine shellfish are from Tsutaya et al. (2013) and Yoneda et al. (2004a), respectively.

Stable isotope ratios of marine fish range from −15.1‰ to −13.1‰ for carbon and from 11.3‰ to 15.0‰ for nitrogen. The isotope ratios from marine mammals range from −14.2‰ to −13.6‰ for carbon and from 14.0‰ to 16.6‰ for nitrogen. The δ¹³C and δ¹⁵N values of marine mammals are higher than those of fish, possibly because of trophic level enrichment (Minagawa and Wada, 1984; Bocherens and Drucker, 2003). The δ¹³C and δ¹⁵N values of brackish-water fish bone collagen were similar and at the lower end of those of marine fishes, respectively (Figure 2).

Dogs

Dogs showed relatively different isotope ratios compared to the other faunae. Two dog individuals have δ¹³C and δ¹⁵N values similar to marine mammals. Assuming that the dogs and the humans consumed similar foods, the Bayesian mixing model indicates that the 95% credible interval (CI) of dietary protein contributions from brackish-water fish, marine fish, and marine mammals are lower than 45%, whereas those from terrestrial foods are lower than 25% (Table 2). The data indicate that the dogs were predominantly fed with brackish-water fish, marine fish, and marine mammals. The posterior distributions of the dietary proportions are summarized in Table 2 and are shown in Figure 3A.

Table 2 Mode and lower and higher 95% credible interval ranges of the relative dietary contribution (%) for the Moyoro dog and human calculated by SIAR.

		BF	C3	MF	MM	MS	TC	TH
Dog	Mode	20.5	1.5	19.5	19.5	15.5	1.5	1.5
	Lower	2.0	0.5	3.3	4.7	1.6	0.9	0.5
	Upper	32.7	20.1	39.1	44.8	30.9	25.0	20.8
Human	Mode	0.5	0.5	0.5	86.5	0.5	0.5	3.5
	Lower	0.1	0.1	0.3	80.2	0.0	0.3	0.6
	Upper	4.2	2.4	9.9	90.2	2.0	8.7	6.6

BF, brackish-water fish; C3, C₃ plants (Tsutaya et al., 2013); MF, marine fish; MM, marine mammals; MS, marine shellfish (Yoneda et al., 2004a); TC, terrestrial carnivores; TM, terrestrial herbivores.

Figure 3

Posterior distribution of the dietary proportion for (A) dogs and (B) adult humans calculated by SIAR.

Humans

The δ¹³C and δ¹⁵N values of adult human bone collagen are summarized in Table 1 and are shown in Figure 4. The stable isotope ratios of adult females range from −14.8‰ to −12.7‰ for carbon and from 16.3‰ to 20.6‰ for nitrogen. The isotope ratios of adult males range from −16.0‰ to −12.7‰ for carbon and from 17.8‰ to 20.8‰ for nitrogen. A Mann–Whitney U-test indicated no significant difference between females and males (δ¹³C values: U = 300, P = 0.937; δ¹⁵N values: U = 264.5, P = 0.437). However, the variances of the δ¹³C values of males were significantly greater than those of females (F-test: F = 2.580, P = 0.032), whereas the δ¹⁵N values between the sexes did not differ significantly (F-test: F = 0.942, P = 0.867).

Figure 4

δ¹³C and δ¹⁵N values for adult human bone collagen from the Moyoro site.

There is no systematic time change in isotope ratios. Spearman rank-correlation tests indicated that the calibrated ¹⁴C ages did not correlate significantly with δ¹³C (R = −0.113, P = 0.481) and δ¹⁵N (R = −0.032, P = 0.845) in adult human individuals. The 95% CIs of calibrated ¹⁴C ages range from 1095 to 334 calBP (Table A2).

Applying the Bayesian mixing model, the 95% CI of dietary protein contribution from marine mammals is calculated to be approximately 80–90% in humans (Table 2). On the other hand, the summed dietary protein contribution from terrestrial foods is <16% (Table 2). The data suggest that the Moyoro human population depended heavily on marine mammals for dietary protein. The posterior probabilities of the dietary proportions are summarized in Table 2 and are shown in Figure 3B.

Discussion

Dog diet

Unlike other terrestrial carnivores, the isotope ratios of the two dogs suggest marine protein consumption (Figure 2). These dogs would have been to some extent fed with marine food caught by humans. However, the Bayesian mixing model showed that the diet composition of the dogs differed from that of the humans; dietary proteins from marine mammals are 80–90% of the consumed proteins for humans but only 5–45% for dogs in the 95% CI. The protein contribution of brackish-water and marine fish was apparently greater in dogs compared to humans and this contribution from brackish-water and marine fishes is possibly comparable with that from marine mammals in dogs (Table 2, Figure 3A). Humans and dogs lived sympatrically in the Moyoro site, and their diets were somewhat different (Table 2), and thus significant overlap in dietary protein intake would have been avoided.

Several ethnographic studies have reported dog use and dog diet of the indigenous Ainu people in Hokkaido and Sakhalin, and fishers in Kamchatka. Ethnographic accounts of the Ainu population in Hokkaido (Starr, 1904) and Sakhalin (Nishitsuru, 1942; Kasai, 1943) in the beginning of the twentieth century and those in Hokkaido in the nineteenth century (Batchelor, 1892) reported that dogs were used for hunting of terrestrial mammals and sledging. Dog skins were used to make clothes and shoes (Batchelor, 1892). Several studies also recorded that the Ainu people fed their dogs with low-sodium trout (Nishitsuru, 1942; Kasai, 1943). The isotope ratio of trout in Hokkaido is similar to that of brackish-water fish (Tsutaya et al., 2013). Shnirelman (1994) reported that fishers in Kamchatka in the late nineteenth century fed their domesticated dogs with dried or fermented fish and used them to pull sledges. Although the cultural traits reported in modern ethnographic studies are not directly comparable with those in the ancient Okhotsk population, such ethnographic observations agree well with the isotopic results in this study.

Carbon and nitrogen isotopic similarities between dogs and humans have been reported from various archaeological sites around the world (e.g. Cannon et al., 1999; Fischer et al., 2007; Fornander et al., 2008; Choy and Richards, 2009). Furthermore, it has been assumed that the isotope ratios of dogs are a proxy for human diet. On the other hand, there are several differences between the dog and human isotope ratios, and some have assumed that the isotopic differences indicate dietary differences between dogs and humans (Rick et al., 2011; Losey et al., 2013). However, none of these studies has shown what the dogs actually ate because of insufficient faunal isotope ratios. This study has found a lesser contribution of marine mammal protein in the dog diet of the Moyoro site by considering the feeding ecology of the Okhotsk dogs and humans. Thus, it would be better to cover other faunal isotope ratios as well when assessing the isotopic evidence for domesticated dog diet.

Human diet

The mean isotope ratios did not differ by sex; however, the variance of δ¹³C value was greater for human males (Figure 4). This variance is not the result of a change in diet over time. The larger variance of the male isotope ratios suggests that the male diets were more heterogeneous than female diets in the Moyoro population. Chisholm et al. (1992) reported that the mean δ¹³C values of human bone collagen for males were more negative than females in several prehistoric Hokkaido sites; furthermore, they inferred that the males would have eaten more terrestrial food. Considering that the δ¹³C values of several adult males are more negative than those of most Moyoro adults, there were apparently some adult males who preferentially consumed ¹³C-depleted food such as terrestrial food.

The wide dietary range of the Moyoro males has been discussed from several aspects. First, accessible food sources might be greater for males due to cultural or behavioral reasons. While Ainu females mainly handled labor around the household such as making clothing, cooking, and childcare, males were in charge of hunting and fishing (Batchelor, 1927; Kasai, 1943; Nishitsuru, 1942; Ohnuki-Tierney, 1974). The greater the ranges of activity, the more diverse the food sources that can be obtained from different ecosystems. Furthermore, some researchers have assumed that the status of females in the eastern Okhotsk culture declined (Yamaura, 1982; Hudson, 2004), which could correlate with the availability of food. The above-mentioned considerations should be evaluated from the standpoint of archaeology and ethnography in the future. Second, male-biased immigration could also be a possible reason, although patrilocal-like marital systems have been described in an ethnographic study of the Sakhalin Ainu (Ohnuki-Tierney, 1974). Adult males who moved in from other villages may have retained the isotope ratios of their ecosystems of origin because it takes more than 10 years for adult bone collagen to fully turn over and reflect a change in new diet (Hedges et al., 2007). We anticipate that immigrants to Moyoro can be identified by using oxygen and strontium isotopes of tooth enamel (Waseda and Nakai, 1983; Kusaka et al., 2012).

Although the mixing model provides quantitative estimates for dietary reconstruction, we need to consider two major drawbacks. First, we cannot evaluate the relative contributions from food sources that are not included in the model. Ordinarily, we cannot calculate the contribution from food sources that do not remain in the archaeological site. To overcome this problem, the isotope ratios of modern C₃ plants and marine shellfish were included in the SIAR models. A small amount of carbonated boiled barley and specific quantities of several marine shellfish remains were excavated from the Moyoro site (Abashiri City Board of Education, 2009); however, the carbon and nitrogen isotope ratios of their edible parts cannot be measured. Although a small amount of carbonated grains of C₄ plants was also excavated, we excluded the C₄ plants from the SIAR analysis, because the C₄ grains were assumed based on their excavation conditions to have been used in rituals (Abashiri City Board of Education, 2009). Although the food sources examined in this study cover the range of excavated food items, it is always possible that other unexcavated food sources were consumed. Second, we cannot evaluate the relative contributions from food sources that indicate almost the same isotopic signatures. Protein contributions from dogs in human diet cannot be evaluated in this study because the δ¹³C and δ¹⁵N values of dogs and marine mammals are quite similar (Figure 2). Although 80–90% of dietary proteins are derived from marine mammals, this probably includes the contribution from dogs as well. However, we believe that the contribution from dogs is not that large because the excavated skeletons of dogs are apparently smaller than those of marine mammals in the Moyoro site although the exact quantity has never been estimated (Abashiri City Board of Education, 2009). Furthermore, in the Kafukai site of northern Hokkaido, dogs and pigs together contribute only 2.9% of the estimated caloric intake based on the quantitative evaluation of the excavated remains (Nishimoto, 1978). Although such drawbacks would not affect the overall conclusions, we should be aware of them.

The Bayesian mixing model of carbon and nitrogen isotopes showed that the Moyoro adults obtained at most approximately 80–90% of dietary proteins from marine mammals (Table 2, Figure 3B), which suggests a significant contribution of marine mammals to the dietary protein sources of the Okhotsk people in eastern Hokkaido. Amino acid-specific nitrogen isotope analyses revealed that the fur seal protein contribution was 78–80% in the Moyoro humans (Naito et al., 2010a), which is slightly lower than the contribution in this study probably because of the above-mentioned reasons. Furthermore, it is suggested that the relative contributions of fur seals were lower in northern Hokkaido (i.e. 0–24% for the Kafukai 1 and 0–11% for the Hamanaka 2 humans), whereas that of marine proteins was equally high (i.e. 76–94% for the Kafukai 1 and 60–76% for the Hamanaka 2 humans). Nishimoto (1978) calculated the relative caloric contributions of edible parts based on the abundance of faunal remains from the Kafukai 1 site (northern Hokkaido), showing that the contribution was greater than 80% for marine fish and 6.5% for marine mammals. Carbon and nitrogen isotope analysis with a stochastic mixing model for three Jomon, Epi-Jomon, and early modern (Ainu) skeletal populations in Hokkaido suggested that the protein contribution was 28–35% for marine fish and 42–51% for marine mammals (Minagawa, 2001). Carbon and nitrogen isotopic data from the Usu-moshiri skeletal population in Epi-Jomon Hokkaido suggested that the protein contributions from marine fish and marine mammals were 12–42% and 45–76%, respectively (Tsutaya et al., 2013). Comparing these findings with our results, the dietary protein dependence of the Moyoro adult humans on marine mammals would be greater than that of the other Okhotsk populations in northern Hokkaido and other prehistoric Hokkaido cultures. This supports the archaeological hypothesis of a strong dependence of the Okhotsk people in eastern Hokkaido on marine mammal hunting (Ono, 1996a, b).

The isotope ratios of the Moyoro adults are higher than that in other Japanese hunter-gatherers, which suggests a strong dependence on marine mammals for dietary protein. The δ¹³C and δ¹⁵N values of mainland Jomon hunter-gatherers are typically within −20‰ to −15‰ and 6‰ to 15‰, respectively (Minagawa, 2001; Yoneda et al., 2004a; Kusaka et al., 2008, 2010; Naito et al., 2013). Mainland Jomon hunter-gatherers mainly depended on C₃ terrestrial ecosystems, and their isotope ratios vary according to the marine protein consumption. Although the Hokkaido Jomon hunter-gatherers have more elevated isotope ratios because of greater marine protein consumption, −17‰ to −13‰ for δ¹³C and 16‰ to 19‰ for δ¹⁵N values (Minagawa, 2001; Naito et al., 2010b), the δ¹³C and δ¹⁵N values of the Moyoro adults are even higher (Table 1).

Several studies have suggested the Okhotsk people enjoyed a better nutritional status, possibly related to their maritime dietary adaptation. Comparing with the mainland Jomon hunter-gatherers, the Okhotsk populations had better oral health (Oxenham and Matsumura, 2008), lower frequency of linear enamel hypoplasia (Oxenham and Matsumura, 2008), greater demographic proportion of elderly individuals (Nagaoka et al., 2012), and larger body height (Kudaka et al., 2013). Marine mammals provided abundant fat, which is one of the most important energy sources for northern hunter-gatherer populations (Cordain et al., 2000). Although several studies have suggested the frequent appearance of cribra orbitalia (Oxenham and Matsumura, 2008) and heavy workload (Shimoda et al., 2012) in the Okhotsk culture compared with the skeletal populations of mainland Japan, the maritime dietary adaptation apparently enabled the Okhotsk people to settle in the harsh northern environment along the coast of the Sea of Okhotsk.

Conclusions

Carbon and nitrogen isotope ratios suggest that the Moyoro humans and dogs heavily depended on marine foods for their dietary protein intake (Figure 2). The Bayesian mixing model suggests that marine mammals may at most have contributed 80–90% to the human dietary proteins (Table 2, Figure 3B). Previous archaeological and isotopic studies have suggested that the Okhotsk people in Sakhalin and northern Hokkaido relied on marine fish, whereas the population on the east coast of Hokkaido had difficulty in catching marine fish during winter because of drift ice and thus relied on marine mammals (Ono, 1996a, b; Amano 2003; Naito et al., 2010a). Our results are consistent with this suggestion. On the other hand, the Bayesian mixing models suggested that the dietary protein contribution of marine mammals in domesticated dogs was lower than that of humans (Table 2, Figure 3A), suggesting an avoidance of significant dietary overlap between sympatrically living humans and dogs at the Moyoro site. Although the mean adult human isotope ratios did not differ between sexes, the variance of the carbon isotope ratios was significantly greater in males.

Acknowledgments

We are grateful to Koudai Umeda for providing faunal samples, and to Tetsuya Amano and Hiroko Ono for access to and assistance with the Moyoro human skeletal remains. We acknowledge Kyoko Ohara-Kanesawa and Hitoshi Mukai for assistance of some sample preparations and stable isotope analysis, respectively. The authors thank the editors and the anonymous reviewers who provided comments that improved this article. This study was supported in part by Grants-in-Aid for Scientific Research (KAKENHI: 17107006, 18500769, 20370095, 20255007, and 24-785) from the Japan Society for the Promotion of Science.

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