Aztec diets at the residential site of San Cristobal Ecatepec through stable carbon and nitrogen isotope analysis of bone collagen

Abstract

This paper presents the first systematic study of Aztec diets using bone collagen stable carbon and nitrogen isotope analysis. The objective was to identify the dietary patterns of an Aztec community living in the Basin of Mexico. The collection analyzed includes adult and subadult skeletons from the residential site of San Cristóbal Ecatepec (in present-day Mexico State) (A.D. 900–1521). The isotopic data were compared with available ethnohistoric and archeological information about Aztec diets. We conclude that Ecatepec residents consumed mostly C₄ and CAM plants (as opposed to C₃ plants), terrestrial animals, and some lacustrine resources. They shared similar diets and foodways, as indicated by the fact that they belonged to the same socio-economic group and that there were no substantial sex-based differences indicated by the isotopic data. The slightly higher carbon isotope compositions of the men compared with the women could be evidence of pulque consumption by the men—as noted in the ethnohistoric record. The nitrogen isotope compositions of the subadults, however, show some variability related to breastfeeding and weaning practices, with infants weaning between 2 and 4 years of age.

Introduction

The application of stable carbon and nitrogen isotope analysis of human skeletons has been a powerful tool for gaining insights into Mesoamerican foodways and paleodiets. Previous studies include the Maya region (e.g., Metcalfe et al. 2009; Powis et al. 1999; Tykot 2002; White and Schwarcz 1989; White et al. 1993, 2001) and Central Mexico at the Classic period site of Teotihuacan (Casar et al. 2017a, b; Morales Puente et al. 2012; Nado et al. 2017; White et al. 2004a, b). In the case of the Aztecs, most of the evidence about their diets and foodways has been provided by ethnohistoric, iconographic, and archeological records. Direct evidence of Aztec diets via stable isotope analyses of human skeletons has been lacking. This paper presents the first systematic paleodietary study of Aztec diets during the Postclassic period (A.D. 900–1521) via bone collagen stable carbon and nitrogen isotope analysis of adult and subadult skeletons from the residential site of San Cristóbal Ecatepec (in present-day Mexico State) (Fig. 1). Our first objective is to identify the overall dietary patterns of an Aztec community living in the Basin of Mexico using stable isotopes and to compare these patterns with the ethnohistoric and archeological evidence available on Aztec diets. Our second objective is to assess dietary variability within this Aztec community by exploring dietary similarities and differences by sex and age.

Fig. 1

Locations of the site of Ecatepec (star) in the Basin of Mexico and other Central Mexico archeological sites mentioned in the text

San Cristóbal Ecatepec (henceforth referred to as “Ecatepec”) is located northeast of Tenochtitlan, near the Texcoco and Xaltocan lakes (Fig. 1), and is a residential site associated with lacustrine activities that features an ancient open-air artificial mound (tlatel); excavations in 2004–2005 recovered the remains of a house with eight rooms, a patio, a hearth (tecuil), and two pre-Hispanic sweat lodges (temazcales) (García Chávez et al. 2006; Trejo Rangel 2014). The excavations revealed that the site was occupied during the Classic (Tlamimilolpa phase (A.D. 200–350)) and Postclassic periods (Mazapa (A.D. 800–1150), Aztec III (A.D. 1430–1500), and Aztec IV phases (A.D. 1500–1521)). The site may have specialized in the production of salt and textiles, which eventually made it to Tenochtitlan as tribute (Trejo Rangel 2014). According to Trejo Rangel (2014), based on the ethnohistoric sources (i.e., Matricula de Tributos, Lamina 5), this site belonged to the lowest settlement level (i.e., calpulli) of the altepetl system¹ within the Basin of Mexico.

Thirty-four human burials composed of forty-five individuals including infants, children, and adults were found underneath the house floor throughout the site’s occupation (García Chávez et al. 2006; Trejo Rangel 2014). Thirteen burials are associated with the Tlamimilolpa, one burial with the Mazapa, and twenty burials with the Aztec III–IV phases. For the purpose of this study, only Postclassic individuals from primary burials were sampled: ten infants (birth–3 years), three children (4–14 years), and eleven adults (15+ years) (six males, five females), two dating to the Mazapa and twenty-two to the Aztec III–IV phases. Dental pathologies, such as caries, dental calculus, and alveolar resorption, were present in the adult individuals but in low frequencies and at a low level of severity (Trejo Rangel 2014). Other pathologies include osteoarthritis (particularly in the spine) and infectious diseases, expressed in long bones and the clavicle. Given that these individuals resided in a single house, we assume that they belonged to the same socio-economic group. These individuals were long-term residents of the Basin of Mexico, based on the results of their bone phosphate-oxygen isotope analysis (Moreiras Reynaga 2019). Based on the geographic location of this site, the ethnohistoric information available on Aztec diets, and zooarchaeological and paleobotanical evidence recovered from nearby sites, we hypothesize that the individuals at Ecatepec had a diverse and well-rounded diet that included plant and animal protein from terrestrial as well as lacustrine ecosystems.

Stable carbon and nitrogen isotope systematics

Stable carbon (¹³C/¹²C) and nitrogen isotope ratios (¹⁵N/¹⁴N) are expressed in delta (δ) notation relative to an internationally accepted standard in parts per thousand (per mil; ‰): e.g., δ¹³C = (¹³C/¹²C_sample−¹³C/¹²C_standard)/(¹³C/¹²C_standard). The standard (VPDB for δ¹³C and AIR for δ¹⁵N) is assigned a value of 0‰ exactly.

Stable carbon and nitrogen isotope analyses of skeletal remains have been extensively applied in bioarchaeology to reconstruct the diets of ancient populations. Plants, animals, and humans obtain their carbon and nitrogen isotope compositions based on their trophic position in the food web and the foods they consumed during their lifetime. This allows differentiation between consumers (e.g., herbivores, carnivores) and their respective diets. The δ¹³C and δ¹⁵N in bone collagen are commonly preserved after death and hence useful for paleodietary reconstructions—except when chemical alteration or contamination occurs after death.

The main drivers of isotopic variation at the base of food webs in terrestrial and aquatic ecosystems include multiple environmental and climatic factors and the photosynthetic pathways used by plant species (Dawson et al. 2002; Farquhar et al. 1989; Kohn and Cerling 2002; Lee-Thorp 2008). Most plants (e.g., wheat, trees, vegetables, fruits) follow the C₃ photosynthetic pathway, which preferentially incorporates ¹²C. As such, they are characterized by very negative δ¹³C (modern mean = − 27‰; range = − 35 to − 20‰) (Dawson et al. 2002; Kelly 2000; Kohn and Cerling 2002). Conversely, plants following the C₄ photosynthetic pathway incorporate less ¹²C than C₃ plants, and as such C₄ plants have less negative δ¹³C (modern mean = − 12‰; range = − 15 to − 10‰) (Kohn and Cerling 2002). Plants following the CAM photosynthetic pathways (e.g., nopal, maguey) include obligate and facultative varieties and are the most complex isotopically since light, temperature, and other environmental conditions drive their wide-ranging δ¹³C (modern range = − 27 to − 12‰) (O’Leary 1981; White et al. 2004b). Of the CAM plants, the Aztecs mainly consumed prickly pear cactus (Opuntia sp.) and maguey (Agave sp.) in the form of pulque, which have a δ¹³C within the C₄-plant range; as such, these two plant types are commonly grouped together isotopically (i.e., C₄/CAM) in paleodietary studies of Central Mexico (e.g., White et al. 2004b; Nado et al. 2017).

Marine plants tend to resemble the δ¹³C of C₄ terrestrial plants more closely than the C₃ plants, and so the former plants are commonly grouped together in the interpretation of diets as “C₄/CAM or marine sources”. Freshwater ecosystems, however, are very complex and have δ¹³C that are much more variable than marine ecosystems (Casey and Post 2011). Once an individual consumes C₃ and/or C₄/CAM plants, or animals that fed on them, their respective δ¹³C signals become incorporated into that individual’s bone collagen (Lee-Thorp 2008). The carbon isotope composition of bone collagen (δ¹³C_col) primarily reflects the intake of dietary protein (plus a small enrichment in ¹³C, i.e., fractionation) (Ambrose and Norr 1993; Chisholm 1989; Krueger and Sullivan 1984). While a lot of variation in the degree of isotopic fractionation between diet and bone collagen has been reported (e.g., Ambrose and Norr 1993; Froehle et al. 2010), the generally accepted spacing ranges from ~ 1 to 5‰ (Keegan and DeNiro 1988; DeNiro and Epstein 1978, 1981; van der Merwe 1982). In summary, carbon isotope ratios can distinguish three types of diets in consumers (animals and humans): (1) mainly C₃ diet; (2) mainly C₄/CAM or marine diet; or (3) a mixed diet composed of C₃ and C₄/CAM.

Bone collagen nitrogen isotope ratios can inform us about the local food web, the trophic level of consumers, the kind of protein sources consumed, and the level of meat consumption in a consumer’s diet. While multiple climatic and environmental factors can influence plant δ¹⁵N (the base of food webs) (Ambrose 1991, 1993; Szpak et al. 2012, 2013), the overall plant δ¹⁵N range from approximately − 3 to + 6‰ (including N-fixing plants) (Ambrose 1993; Hastorf and DeNiro 1985). Early studies found a “stepwise trophic shift” in food webs involving an increase of 2–6‰ from plants to herbivores, herbivores to omnivores, and omnivores to carnivores (DeNiro and Epstein 1981; Schoeninger and DeNiro 1984; Schoeninger et al. 1983). This positive shift is variable and depends on the specific habitat and food web under consideration (Ambrose 1991, 1993). In the case of humans, those with vegetarian diets have low δ¹⁵N, usually between + 3 and + 7‰ while meat eaters have δ¹⁵N > + 7‰ (Nado et al. 2017; Wright 1994). The marine food webs begin at higher δ¹⁵N and have more trophic levels compared with terrestrial ecosystems. As such, marine food sources have higher δ¹⁵N relative to terrestrial ecosystems (Kelly 2000; Lee-Thorp 2008; Metcalfe et al. 2009). Individuals with a primarily marine diet can have a δ¹⁵N of + 14 to + 17‰ (Lee-Thorp 2008; Nado et al. 2017; Schoeninger et al. 1983). Freshwater ecosystems experience much δ¹⁵N variability so they tend to have higher δ¹⁵N than terrestrial environments but not as high as those of marine ecosystems (Casey and Post 2011; DeNiro and Epstein 1981; Metcalfe et al. 2009). As with bone collagen carbon isotope compositions, the nitrogen isotope composition of bone collagen also reflects the protein content in a consumer’s diet. The fractionation between collagen and diet (Δ¹⁵N_{col − diet}) has been estimated to be + 2.5 to + 3‰ (DeNiro and Epstein 1981; Keegan and DeNiro 1988; Metcalfe et al. 2009). The δ¹⁵N trophic level shift also applies to breastfeeding infants, whose main source of protein is breast milk. Given that they are situated about one trophic level above their mothers, their δ¹⁵N tends to be 2–3‰ higher compared with that of women of reproductive ages from the same population (Fogel et al. 1989; Fuller et al. 2006; White et al. 2001; Wright and Schwarcz 1999). Other factors such as climate, anthropogenic nutrient inputs like human sewage in lakes, physiology, water deprivation, stress, and disease can also influence the δ¹⁵N signal incorporated into human bone collagen (Ambrose 1991; Katzenberg and Lovell 1999; Reitsema 2013; Olsen et al. 2014, 2018).

Bone continues to remodel by resorption of older bone and the formation of new bone throughout an individual’s lifetime (Sealy et al. 1995). The precise turn-over rates for bone of healthy individuals are not certain, but we know that bone remodeling depends on age as well as bone type (e.g., rib vs. femur) and its composition: cortical vs. trabecular (Cox and Sealy 1997; Sealy et al. 1995; Tsutaya and Yoneda 2013). Infants experience the highest turn-over annually (100%), followed by children (70–50%), juveniles (30–10%), and adults (0.3–3%) (Cox and Sealy 1997; Martin et al. 2015; Tsutaya and Yoneda 2013; Welle 1999). As a result, the bulk bone collagen δ¹³C and δ¹⁵N provide an average isotope composition of an individual’s lifetime—at least the last 10 or more years of life in adults and less time in children depending on age at death (Ambrose 1993; Kohn and Cerling 2002; White et al. 2000, 2004b).

Aztec diets based on ethnohistoric and archeological evidence

While Spanish chroniclers, like Bernardino de Sahagún, Diego Durán, Hernán Cortés, and Bernal Díaz del Castillo, among others, wrote about the Aztecs with particular intentions and biases, these ethnohistoric accounts provide researchers with valuable information about Aztec foodways and their connection to social, economic, political, and religious practices (Coe 1994; Staller and Carrasco 2010). Moreover, archeological studies in the Basin of Mexico and surrounding areas throughout the Postclassic period (A.D. 900–1521) have uncovered a wide range of data on Aztec foodways, including information on food processing, cooking techniques, and related utensils and serving vessels (Brumfiel 2009; Parsons 2010; Smith and Price 1994; Smith et al. 2002; Smith 2016).

Here we first provide an overview of the ethnohistoric and archeological evidence for food sources or menu items available to the Aztecs in their everyday diets. This helps to establish a dietary baseline against which to evaluate the diets of the Aztec residents at Ecatepec as determined from their collagen stable isotope compositions.

Overview of ethnohistoric evidence of Aztec foods

Maize (Zea mays) was the principal Aztec staple consumed in a variety of solid (e.g., tortillas, tamales) and liquid (e.g., gruel (atole)) forms, and it was highly regarded in ceremonial and symbolic contexts (Berdan and Anawalt 1997; Sahagún 1932; Smith 1996). Beans (Phaseolus vulgaris), another Aztec staple, were consumed with maize. This food combination provides full protein intake since beans contain amino acids lacking in maize (i.e., lysine and tryptophan) (Bressani et al. 1962; Coe 1994; Deng et al. 2017). Other common plants consumed by the Aztecs included amaranth (Amaranthus sp.), tomato (Physalis), avocado (Persea), squash (Cucurbita sp.), chili peppers (Solanaceas), chia (Salvia sp.), prickly pear cactus (nopal) and its fruit tuna (Opuntia), and maguey (Agave sp.; as a nutritious beverage or fermented drink called octli in Nahuatl and pulque in Spanish, and in its raw and fermented form called aguamiel in Spanish) (Smith 1996; Soule 1979).

Díaz del Castillo’s (2008: 174–175) description of the Tlatelolco market (tianquitz) mentions a type of blue-green spirulina algae (tecuitlatl) that grew on lake surfaces, which was collected by fishers and prepared for consumption: “…and the fisherwomen and others who sell some small cakes made from a sort of ooze which they get out of the great lake, which curdles, and from this they make a bread having a flavour something like cheese”. Smith (1996) and others (Furst 1978; Parsons 2010; Ortíz de Montellano 1990) point out that the algae is very nutritious, rich in protein, and easily accessible for those who resided near the Basin of Mexico lakes. Cortés mentions that at the same market, “there is every sort of vegetable, especially onions, leeks, garlic, common cress and watercress, borage, sorrel, teasels and artichokes; and there are many sorts of fruit…” (1986:103–104).

Several plant foods were also imported to the Basin of Mexico through the Aztec tribute system. For instance, the Mesoamerica region referred to as the “hot country” by Durán (1964:129) (i.e., Southern Mexico and Central America) sent a range of fruits, including “pineapples, numerous types of sweet fruits such as guavas, wild pear-like fruits, yellow, black, and white sapotas, avocados, and two or three kinds of yams”. Aztecs also used a range of plants as condiments, including coriander, fennel, and anise-like vegetables, and epazote (Chenopodium spp.), which was added to flavor drinks and tamales (Picó and Nuez 2000).

Aztec foods included meat from a range of animal species. According to Sahagún, some of the tortilla fillings included turkey eggs and water-fly eggs, and meats sold at the markets included “rabbit meat, hare, opossum, venison, and “meat of wild beasts”…duck, crane, goose, mallard, quail, and eagle” (Berdan and Anawalt 1997:155, 218). Díaz del Castillo (2008:174) mentions that the Mexicas at Tlatelolco “sold fowls, cocks with wattles, rabbits, hares, deer, mallards, and young dogs,” while Cortés (1986:103–104) provides an even more detailed account of the animals sold for consumption at the same market:

There is a street where they sell game and birds of every species found in this land: chickens [turkeys], partridges and quails, wild ducks, flycatchers, widgeons, turtledoves, pigeons, cane birds, parrots, eagles and eagle owls, falcons, sparrow hawks and kestrels…They sell rabbits and hares, and stags and small gelded dogs which they breed for eating…They sell chicken [turkey] and fish pies, and much fresh and salted fish, as well as raw and cooked fish. They sell hen and goose eggs, and eggs of all the other birds I have mentioned.

Durán describes the vast number of dogs that were sold at the Acolman market for consumption (Berdan and Anawalt 1997). The markets of Xochimilco, Zumpango, and Toluca sold the Mexican salamander or axolotl (Ambystoma mexicanum) that lived in the Basin lakes, alive or roasted for consumption (Tate 2010). Durán (1964:129) also mentions the terrestrial and aquatic species brought to Tenochtitlan as part of tribute payments:

From other places were brought deer and rabbits and quails, some uncooked and other in barbecue. As tribute also came gophers, weasels, and large rodents, which thrive in the woods. Toasted locusts, winged ants, large cicadas and little ones, in addition to other small animals. Those living near lagoons sent everything that thrives in the water, such as algae, a certain type of insect that walks upon the water, and small worms…

In summary, the chroniclers’ descriptions include food from the following groups: (1) grains, (2) legumes, (3) vegetables and fruits, (4) birds and waterfowl (terrestrial and lacustrine), (5) small game and insects, (6) lacustrine plants and animals, and (7) marine resources. This information reveals the wide variety of fauna and flora available for consumption in the Basin of Mexico (and beyond), even though it is likely that not everyone in Aztec society had access to these foods or consumed them with the same frequency due to socio-economic differences.

Overview of archeological evidence of Aztec menu items

Carbonized plants such as maize and beans and skeletal remains of various animal species have been recovered from domestic contexts at several Postclassic Aztec city-states (Smith 2008). While this survey is not exhaustive, our goal here is to discuss the main animal and plant “menu” items that were part of the everyday Aztec “food plate” at multiple Postclassic Central Mexico archeological sites—Calixtlahuaca, Capilco, Chalco, Cuexcomate, El Japón, El Tlatoani, Melones, Mexicaltzingo, Xaltocan, Yautepec, and Zultepec-Tecoaque (Fig. 1). A summary of the animal and plant species recovered by archeological site is provided in Supplementary Tables 1 and 2.

At the in-land sites of Calixtlahuaca (near Toluca), Zultepec-Tecoaque (Tlaxcala), El Tlatoani, Yautepec, Capilco, and Cuextomate (Fig. 1) mammals such as dog and white-tailed deer as well as birds such as turkey were the most abundant fauna recovered (Corona et al. 2015; Fauman-Fichman et al. 2019; Heath-Smith and Wharton 2015; Manin 2017; Valadez Azúa and Rodríguez Galicia 2014). Smaller fauna such as rabbit, peccary, fox, and armadillo were also present at most of these sites. Turtle was a common lacustrine fauna encountered at Calixtlahuaca, Capilco, Cuextomate, and Yautepec, but only the latter two sites had evidence of fish consumption.

Dog, turkey, rabbit, and, to a lesser extent, white-tailed deer were also recovered from archeological sites closer to the Basin of Mexico lakes including Mexicaltzingo, El Japón, Chalco, Melones, and Xaltocan (Fig. 1) (Ávila López 2006; Corona 1997; Guzmán and Polaco 2008; Valadez Azúa and Rodríguez Galicia 2005, 2014). At the Melones site, Valadez Azúa and Rodríguez Galicia (2014) encountered turkey remains while excavating a kings’ palace with domestic areas. The thighs, legs, and breasts were found in the kings’ spaces, while head, feet, uropygium (parson’s nose), and thoracic cavity remains were recovered from kitchen areas. Moreover, a wider range of fauna, including lacustrine species, were recovered at these sites. At these Basin of Mexico sites (except for Melones), amphibians (e.g., frogs), reptiles, a range of lacustrine waterfowl (e.g., ducks), fish, and marine mollusks and oysters were recovered (Supplementary Table 1). In summary, since these sites are located near the Basin of Mexico lakes, it appears that the Aztecs took advantage of their environment by consuming a variety of lacustrine species, in addition to more common terrestrial species.

Macro- and/or micro-botanical (e.g., pollen) remains of edible plants were recovered from the Aztec sites of Xaltocan, Mexicaltzingo, Capilco, Cuexcomate, Yautepec, and Chalco (only maize cobs were found at this site) (Fig. 1; Supplementary Table 2) (Ávila López 2006; Fauman-Fichman and Smith 2015; Fauman-Fichman et al. 2019; Guzmán and Polaco 2008; Heath-Smith and Wharton 2015; McClung de Tapia and Martínez Yrizar 2005; Valadez Azúa and Rodríguez Galicia 2005). The main C₄ plant was maize (Zea mays), while at most sites, varieties of another C₄ plant, amaranth (Amaranthus sp.), and its genetic relatives such as quelite (Amaranthus hybridus) and seepweed (Suaeda torreyana), were also present. The most common CAM plants are members of the Opuntia, Cactaceae, and Agavaceae families. These include plants such as prickly pear cactus and maguey. At Xaltocan, bilberry cactus (Myrtillocactus geometrizans) was also found, while a succulent known as purslane or verdolaga (Portulaca oleracea) was present at Xaltocan, Capilco, Cuexcomate, and Mexicaltzingo.

Several C₃ plants were recovered from these sites (Supplementary Table 2) (Ávila López 2006; Fauman-Fichman and Smith 2015; Fauman-Fichman et al. 2019; Guzmán and Polaco 2008; Heath-Smith and Wharton 2015; McClung de Tapia and Martínez Yrizar 2005; Valadez Azúa and Rodríguez Galicia 2005). Legumes (Leguminosae) were encountered, including beans (e.g., Phaseolus coccineus, Phaseolus vulgaris). Varieties of tomato (e.g., groundcherry, red tomato, tomatillo) and chili and bell peppers (Physalis and Solanaceas) were also found. Fruits of Chenopodium such as epazote were recovered at most sites. At Xaltocan and Mexicaltzingo, chia (Salvia sp.) and lacustrine plants such as bulrush (Scirpus), pondweed (Potamogeton), and spiral ditch grass (Ruppia) were identified, consistent with proximity to the Basin of Mexico lakes. At Capilco, Cuexcomate, and Xaltocan, mesquite from the leguminous family (Prosopis) was recovered. Fruits such as avocado (Persea), cherry (e.g., Prunus seretina), and soursop (Annona) as well as other vegetables like squash (Cucurbita sp.) and potato (e.g., Solanum rostratum) were found at Capilco, Cuexcomate, and Mexicaltzingo. Other edible C₃ flora recovered from at least one of these Aztec sites include nettles or ortiga (Celtis), zapote (Casimiroa zapote), grape (Ribes), chestnuts (Fagaceae), and cacao (Theobroma cacao). Soursop and cacao are exotic to the Basin since they grow in sub-tropical and tropical areas of Southern Mexico as well as Central and South America.

Based on the evidence discussed above, several aspects of the Aztec diets stand out. First, a wide diversity of animal and plant species were used in Central Mexico. This reflects the diverse ecosystems in this region that allowed many species to flourish as well as the range of Aztecs’ activities, including hunting, fishing, farming, and managing domesticated animals (i.e., dog, turkey). Hence, there were likely plenty of meat and plant sources in Central Mexico available to the Aztecs. While some exotic plant and animal foods were identified at the Aztec markets and at a few archeological sites (e.g., Xaltocan, Chalco), the vast majority of food items consumed by the Aztecs in everyday life were indigenous to Central Mexico (or temporary residents in the case of migratory birds).

Second, the faunal results illustrate that the staple animals in the Aztec food plate included turkey, dog, rabbit, and white-tailed deer, and in some cases, waterfowl species (terrestrial and lacustrine), and a few reptiles and amphibians. The paleobotanical evidence available suggests that the Aztecs incorporated a variety of Central Mexico C₃, C₄, and CAM plants into their everyday food plate such as maize, beans, amaranth, chili peppers, tomato, squash, cacti, maguey, chia, avocado, and a variety of fruits and condiments. Third, the archeological evidence indicates that the Aztecs also took advantage of other food sources that were available depending on their particular ecological setting. For instance, Chalco, Mexicaltzingo, and Xaltocan are located near the Basin of Mexico lakes, which were home to a variety of readily accessible lacustrine species, including fish, waterfowl, and lacustrine plants.

The absence of fish remains at some sites (e.g., El Japón) may be a result of taphonomic processes (Serra and Valadez 1986) or different data collection methods. Additionally, species of small fish (e.g., charales (Christoma)) inhabit the Basin lakes and their bones are thin and soft enough for people to consume them whole in present-day Mexico (Muñoz Zurita 2012) and they may have done so in the past (Widmer and Storey 2017). While the zooarchaeological evidence is limited for most of the Central Mexico sites reviewed, consumption of lacustrine foodstuffs, including fish, is supported by the ethnohistoric and historic records. Sahagún mentions that lacustrine resources were important for the Aztecs, and more specifically for those from the lower socio-economic classes. A wide diversity of edible lacustrine species that inhabited the Basin of Mexico lakes could be obtained with little effort. Also, there was open access to the lakes, allowing everyone to easily acquire fish and other lacustrine food sources (Parsons 2010; Valadez Azúa and Rodríguez Galicia 2014).

It has been argued that access to some foodstuffs (e.g., meat) during the Classic period was largely determined by socio-economic status in large urban centers in Mesoamerica, such as Teotihuacan (Morales Puente et al. 2012; Nado et al. 2017; Valadez Azúa and Rodríguez Galicia 2014). This pattern seems to have continued through the Postclassic period, as exemplified by the Melones site, where the kings had access to meat products (the meatier cuts of turkey) (Valadez Azúa and Rodríguez Galicia 2014). Valadez Azúa and Rodríguez Galicia (2014:155) argue that the kings hosted “feasts at the plazas where they had different kinds of dishes that were shared with the community”. This suggests that the elite members of Aztec society (e.g., city-state rulers, priests, the Aztec Emperor and his advisors) could consume meat frequently and in large quantities since they were wealthy and could purchase a variety of animal products. As for the lower socio-economic classes (e.g., commoners, slaves), they mainly consumed foods that they could access (mostly plant-based as well as lacustrine foods from the Basin lakes).

Materials and methods

Sample preparation and collagen extraction

Stable carbon and nitrogen isotope analysis was carried out on 24 individuals (2 Mazapa and 22 Aztec III–IV) from the site of Ecatepec. Ribs (n = 22) and femoral fragments (n = 2) were sampled from the skeletons. Bone sample weights ranged from 1.4 to 6.0 g, and a Dremel® rotary tool was used to extract the femoral fragments. Trabecular bone was mechanically removed using a dental pick leaving only cortical bone for analysis; the latter remodels more slowly than the former and is less likely to have undergone post-mortem alteration (Cox and Sealy 1997; Kohn and Cerling 2002; Sealy et al. 1995). Samples were cleaned with a toothbrush and deionized water followed by sonication in 40-mL beakers for ~ 15 min to remove remaining debris and dried overnight at 90 °C. The samples were then ground with a ceramic mortar and pestle to obtain a uniform size (< 850 μm) for collagen extraction.

Bone collagen was extracted following a modified Longin (1971) method as described in Olsen et al. (2018). To remove lipids, each sample (~ 500 mg) was soaked three times in 2:1 chloroform:methanol for 15 min and subsequently dried at room temperature overnight (Folch et al. 1957). The inorganic component of samples was dissolved in a 0.25-M HCl for 24 h, followed by a 0.5-M HCl solution until they reached full demineralization (8–10 days). The amount of humics was initially substantial in 16 out of the 24 samples since 5 to 7 rinses with 0.1 M NaOH were required to remove these contaminants. Once the collagen was fully solubilized and dried, ~ 0.4 mg was weighed into tin capsules for isotope analysis.

Isotopic analysis

All isotopic analyses were carried out in the Laboratory for Stable Isotope Science (LSIS) at The University of Western Ontario, London, Ontario, Canada. Bone collagen samples and standards were combusted at 1020 °C using a Costech 4010 Elemental Combustion System (EA). The produced gases were swept through a heated (60 °C) GC separation column using helium as the carrier gas to a Thermo Scientific™ DELTA^plus XL® isotope ratio mass spectrometer (IRMS) operating in continuous-flow (CF) mode via a Conflo III.

The carbon and nitrogen isotope compositions were calibrated to the international standards Vienna PeeDee Belemnite (VPDB) and atmospheric N₂ (AIR), respectively. The calibration standards used for δ¹³C_col and δ¹⁵N_col were USGS40 (L-glutamic acid, accepted δ¹³C = − 26.39‰, δ¹⁵N = − 4.52‰) and USGS41 (L-glutamic acid, accepted δ¹³C = + 37.63‰, δ¹⁵N = + 47.57‰) (Qi et al. 2003). The international standard IAEA-CH-6 (accepted δ¹³C = − 10.45‰; Coplen et al. 2006) and the LSIS internal keratin standard (MP Biomedicals Inc., Cat No. 90211, Lot No. 9966H) (accepted δ¹³C = − 24.04 ± 0.12‰, δ¹⁵N = + 6.36 ± 0.15‰; LSIS running value) were analyzed to monitor analytical accuracy throughout each session. Every 10–12 collagen samples were analyzed in duplicate to monitor analytical precision and methodological consistency (based on independent sample aliquots).

Instrument precision based on the calibration standards (n = 9) was ± 0.10‰ and ± 0.41‰ for δ¹³C and δ¹⁵N, respectively. The repeatability of isotopic measurements assessed with sample duplicates (n = 3) was ± 0.1‰ or better. The average reproducibility of the same sample using an independent extraction of collagen was better than ± 0.1‰ (n = 3) for δ¹³C and δ¹⁵N. A mean δ¹³C of − 10.34 ± 0.05‰ (n = 3) was obtained for IAEA-CH-6 and mean values of δ¹³C = − 24.00 ± 0.04‰ and δ¹⁵N = + 6.36 ± 0.05‰ (n = 5) were obtained for the LSIS keratin standard, all of which compare well with their accepted values. Based on these check standards, analytical accuracy is better than ± 0.04‰ for δ¹³C and ± 0.05‰ for δ¹⁵N.

Data treatment and floral and faunal baseline isotope data

While the fractionation between human collagen and diet (Δ ¹³C_{col − diet}) in mixed diets has been found to be quite variable (e.g., Froehle et al. 2010), in this study, we use the standard fractionation of + 5‰ in our interpretation (Table 1; DeNiro and Epstein 1978, 1981; Keegan and DeNiro 1988). Similarly, we have corrected our human δ¹⁵N_col data by − 3.0‰ to estimate δ¹⁵N_diet (Table 1). We also compiled the available floral and faunal carbon and nitrogen isotope data for the Basin of Mexico (e.g., Xochimilco, Texcoco, Xochicalco, Teotihuacan) to aid our interpretation of the isotope data for the Ecatepec individuals (Casar et al. 2017a; Lounejeva-Baturina et al. 2006; Manin et al. 2018; Morales Puente et al. 2012; Somerville et al. 2017). For comparison with the Ecatepec diets (i.e., δ¹³C_diet and δ¹⁵N_diet), we have corrected the faunal collagen δ¹³C by − 2.0‰ to reflect flesh δ¹³C (following DeNiro and Epstein 1981) (Table 2). There is no significant difference between flesh and collagen δ¹⁵N; hence, no further correction is required (DeNiro and Epstein 1981). The modern fauna and flora δ¹³C have been corrected by + 1.5‰ to account for the “Suess effect”.² The baseline isotope data are presented in Table 2 and illustrated in Fig. 2.

Table 1 Carbon and nitrogen isotope data and collagen quality indicator results for the Ecatepec collection

Table 2 Summary δ¹³C and δ¹⁵N data for modern and archeological flora and fauna^a

Fig. 2

Ecatepec δ¹³C_diet and δ¹⁵N_diet plotted alongside results for humans from other Basin of Mexico archeological sites, as well as Basin of Mexico plant and animal δ¹³C and δ¹⁵N data. Isotope data for humans as well as flora and fauna are plotted with their respective mean and range (SD, whiskers). “M” = modern and “A” = archeological flora and fauna. Color of means for flora and fauna indicates ecosystem (dark blue = marine; light blue = riverine; orange = terrestrial). The δ¹³C for modern C₃, C₄, and CAM plants, insects, and riverine and marine fish has been corrected for the “Suess effect” by + 1.5‰. The δ¹³C for collagen from archeological fauna has been corrected by − 2.0‰ to reflect flesh values. Repeatability of individual measurements was generally better than ± 0.1‰ (SD) for both carbon and nitrogen isotopes

While there is ethnohistoric and archeological evidence to suggest that the Aztecs consumed food sources from the Basin of Mexico lakes, there are several challenges in reconstructing this aquatic ecosystem’s food web, and consequently, isotopic compositions during the Postclassic period. First and foremost, this water body of interconnected lakes has been mostly drained after the Spanish invasion and subsequent settlement of a growing population in the territory that today encompasses Mexico City. As a result, the original lake system has been lost to human settlement activities over the past ~ 400 years. Second, the surviving body of water, Xochimilco Lake, has undergone massive hydrological and environmental alteration, primarily due to agricultural production, use of fertilizers, groundwater pumping, wastewater inputs, and the introduction of exotic fish species such as carp (Cyprinus carpio) and tilapia (Oreochromis niloticus) (Zambrano et al. 2010). While there are isotopic data for modern species within this lake (Zambrano et al. 2010), these results, especially for δ¹⁵N, likely do not provide a representative baseline for reconstructing Aztec diets, as has been demonstrated previously for other anthropogenically impacted terrestrial and lacustrine localities (e.g., Guiry 2019). Others have recovered archeological aquatic fauna from Aztec sites but a systematic stable isotope analysis of these samples has yet to be undertaken. Below, we include available δ¹³C and δ¹⁵N of modern freshwater and marine fish (Table 2) collected by Morales Puente et al. (2012) for their paleodietary study at Teotihuacan that are not from the Xochimilco Lake system and do not seem to have been affected by anthropogenic factors, as a way to tackle some of these challenges by incorporating some aquatic baseline isotope data in our paleodietary study.

Most Aztec plant foods including legumes, vegetables, and fruits fall into the C₃ category. The Valley of Mexico C₃ plants have a δ¹³C ranging from − 26.8 to − 18.3‰ (Table 2; Lounejeva-Baturina et al. 2006; Morales Puente et al. 2012). Aztec C₄ plant staples include amaranth (δ¹³C_mean = − 12.6 ± 1.2‰) and maize (δ¹³C_mean = − 10.2 ± 0.4‰). The CAM plants (e.g., prickly pear cactus, tuna fruit, maguey) have δ¹³C (range = − 14.3 to − 10.2‰) that overlap with the C₄-plant range (− 13.6 to − 9.9‰) in this region (Table 2). It is evident isotopically that cougars (δ¹³C_mean = − 21.1 ± 0.6‰) had a C₃ diet and consumed animals with C₃ diets. There is much more variability in the diets of rabbits and hares; some had a C₃ diet, while others had a mixed C₃/C₄ diet (Table 2; Casar et al. 2017a; Somerville et al. 2017). The opossum (tlacuache) (δ¹³C = − 14.6‰) and the domesticated turkey (δ¹³C_mean = − 14.0 ± 3.3‰) both consumed C₃ and C₄/CAM plants (Table 2; Manin et al. 2018; Morales Puente et al. 2012). The domesticated dog (δ¹³C_mean = − 9.7 ± 0.3‰) mostly consumed C₄/CAM food sources. Modern insect samples from the Oaxaca Valley can be divided into C₃ and C₄/CAM consumer groups (Table 2; Warinner 2010). Modern muscle samples of marine and riverine fish, which would have been exotic to Teotihuacan, have a δ¹³C_mean = − 15.3 ± 1.2‰ and − 21.0‰, respectively (Table 2; Morales Puente et al. 2012).

A modern bean sample has a δ¹⁵N of − 1.5‰, while the rest of the C₃ non nitrogen-fixing plants have a δ¹⁵N_mean of + 3.7 ± 3.1‰ (range = − 1.0 to + 8.4‰) (Table 2; Lounejeva-Baturina et al. 2006; Morales Puente et al. 2012). The C₄/CAM plants have a δ¹⁵N_mean of + 2.9 ± 2.9‰ (range = − 2.0 to + 9.2‰). Herbivores such as rabbits have a δ¹⁵N_mean = + 5.6 ± 2.2‰ and an omnivore like the opossum has a δ¹⁵N = + 8.9‰ (Table 2; Morales Puente et al. 2012; Somerville et al. 2017). Insects consuming C₃ or C₄/CAM plants have δ¹⁵N_means = + 6.1 ± 1.6‰ and + 5.5 ± 1.0‰, respectively (Table 2; Warinner 2010). Two domesticated animals, turkey and dog, have δ¹⁵N_means = + 6.6 ± 1.7‰ and + 9.7 ± 0.9‰, respectively (Table 2; Manin et al. 2018; Morales Puente et al. 2012). The dogs’ δ¹³C and δ¹⁵N indicate that they were likely given similar foodstuffs as those consumed by the Teotihuacanos (Morales Puente et al. 2012). The two riverine fish have a δ¹⁵N = + 7.7 and + 10.5‰, while the δ¹⁵N of marine fish ranges from + 12.2 to + 12.7‰ (Table 2; Morales Puente et al. 2012). The freshwater fish have higher δ¹⁵N than terrestrial animals, while marine fish have even higher δ¹⁵N.

Results

Bone collagen preservation

We considered the common quality indicators in tandem to evaluate the preservation of the bone collagen before the δ¹³C_col and δ¹⁵N_col analysis and interpretation of these data (Table 1). Well-preserved collagen has elemental abundances of carbon (wt% C) and of nitrogen (wt% N) of at least 35% and 11%, respectively (Ambrose and Norr 1992; van Klinken 1999). The bone collagen samples average 41.9 ± 1.5 wt% C (range = 38.0–44.2%) and 15.0 ± 0.6 wt% N (range = 13.6–16.0%). The atomic C:N ratio for well-preserved collagen has been considered to range between 2.9 and 3.6 (Ambrose 1993; DeNiro 1985), but more recently van Klinken (1999) proposed a tighter range of 3.1 to 3.5. Our collagen samples fit well within this tighter range, averaging 3.25 ± 0.0 and ranging from 3.2 to 3.3. A third quality control measure is collagen yield (wt.% col), given that it decreases variably from the time the bone was buried, depending on burial environment (e.g., wet, dry, sub-tropical) (van Klinken 1999). Modern bone contains 22–25% collagen by weight. Archeological bones tend to have collagen yields ranging from 20 to 1%; at < 1%, the collagen is likely too degraded for the preservation of original δ¹³C_col and δ¹⁵N_col (Ambrose 1993; van Klinken 1999). Our sample set has a mean collagen yield of 10.2 ± 5.4 wt.% (range = 3.0–17.5 wt.%). Based on these quality indicators, all 24 bone samples analyzed have well-preserved collagen for which the δ¹³C_col and δ¹⁵N_col can be considered original. Table 1 presents a summary of the Ecatepec bone collagen samples and their carbon and nitrogen isotope results and quality indicators.³

Carbon and nitrogen isotope data

Figure 3 illustrates Ecatepec bone collagen carbon and nitrogen isotope results (Table 1) plotted by age group. This collection has an overall mean δ¹³C_col = − 8.1 ± 0.7‰ (n = 24) (range = − 9.4 to − 6.7‰). The children (n = 3) and adults (n = 11) have a similar mean δ¹³C_col (− 8.7 ± 0.4‰ and − 8.2 ± 0.6‰, respectively) but the adults have a slightly wider range (− 9.2 to − 7.2‰) compared with the children (− 9.0 to − 8.3‰). This suggests that there was more variability in the intake of C₃ and C₄/CAM foods by the adults, while the children consumed slightly less C₄/CAM foods than some adults (Fig. 3). The lack of δ¹³C_col variability in the children group may also arise from the smaller sample size compared with the adult group. The infants (n = 10) have a slightly less negative mean δ¹³C_col (− 7.8 ± 0.7‰) and the widest range within this collection (− 9.4 to − 6.7‰) (Fig. 3), likely because of different feeding practices.

Fig. 3

Ecatepec bone δ¹³C_col and δ¹⁵N_col plotted by age group. Repeatability of individual measurements was generally better than ± 0.1‰ (SD) for both carbon and nitrogen isotopes

This collection has an overall δ¹⁵N_col mean of + 12.3 ± 1.6‰ (range = + 9.0 to + 15.6‰). The children and adults have similar δ¹⁵N_col means (+ 11.6 ± 1.0‰ and + 11.3 ± 0.9‰, respectively), but the adults have a wider δ¹⁵N_col range (+ 9.0 to + 12.2‰) relative to the children (+ 10.8 to + 12.7‰) (Fig. 3). One 4-year-old child (DM 90) has a δ¹⁵N_col that is + 1.4‰ higher than the adult δ¹⁵N_col mean (+ 11.3‰) (Fig. 3), which may be indicative of a partial nursing signal and reflect a weaning transition. There are differences in δ¹⁵N_col within the adult group, which suggest that some adults consumed less animal protein than others; DM 88 is the adult male who consumed the least amount of meat, based on the lowest δ¹⁵N_col (+ 9.0‰) (Fig. 3). The infants have the highest mean δ¹⁵N_col + 13.7 ± 1.4‰ (range = + 12.1 to + 15.6‰) (Fig. 3). The difference between the infant and adult δ¹⁵N_col means suggests that many infants (birth to 3 years) were nursing up until death.

Discussion

What did the Aztecs consume at Ecatepec?

The Ecatepec individuals’ δ¹³C_diet and δ¹⁵N_diet are plotted alongside the Basin of Mexico floral and faunal δ¹³C and δ¹⁵N in Fig. 2. These Aztecs primarily consumed C₄/CAM foods (e.g., maize, amaranth, nopal, pulque), which have higher δ¹³C (δ¹³C_C4/CAM = − 14.3 to − 9.9‰). There is a δ¹³C_diet variability of approximately ± 2‰ among the adults. Those who consumed more C₃ foods (and perhaps some lacustrine foods with more negative δ¹³C relative to those of C₄/CAM foods) have slightly more negative δ¹³C_diet (e.g., DM 84 = − 13.4‰; DM 85 = − 13.4‰; DM 88 = − 14.1‰; DM 94 = − 14.2‰) (Table 1). As with these adults, the three children also incorporated more C₃-type foods into their diets. As for the infants, their wide range of δ¹³C_diet (Table 3) is indicative of the variability in their intake of C₃ and C₄/CAM foods, directly or via breast milk.

Table 3 Summary of δ¹³C and δ¹⁵N data for humans from Ecatepec and other Basin of Mexico archeological sites^a

Most Ecatepec individuals and terrestrial omnivores, such as the dogs and the opossum, have similar δ¹⁵N (Fig. 2), which could be reflecting the consumption of these animals by humans but also that such animals thrived in human-modified environments eating the foods that humans left behind (e.g., scraps, organic refuse). These results indicate that (1) most individuals (except for an adult male (DM 88)) incorporated sufficient animal protein in their everyday diets, and (2) the animals consumed include those from the terrestrial ecosystem such as opossum, dog, turkey, insects, and perhaps in some cases from the Basin of Mexico lacustrine ecosystem. Based on the δ¹³C_diet and δ¹⁵N_diet, it is very likely that these Aztec residents consumed C₄/CAM plants directly, as well as through consuming C₄/CAM-fed terrestrial animals, particularly turkey and dog (Fig. 2). As for the infants, at least five were nursing, as indicated by their higher δ¹⁵N_col relative to the other individuals (Figs. 3 and 4) (see discussion below). These infants also have less negative δ¹³C_col. This result could suggest a maternal diet primarily based on C₄/CAM foods that was passed on to the infants through breast milk, as well as a trophic level effect of about + 1‰ (Fuller et al. 2006). In summary, the diets of Aztecs at Ecatepec incorporated mainly C₄/CAM plants, some C₃ plants, C₄/CAM-fed terrestrial animals, and perhaps some lacustrine foods. These results correlate well with the overall zooarchaeological and paleoethnobotanical menu items recovered at other Postclassic Aztec sites.

Fig. 4

Bone δ¹³C_col and δ¹⁵N_col of the Ecatepec subadults plotted by age. Repeatability of individual measurements was generally better than ± 0.1‰ (SD) for both carbon and nitrogen isotopes

Could the Aztecs have consumed lacustrine foods at Ecatepec?

As discussed above, anthropogenic alteration of the Basin of Mexico lakes makes it challenging to assess pre-Hispanic stable carbon and nitrogen isotope compositions using what is left of the ecosystem in present-day Xochimilco Lake. The absence of δ¹³C and δ¹⁵N data for archeological lacustrine species indigenous to the Basin of Mexico makes it difficult to evaluate quantitatively the possible consumption of lacustrine foods by the Ecatepec residents. Even so, the isotope results collected thus far are sufficient to hypothesize that there was at least some consumption of lacustrine foods.

The δ¹³C_diet of the Ecatepec children and adults (range = − 14.0 to − 12.2‰) (Table 3) is more negative than the δ¹³C of dogs, which had a 100% C₄ diet (Fig. 2); the dogs likely ate the old crusty tortillas that people gave them (as is often the case today) but no other vegetables and condiments (Morales Puente et al. 2012). These Ecatepec residents also have δ¹⁵N_diet (range = + 7.8 to + 9.7‰) (except for DM 88) (Table 3) situated within the δ¹⁵N range (+ 7.7 and + 10.5‰) of freshwater fish from a riverine context (Fig. 2). However, these riverine fish, albeit from outside of the study area, have much lower δ¹³C (range = − 22.1 to − 19.8‰) (Table 2). There is some evidence, however, that the lacustrine food web within the study area may have a high δ¹³C. A pre-agricultural (12,683 cal. BP⁴) adult woman from Peñón III (Peñón de los Baños), an island in Lake Texcoco (Fig. 1) who had a diet primarily based on freshwater/lacustrine foods, has a δ¹³C_diet of − 16.6‰ and a δ¹⁵N_diet of + 10.6‰ (Fig. 2; Table 3) (González et al. 2003; Casar et al. 2017b). Given that this woman did not incorporate C₄/CAM plants (particularly maize) heavily into her diet, her δ¹³C_diet suggests that the lacustrine food web in the Basin of Mexico likely had higher δ¹³C, not that dissimilar from the δ¹³C of a diet dominated by C₄/CAM foods. Additionally, her δ¹⁵N_diet is higher than all but one δ¹⁵N of the terrestrial omnivores and only slightly more elevated (by ~ 2‰) than the mean δ¹⁵N_diet of the Ecatepec residents (Fig. 2; Table 3).

To further test for the presence of lacustrine foods in the Ecatepec residents’ diets, we compared their δ¹³C_diet and δ¹⁵N_diet with those of four other pre-Hispanic human groups recovered from other Basin of Mexico archeological sites between the Classic (AD 200–550) and Postclassic (AD 900–1521) periods. These groups include Xaltocan and the Teotihuacan (Fig. 1) residential districts (barrios) of Teopancazco, La Ventilla, and San José 520 (Fig. 2, Table 3) (Casar et al. 2017b; Danforth 2000; Morales Puente et al. 2012; Nado et al. 2017). While all of these groups have overlapping mean δ¹³C_diet, which reflect the primary consumption of C₄/CAM foods (60–70% of the overall diet and 50% C₄ protein), there are clear differences in δ¹⁵N_diet (Fig. 2, Table 3). Xaltocan and the three Teotihuacan districts all have mean δ¹⁵N_diet, ranging from + 5.3 to + 7.1‰. By comparison, the Ecatepec adults and children (excluding the nursing infants) have means δ¹⁵N_diet of + 8.3‰ and + 8.6‰, respectively, which positions them a trophic level above the individuals from the other Basin of Mexico archeological sites (Fig. 2, Table 3). This suggests that while the Xaltocan and Teotihuacan individuals had mostly terrestrial diets, the Ecatepec group likely included lacustrine foods in their diets in addition to terrestrial resources. At Teopancazco, there are two outliers (burials 105 and 112), who likely consumed more C₃ (50% of the overall diet) and marine foods, based on their lower δ¹³C_diet (− 15.4‰ and − 15.9‰, respectively) and even higher δ¹⁵N_diet (+ 12.8‰ and + 16.4‰, respectively) compared with the Ecatepec group (Fig. 2, Table 3). These observations allow us to more confidently assert that the Aztecs at Ecatepec consumed lacustrine foods in addition to the terrestrial plants and animals that were also part of their everyday diets.

The incorporation of lacustrine foods into the Aztec diets is supported by ethnohistoric and zooarchaeological evidence. Ethnohistoric accounts detail a wide variety of lacustrine foods available in the Basin of Mexico. For instance, in the Florentine Codex, Sahagún (1981) describes the wide variety of lacustrine sources in the Basin and provides several illustrations of Aztec hunting and fishing practices. Likewise, a mid-sixteenth century map depicts different fishing and waterfowl-netting activities at Lake Texcoco (Apnes 1947 in Parsons 2010). These ethnohistoric sources also note how the Aztecs actively controlled the water depths across the Basin, using each lake area for different lacustrine activities (e.g., hydraulic management, aquatic infrastructure) (Parsons 2010). The zooarchaeological evidence presented above further suggests that communities situated near the Basin lakes, such as Chalco, Xaltocan, and Mexicaltzingo, consumed a variety of lacustrine foods including waterfowl, turtle, frogs, and to a lesser extent fish and shellfish. As such, it is very likely that the Aztecs at Ecatepec, living so close to the Texcoco and Xaltocan lakes, also exploited the lacustrine landscape, as was hypothesized by Trejo Rangel (2014).

There is no ethnohistoric evidence to suggest that the elites had control over the lacustrine landscape or its products (Parsons 2010). This would have allowed lower status Aztecs to access lacustrine food sources with little effort and without restriction from the elites. According to Trejo Rangel (2014), this community could have belonged to a lower socio-economic group within the Basin of Mexico socio-political system. As such, it is possible that these Aztecs of low socio-economic status used lacustrine foods to fulfill some of their dietary needs, as described by Sahagún and later recorded during the historic period (Valadez Azúa and Rodríguez Galicia 2014).

The lack of major pathological conditions commonly associated with dietary deficiencies and nutritional stresses within this Aztec group suggests that, overall, they were healthy and well nourished (Trejo Rangel 2014). Access to a wide range of lacustrine food items, high in protein and overall nutrition, would have allowed the Aztecs at Ecatepec to fully take advantage of their surrounding environment and fulfill their dietary needs regardless of socio-economic standing. Conversely, a diet rich in carbohydrates, such as the heavy consumption of C₄ grains (e.g., maize, amaranth), causes the development of lactic acid when metabolizing sugars and starches, which increases the prevalence of bacteria and dental caries (Hillson 2008; Larsen 1997). This further supports the hypothesis that these residents did not rely solely on C₄ grains and that lacustrine foods were likely a component of their diets.

Dietary variability based on sex

The Aztec members of the Ecatepec community likely resided within the Basin of Mexico throughout most of their lives (Moreiras Reynaga 2019). Accordingly, we have assessed whether there were sex-based differences visible in their diets based on the δ¹³C_col and δ¹⁵N_col results (Fig. 5). The two Mazapa phase adults, a male (DM 84) and a female (DM 85), have the same δ¹³C_col, but the male has a higher δ¹⁵N_col, which is similar to the δ¹⁵N_col of a few males from the Aztec III–IV phase group (Fig. 5). The Mazapa individuals have lower δ¹³C_col compared with the Aztec III–IV phase group, suggesting that they may have consumed less C₄/CAM foods in the early phase of the Postclassic period.

Fig. 5

Bone δ¹³C_col and δ¹⁵N_col of the Ecatepec adults plotted by sex. Individuals DM 84 and 85 date to the Mazapa phase, while the rest of the individuals date to the Aztec III–IV phase. Individuals DM 88 and 94 have the most distinct diets within the Aztec III–IV group. Repeatability of individual measurements was generally better than ± 0.1‰ (SD) for both carbon and nitrogen isotopes

In terms of the Aztec III–IV adults, the females consumed slightly less C₄/CAM foods compared with the males based on their lower δ¹³C_col (Fig. 5). Based on δ¹⁵N_col, however, the female and male adults seem to have consumed similar amounts of animal protein. These results suggest that the men in this group may have consumed pulque in addition to C₄ grains compared with the women. Based on the ethnohistoric record, we know that adult men had the freedom to consume pulque in moderation while women (up to the age of 60 years) were not allowed to drink or even sell pulque (Coe 1994; Soustelle 1961). Aside from this sex difference, their overall diets were quite similar as inferred from the isotopic data. Only the lower δ¹⁵N_col (+ 9.0‰; Fig. 5) of an adult male (DM 88) provides evidence of less animal protein consumption compared with the rest of the group. This man likely consumed meat occasionally, while the rest of the group consumed meat more frequently. This adult male (DM 88) and an adult female (DM 94) also consumed the least amount of C₄/CAM foods within this group based on their low δ¹³C_col. These results suggest that this community shared similar diets overall. This trait is reflective of an Aztec population that shared similar foodways including their everyday consumption habits, availability of food products for the community as a whole, and access to specific foods based on their proximity to the Texcoco and Xaltocan lakes.

An example of Aztec feeding and nursing practices

The infant and child δ¹³C_col and δ¹⁵N_col data provide insights into the breastfeeding and weaning practices of this Aztec community. We infer that five infants between the ages of 1 and 2 years (DM 78, 82, 83, 86, and 91) were breastfed up to the time of death, given that their mean δ¹⁵N_col (+ 14.9 ± 0.5‰) is higher by 3.7‰ than the adult females of reproductive ages (δ¹⁵N_col + 11.2 ± 0.4‰) (Figs. 3 and 4). It is possible that their mother’s milk was mostly composed of C₄/CAM protein. A 1.5-year-old infant (DM 76), a 3-year-old (DM 80), and a 4-year-old child (DM 90) have δ¹⁵N_col (range = + 12.6 to + 12.7‰) intermediate to the ranges of the breastfeeding infants and the adult females (Fig. 3), which could indicate that these subadults were being weaned (Fig. 4). Three infants (DM 81, 98, and 105) between the ages of 2 and 3 years have δ¹⁵N_col within the adult δ¹⁵N_col range (Figs. 3 and 4), which suggests that some infants could have been weaned as early as 2 years of age. This is a small sample, however, and we do not know the cause of death of these individuals. Nonetheless, these results likely point to some variability in this community’s subadult feeding practices. Some infants were likely fully weaned early on during infancy (between ages 2 and 3 years), while other subadults continued to breastfeed until about 4 years of age. This extended nursing practice has been previously reported at other Mesoamerican sites such as Teotihuacan (e.g., White et al. 2004a; Wright and Schwarcz 1998, 1999).

Conclusions

This study has shed light on several aspects of the dietary practices of the Aztec residents from Ecatepec. The carbon and nitrogen isotope results revealed overall dietary patterns that correlate well with the zooarchaeological and paleoethnobotanical data recovered at other Aztec sites located near the Basin of Mexico lakes (e.g., Xaltocan, Mexicaltzingo). The Aztecs at Ecatepec incorporated mainly C₄/CAM plants as well as some C₃ plants, terrestrial animals, and probably lacustrine foods into their diets.

While C₄/CAM foods were important at this site, lacustrine foods likely also played a role in the Ecatepec residents’ diets. Adult females consumed slightly less C₄/CAM foods compared with the males, perhaps suggesting that they likely did not consume pulque, but all the adults (except for one) had similar amounts of animal protein in their diets. The similarity in the Ecatepec diets, based on the Aztec III–IV group, could be due to sharing similar foodways as a community, the accessibility of specific food products based on their shared socio-economic status, and proximity to the Basin of Mexico lakes. We found that the two adults dating to the Early Postclassic period consumed less C₄/CAM foods compared with the majority of the adults from the Aztec III–IV phase, pointing to possible dietary differences across time periods.

Half of the infants in this group had nitrogen isotope compositions that signal breastfeeding, while the other half underwent the weaning transition as early as 2 years of age. Two infants and one child showed signs of the weaning transition between the ages of 2 and 4. To our knowledge, this is the first time that bone collagen isotope compositions have been measured systematically for a group of Aztec residents to reconstruct their diets. This study provides an initial baseline for everyday diets for Aztecs living close to the Basin of Mexico’s lacustrine environment during the Postclassic period.