Introduction

Urban ecosystems have become habitats for an important number of animals, such as large mammals as Puma concolor, and other mammals as Bassariscus astutus, Didelphis virginiana (Wright et al. 2012; Cisneros-Moreno and Martínez-Coronel 2019; Robins et al. 2019). Those ecosystems provide different types of habitats than wilderness; for example, larger and more intense heat islands (Ackley et al. 2015), higher noise levels (Slabbekoorn and Peet 2003), and different illumination patterns (Hopkins et al. 2018), and animals had to adapt to these new conditions by changing their behavior, for example birds change their singing pitch to avoid interference by the urban noise (Slabbekoorn and Peet 2003). One of the critical changes in the habitat is the increment of food for omnivorous animals (Gardener and Sunquist 2003, Wright et al. 2012).

Urban areas provide many food sources for different animal groups. For example, birds eat seeds from trees and garden plants that grow in cities artificially provided food like seeds and hummingbird feeders (Fuller et al. 2008; Coogan et al. 2018). Mammals also have a great selection of foods ranging from natural prey (even elk and deer; Robins et al. 2019) to human trash (Hopkins and Forbes 1980; Smith 2016), which can be very abundant since worldwide urban food waste is over 450 Gkg per year and is common in cities all around the world (Adhikari et al. 2006). Trash food provides a significant amount and variety of alimentary items for the animals that can take advantage of them.

However, it is difficult to accurately determine the importance of human-provided food for urban wildlife since many of these food items leave little or no identifiable trails in the animal feces (Newsome et al. 2010). At the same time, fecal analyses tend to sub or overestimate the importance of certain alimentary items depending on their size and digestibility (Roth and Hobson 2000; Milakovic and Parker 2011). An alternative approach is based on the stable isotopic analysis that has proved to be an efficient tool to establish the alimentary resources of wild species and provides information about the assimilated alimentary items and not just the ingested ones (Hobson et al. 1996; Herrera et al. 2001). This method has allowed determining the food habitats (Crawford et al. 2008; Wurster et al. 2012), comparing alimentary patterns among populations of the same species, and estimating anthropogenic food use by wild animals (Wurster et al. 2012). There are several studies on the alimentary patterns in urban environments of species like foxes, bears, coyotes, and raccoons (Merkle et al. 2011; Murray et al. 2015; Scholz et al. 2020; Nicholson and Cove 2022). However, studies regarding other species, such as opossums in urban environments, are lacking (although there are studies on the relation between opossums and their environment including several anthropogenic factors; Walsh and Tucker 2020, 2023).

Therefore our study model is the Virginia opossum (Didelphis virginiana), an omnivorous mammal that has greatly expanded its distribution range thanks to human activity (Walsh and Tucker 2020) and is well adapted to live inside urban areas (Wright et al. 2012). Its success in colonizing anthropogenic habitats has mainly been attributed to omnivorous habits and its ability to consume human waste (Gardener and Sunquist 2003, Wright et al. 2012) to the point that opossums in urban areas have consistently smaller home ranges since the don’t need much area to search for food (13.4–37.3 ha for urban males, 5.1–18.8 ha for females compared to 78.6–141.6 ha and 28.0–64.4 ha of the rural animals; Wright et al. 2012). This makes opossums the perfect model of successful use of human habitats by mammals. However, it is yet to be fully understood how these new habitats modify opossums’ diet. At the same time, since it is a very common urban animal (Wright et al. 2012), the knowledge of its behavior and ecology in urban ecosystems will be a keystone in urban wildlife conservation. Although there have been some studies related to its diet in urban areas, those studies are surprisingly scarce and concentrated on the northern portion of the species distribution (Hopkins and Forbes 1980; Smith 2016).

This project aims compare the alimentary niche of Didelphis virginiana in natural and urbanized locations inside Mexico City using the stable isotope analysis. This data is expected to provide information on the species’ behavior in urban environments and help create more efficient management programs for opossums.

Methods

The study was performed on the Pedregal de San Angel lava field left by the eruption of Xitle (in 280 ± 35 A. D.; Siebe 2000) in Mexico City (henceforth Pedregal). The original location is characterized by a very heterogeneous terrain and xerophilous shrub vegetation (Rzedowski 1954). Most of the lava field has been urbanized in the last decades, yet some remnants of the natural vegetation remain, and some of those are well preserved, consisting of up to several hundreds of hectares (Lot and Camarena 2009). Therefore, the study location is a very particular mosaic of urban and natural areas in various states of conservation that allows for comparisons between natural and urban areas and isolated natural remnants.

Opossums were live-trapped in eight locations inside the Pedregal area (Fig. 1) using Tomahawk and pit-fall traps (description provided in Glebskiy and Cano-Santana 2021) in 2018 and 2020. Samples of hair were taken from the trapped animals. In some cases, 0.2–0.5 ml blood samples were taken by puncturing the coccygeal vein. Samples were handled according to the instructions provided by the analyzing laboratories; Stable Isotope Laboratory at the University of Davis, California, for 2018 samples and Laboratorio de Análisis de Isótopos Estables, UNAM for 2020 samples. The δ of 14 C and 15 N isotopes and the proportion of C:N was calculated for each sample (for details on the method see: https://stableisotopefacility.ucdavis.edu/carbon-and-nitrogen-solids and Sharp 2017).

Fig. 1
figure 1

Location of the sampling points

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The isotopic niche (SEAC ‰2) was calculated using the SIBER package (Jackson et al. 2011) for opossums in each location, based on their hair samples. Human hair-based isotopic niche was used to establish human food niche reference values (Newsome et al. 2015). At the same time, a comparison of the δ14C, δ15N and C:N ratio among locations, year of sample collection, and the season (dry or rainy) were performed using a multifactorial ANOVA analysis, and a Tukey HSD as a post-hoc test. The samples of hair and blood collected in 2018 were analyzed using a multifactorial ANOVA using as factors: the location where samples were collected, season (dry or rainy), and type of sample (hair or blood); a Tukey test was used as a post-hoc.

Results

Differences in δ14C between the locations were found (F = 11.543, p < 0.001), specifically, NA1 was different from all other locations (lower δ14C values), and NA5 and NA3 were different between them (p < 0.01 in all cases; see Table 1). The δ15N analysis showed no significant differences. There were significant differences between locations when comparing the C:N ratio (F = 13.517, p < 0.001), NA1 had a higher C:N proportion than all other locations (p < 0.05 in all cases; see Table 1).

Table 1 Characteristics of the sampling sites used for this study. Values given with standard deviations
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The ANOVA test of hair and blood samples δ14C shows that there are differences between seasons, sample types, locations, and the interaction between location and sample type (F = 8.741 p = 0.006, F = 8.686 p = 0.006, F = 6.021, p = 0.006 and F = 10.323, p < 0.001 respectively) but no differences between years were found. Particularly, NA3 area opossums have lower δ14C than UA1 and NA5 (p < 0.05), and NA1 hair samples had lower δ14C than all the other samples in all locations, including the blood sample from the same location (p < 0.05 in all cases). The δ15N value was higher in the NA1 area than in NA3 (F = 3.534, p = 0.041, Tukey p = 0.032). Comparisons of C:N proportions show differences between location (F = 12.544, p < 0.001) and the interaction of location and sample type (F = 13.954, p < 0.001). NA1 hair samples had a higher C:N ratio than all other samples (p < 0.01), including blood samples from NA1; however, there were no differences between NA1 blood samples and samples from other locations.

According to SEAC ‰2 the isotopic niche was 7.03‰2 for NA1 opossums, 3.23‰2 for NA2, 3.23‰2 for NA3 12.45‰2 for NA4, and 4.02‰2 for UA3. The isotopic niche analysis shows a separation of the NA1 from the rest of the locations due to a lower δ14C (Fig. 2) and that the possums from the NA4 zone have the widest isotopic niche.

Fig. 2
figure 2

Isotopic niche area (SEAC ‰2) of opossum groups from different areas. Analysis based on hair samples

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Discussion

Contrary to previous studies (Murray et al. 2015; Scholz et al. 2020; Nicholson and Cove 2022), there are no differences in opossum isotopic data between the urbanized and natural areas. All opossums are grouped in a small isotopic area (except for NA1 area Fig. 2); therefore, opossums do not modify their isotopic niche when they feed on urban areas. At the same time, there were no differences in δ15N, which suggests that opossums maintain similar trophic positions in all locations. Therefore it appears that urbanization has little effect on opossum’s isotopic niche. There are two possible explanations for this: (1) opossums maintain the same diet and search for the same food in urban and natural environments, or (2) the isotopic values of the food available to opossums are similar in both environments. The second hypothesis seems to be the most likely since human populations commonly rely on C4 plants as the primary carbon source (WHO 2003), and in our study locations, C4 and CAM plants are very common and represent an important proportion of primary production (Cano-Santana 1994). Moreover, it is confirmed by the isotopic analysis of human hair that appears close to the opossum’s hair (Fig. 2).

The year of data collection did not affect the isotopic niche (Fig. 2), suggesting that the opossum diet tends to be constant over time. When comparing the isotopic niche amplitude, the widest niche (12.45‰2) was found in a small natural area NA4 this is likely due to the fact that this area is too small (3 ha) to host an opossum population and individuals trapped there had to obtain food in different natural and urban areas that surround NA4 (Fig. 1). On the other hand, opossums in more extensive natural areas tend to have smaller isotopic niches (except NA1), suggesting that they prefer to feed inside those areas when possible.

In both natural and urban areas, opossums appear to consume both C3 and C4/CAM plants in similar proportions. However, the NA1 location stands out in terms of δ14C (Fig. 2) and the C:N ratio. Opossums in this area rely more on C3 plants as their carbon source and have a greater C:N ratio, which suggests a lower-quality diet (Crawfort et al. 2008). This points out an important trait in opossum’s dietary behavior. The NA1 is a mid-size natural area surrounded by urban terrain (generally characterized by C4 diets; Walsh and Tucker 2023) and two of our sampling sites the UA2 and UA3, are located very closely to this area (Fig. 1; approximately 200 m in both cases), and cases of opossums crossing between those areas were registered (by GPS tracking between UA3 and NA1; López-Saldívar in prep. and capturing the same individual in the vicinity of UA2 and inside NA1; pers. obs.). However, there are significant differences in carbon sources and even the quality of food. This suggests that even though opossums can move across different locations and have proven to be omnivorous and opportunistic (McManus 1974); they conserve their feeding area and are unwilling to search for food in other types of ecosystems, even when food is of lower quality and they have access to different food sources. Therefore, there is a separation between opossums in urban and natural areas based on their foraging habits.

At the same time, there were differences in δ14C between blood and hair samples in NA1. Both sample types were collected during the rainy season, but blood samples represent the diet of the last months before collection (Crawfort et al. 2008), while hair samples represent the diet over a more extended time: years or even the entire lifespan of this species since it does not undergo seasonal molt (Walsh and Tucker 2021). Therefore, NA1 Opossums only have their C3 diet during the dry season. This raises two questions: why do opossums mainly consume a C3-based diet, and why does this happen only during the dry season? The heterogenous terrain could explain the dominance of the C3 plants in the diet of the NA1 opossums; the Pedregal vegetation depends heavily on the amount of soil and the water it can store (Cano-Santana 1994; Rzedowski 1954). In particular NA1 is characterized by a more rugged terrain consisting of rocky formations and fissures in the rock that store important amounts of soil, which in turn acts as water storage. This combination increases water availability which gives an advantage to the C3 plants; this is consistent with Cano-Santana (1994), who observed that trees (generally C3) are more common in rugged terrain while plants of the family Poaceae (C4) are more common in the plain terrain. Therefore NA1 opossums have access to C3-based food. However, it appears to be a suboptimal food source since they only resort to it during the dry season when resources are scarce and use their common C4/CAM-based food sources (probably based on annual grasses) when available. This trait is of interest since a previous study showed that the terrain ruggedness is one of the most important factors to predict another mid-size mammal in the region: the eastern cottontail (Glebskiy et al. 2018) suggesting that small-scale terrain characteristics (a factor generally overlooked in ecological studies) could have effect on the animals inhabiting the area.

Conclusions

Virginia Opossums tend to be more conservative in their feeding locations than expected from an opportunistic omnivore generalist. Urbanization has little effect on the isotopic niche of opossums; however, there is a separation between opossums that inhabit urban and natural areas based on their dietary preferences. Furthermore, terrain ruggedness plays an important role in mid-size mammal ecology.