The paleo-synanthropic niche: a first attempt to define animal’s adaptation to a human-made micro-environment in the Late Pleistocene

Synanthropic behavior, i.e., the behavior of wild animals that benefit from a shared ecology with humans, has existed long before the sedentarization of Homo sapiens during the Neolithic, around 10,000 years ago. This study describes and discusses the concept of an older animal-human relationship: paleo-synanthropic behavior and the associated paleo-synanthropic niche. Key features of this new niche are anthropogenic food waste from mobile hunter-gatherers as a stable food base for small opportunistic scavengers and a human-near environment safe from large predators. By linking the niche to human behavior rather than to a specific location or structure, this niche was accessible for a long time, even in the Late Pleistocene. Like modern synanthropic animals, members of the paleo-synanthropic niche experienced an increase in population density and a decrease in home range. This, in turn, made it easier for humans to capture these animals and use them as resources for meat, fur, or feathers, as seen in the zooarchaeological record of many European Late Pleistocene sites. As a disadvantage, diseases such as zoonoses may have spread more easily.


Introduction
Today's urban landscape includes not only cars, bicycles, and pedestrians, but also various mammals and birds that seem to thrive in our human-made environment. Whether it is pigeons that peck at bread crumbs in urban pedestrian areas or red foxes that take up residence in our front yards and forage for food scraps in the streets at night, we are not alone in our cities. Studies of urban foxes have shown that this synanthropic behavior has a strong influence on dietary niches (Contesse et al. 2004;Scholz et al. 2020), as well as on morphology (Parsons et al. 2020), and thus potentially on the genetics of these animals (Wandeler et al. 2003;DeCandia et al. 2019). Urban foxes are more diet-specialized than their rural counterparts and show morphological changes, such as snout shortening, not dissimilar to an early stage of domestication. However, the phenomenon of synanthropic behavior is not new. Even the naming of certain species indicates their proximity to humans, as in the case of the house sparrow (Passer domesticus) or the house mouse (Mus musculus).
Synanthropism refers to a behavior of free-ranging animals (or plants) benefiting from the shared ecology with humans (Klegarth 2016). According to Shochat et al. (2006), species can benefit from humans through both bottom-up and top-down processes. The former refers to the food resources that are available to the synanthropes, and the latter refers to the security provided by the human environment. As a result of synanthropic behavior, the population density, reproduction, and survival advantage of synanthropes increase. Meanwhile, the home range decreases as the animals feed on the monopolized, centralized anthropogenic resources (Gehrt et al. 2011;Hulme-Beaman et al. 2016).
Synanthropes have been further subdivided by some ecologists to account for individual animal behavior (Johnston 2001;McKinney 2006;O'Connor 2013;Hulme-Beaman et al. 2016). Other researchers prefer to use the term commensalism to describe the same relationship between humans and animals in an anthropogenic environment (O'Connor 2013;Hulme-Beaman et al. 2016). However, commensalism (lat. = "sharing the table") is more likely to describe a + /0 relationship between an animal and its host (Johnson et al. 1997;Mathis and Bronstein 2020), which may not necessarily be human. For example, commensalism is attributed to all scavengers, whether they are adapted to an anthropogenic food source or are opportunistic. The other factors described above, such as the security provided by the host, do not play a role in commensalism either. To avoid any confusion of words and their ecological meanings, it is better to speak of commensal or trophic synanthropism when focusing on the food component. If the environmental component, i.e., the use of human-made structures, such as buildings or cities, is to be emphasized, then one can speak of environmental synanthropism (O'Connor 2013), although this does not preclude synanthropic animals from falling into both categories. Another difference between synanthropism and the classical definition of commensalism is the + /0 relationship, which need not be present in synanthropism. A + /0 relationship means that one part (animal) benefits without affecting the other part (human). For animals living in human-made environments, this is not always the case. Conflicts between animals and humans, such as attacks by food-habituated animals (Linnell et al. 2002;Timm and Baker 2007;Behdarvand and Kaboli 2015), severe pest infestations resulting in crop losses (Duplantier and Rakotondravony 1999;Stenseth et al. 2003), or disease transmission and zoonoses (Raoult et al. 2013;Klegarth 2016;Keck and Lynteris 2018) are regularly documented today and throughout history.
Synanthropism, or the commensal synanthropic behavior of different animals, has been studied mainly since the Neolithic period (beginning about 10 kyrs ago in the Levant (Weisdorf 2005; Belfer-Cohen and Goring-Morris 2020)) and the associated formation of settlements. The main focus of research on ancient synanthropism has been on human-animal relationships resulting from the creation of a new landscape structure (e.g., buildings and settlements) (O'Connor 2013;Hulme-Beaman et al. 2016). Since the Neolithic Revolution, humans have built houses, villages, and cities, cultivated crops and vegetables, domesticated livestock, and spread these technologies around the world, with demonstrable and lasting effects on the environment. Sedentarization and the development of agriculture created a stable niche that provided food for a long time. This has been shown to attract first rodents (Frynta et al. 2005;Cucchi et al. 2020) and later cats (Hu et al. 2014;Ottoni and Neer 2020;Krajcarz et al. 2022), which feed on these rodents. Over the decades and centuries, the small settlements grew into villages and towns, forming the first urban environments populated by synanthropic animals.
The aim of this study, however, is to deal with the time well before the Neolithic and the sedentarization of humans. For this purpose, the concept of the paleo-synanthropic niche, i.e., the adaptation of certain animals to the microenvironment of mobile hunter-gatherer societies during the Late Pleistocene (MIS 4 to 2, ca. 71 to 14 kyrs ago (Lisiecki and Raymo 2005)), will be elaborated.

Human group size
The larger the human group, the greater the ecological impact. Larger groups require more food than smaller groups, which translates into more prey to hunt. This has two consequences: first, the hunting pressure on humans' primary prey increases, and second, there is potentially more food waste. Neanderthal group sizes in Europe have been estimated to range from 10 to 30 individuals (Hayden 2012;Bocquet-Appel and Degioanni 2013). By studying the two reference sites Le Rozel (France) and Sidrón Cave (Spain) it was possible to support the previous estimates with direct evidences (Rosas et al. 2013;Duveau et al. 2019). Group sizes of AMHs were strongly dependent on the regions they occupied. In more densely populated regions, such as Belgium or the Swabian Jura, AMH groups of about 40 to 50 persons and up to 150 persons have been estimated for MIS 3 (57 to 29 kyrs ago) (Schmidt and Zimmermann 2019). In particular, for the Swabian Jura (Germany), estimated AMH groups were 10 to 15 times larger than those of the Neanderthals (Conard et al. 2012(Conard et al. , 2013.

Hunting pressure
As mentioned above, food demand increases with group size, and so does hunting pressure. The increased hunting pressure on mammoths in MIS 3 could be demonstrated by the change in the niches of large herbivores in the C-N isospace of Central Europe Münzel et al. 2017;Wißing et al. 2019). The absence of these megaherbivores made food available to other herbivores, such as horses. Many archaeological sites, which were occupied by AMHs, show tons of mammoth bones and ivory, especially in Poland  and the Czech Republic (Wojtal et al. 2012;Wilczyński et al. 2015;Svoboda et al. 2019). Cut marks have been found on the bones, indicating that the animals were at least butchered. Whether humans are solely to blame for the extinction of the European megaherbivores is debatable (Alroy 2001; Wroe et al. 2004;Bulte et al. 2006), although they have had their share (Lorenzen et al. 2011;Haynes 2018). As a consequence of specializing in hunting the now declining megafauna, Pleistocene humans, and AMHs in particular, had to adapt their diet to include more small game (Stiner et al. 1999(Stiner et al. , 2000Stiner 2009;Baumann et al. 2020c). However, in other European Middle Paleolithic sites, especially in the Mediterranean areas, there is evidence for a exploitation of small game resources that cannot be directly linked to overhunting of large mammals (Finlayson et al. 2012;Hardy et al. 2013;Finlayson 2019). It is beyond the scope of this study to discuss this in detail.

The earliest synanthropic behavior in the European Late Pleistocene
The first hypotheses of synanthropic behavior well before sedentarization were made by O'Connor (2013) and were based on zooarchaeological analyses (mainly NISP, i.e., number of identified individuals) of European and American Late Pleistocene sites. It was striking to O'Connor (2013) that certain animals, such as ravens and foxes, were quite common in archaeological sites associated with Neanderthal or early AMH occupation, and he therefore suggested that these animals were commensal to humans. Besides NISP, the analysis of carnivore tooth marks on human prey has a high potential to provide information about possible synanthropic behavior of small carnivores. Several archaeological sites from the Middle to the Upper Paleolithic show tooth marks made by small carnivores (Camarós et al. 2016;Daujeard et al. 2016;Yravedra et al. 2017;Yravedra et al. 2019a, b). Identifying which carnivore is active at a site is a difficult task. There are more than 40 years of taphonomic research on this topic, but almost all authors focus on studying large carnivores such as wolves, felids, bears, and hyenas. Only a few authors studied small carnivore tooth marks, such as of foxes (Cáceres et al. 2011;Andrés et al. 2012;Krajcarz and Krajcarz 2014;Yravedra et al. 2014). In recent years, the use of artificial intelligence has allowed for very promising results. Yravedra et al. (2019a, b) were able to classify dog, fox, and wolf-tooth marks with high accuracy. This method has been further developed to include other carnivores (Courtenay et al. 2019Yravedra et al. 2021). The results of the tooth mark analysis of Courtenay et al. (2023) even allow the hypothesis that interactions between humans and canids already took place in the Lower Paleolithic (older than 300 kyrs BP). However, in this type of analysis it is very difficult to ensure the contemporaneity of human and carnivore activities. It cannot be excluded that carnivores visited the sites much later than humans. Furthermore, there are hypotheses for synanthropic birds in the Late Pleistocene. These refer either to commensal synanthropism, as for ravens from archaeological sites in Moravia or to environmental synanthropism, as for owls at cave sites (Kost and Hussain 2019;Hussain 2021). There is also evidence for commensal synanthropic behavior of foxes in the Near East prior to the Neolithic period. Maher et al. (2011) studied a joint human-fox burial from the Pre-Natufian period, a regional archaeological period (ca. 13-12.3 kyrs BP) in which humans were already partially sedentary (Bar-Yosef 1998). The authors even hypothesized that it had been a fox kept as a pet.
The first evidence of commensal synanthropic behavior was provided by red and Arctic foxes (Vulpes vulpes and Vulpes lagopus) in the Swabian Jura (Germany) (Baumann et al. 2020a, b, c) (Fig. 1). In a study of the stable carbon and nitrogen isotopes (δ 13 C and δ 15 N) from bone collagen of these foxes, their diet was reconstructed. While no anthropogenic influence on the diet of red and Arctic foxes was found in the MIS 4 layers that are associated with Neanderthal occupation, the picture changed in the layers associated with the occupation of AMHs in MIS 3. Starting in the Aurignacian (ca. 42-34 kyrs BP (Conard and Bolus 2008;Richards et al. 2019)), foxes exploited a new dietary niche that arose only as a result of the hunting activities of the AMH (Baumann 2020; Baumann et al. 2020a, b, c). AMHs brought whole small to medium sized prey, such as hares and reindeer, to the cave site and butchered them there (Niven 2007). Large prey, such as mammoths, were butchered directly at the kill site, and only the meat, pre-selected bones, and ivory were brought back to the caves. This created a new food resource for small, opportunistic scavengers, such as foxes. However, large carnivores did not have access to this resource, as indicated by stable isotopes. Therefore, this was the first synanthropic niche that provided both food and protection from predators. This niche was also demonstrated for the Gravettian (ca. 34-30 kyrs BP (Taller and Conard 2019)) of the Swabian Jura in the same study. After the Last Glacial Maximum (LGM), in MIS 2, the niches of foxes changed again, as shown by studies on the trophic niches of wolves, foxes, and dogs in the Magdalenian (ca. 16-14 kyrs BP (Weniger 1987;Albrecht and Berke 1988;Jochim et al. 1999;Taller et al. 2014)) of the Hegau Jura (Germany/Switzerland) and the Central Swabian Jura (Germany) (Baumann et al. 2020a, b, c;Baumann et al. 2021). The synanthropic niche that was occupied by foxes in MIS 3 was occupied by the dogs, and/or genetic wolves, which were influenced by humans during the MIS 2. Only one of the studied foxes from the Hegau Jura was assumed to be synanthropic (Baumann et al. 2020a, b, c). In the archaeological sites of the Central Swabian Jura, no synanthropic foxes were found, which may be related to the low level of human occupation in this region during the Magdalenian (Taller et al. 2014).
Further evidence of commensal synanthropic behavior is provided by the common raven (Corvus corax) during the Gravettian in Moravia (Czech Republic) (Baumann et al. 2022) (Fig. 1). The archaeological sites of Palvov, Předmostí, and Dolní Věstonice are mammoth kill sites with partial structures indicating long-term occupation (Trinkaus and Svoboda 2006;Svoboda 2016;Svoboda et al. 2019). The potential main food resource for a synanthropic niche was mammoth, as tons of mammoth bones were found there Svoboda et al. 2019). Furthermore, the main human diet reconstructed from stable isotopes also comprised of mammoth (Trinkaus and Svoboda 2006;Bocherens et al. 2015;Baumann et al. 2022). Baumann et al. (2022) analyzed sulfur (δ 34 S), which revealed information about the origins of the ravens, in addition to δ 13 C and δ 15 N for the reconstruction of the dietary niches. As a result, the different sulfur groups displayed different diets. The ravens with sulfur values similar to the local herbivores had a high concentration of mammoths in their diet and thus occupied the local synanthropic niche, while the ravens with non-local sulfur values had a mixture of different large mammals in their diet. Since all the ravens studied were found at sites associated with AMHs, it was hypothesized that the nonlocal ravens were attracted by human activities, with isotopic values in bone collagen not yet adapted to the new diet. As with the foxes of the Swabian Jura, the local synanthropic ravens of Moravia showed no niche overlap with large carnivores that would be potentially dangerous to them.

Key characteristics of the paleo-synanthropic niche
The term paleo-synanthropic niche refers to the commensal synanthropic behavior of certain animals in the Paleolithic, the earliest archaeological period. The activities of Pleistocene humans influenced the environment and created a new micro-environment that allowed certain animals to exploit this new ecological niche among humans. Ecological niches are highly complex constructs that are difficult to understand with simple formulas or analyses (Kearney 2006;Pocheville 2015). This is especially true in the field of paleoecological research, where they are even more difficult to understand because there are few methods to study them. The most common of these are trophic niches, which can be studied by analyzing carbon and nitrogen isotopes and the models constructed therefrom (e.g., Bocherens et al. 1991Bocherens et al. , 1994Bocherens et al. , 2011Bocherens et al. , 2015Higashi et al. 1992;Svanbäck and Persson 2004;Layman et al. 2007;Bocherens 2009Bocherens , 2015Münzel et al. 2014;Wißing et al. 2016;Schwartz-Narbonne et al. 2019)). However, even these analyses reveal only a fraction of the full complexity of trophic behavior.
The paleo-synanthropic niche is essentially a niche for animals with commensal synanthropic behavior. Therefore, an easily accessible food resource is one of its key characteristics ( Fig. 2A). The food available to modern (commensal) synanthropic animals is of anthropogenic origin. Ecological studies of modern foxes (Contesse et al. 2004;Savory et al. 2014;Panek and Budny 2017;Reshamwala et al. 2018), coyotes (Fedriani et al. 2001;Murray et al. 2015), black bears (Merkle et al. 2011), and other carnivores (Prange et al. 2004;Rodewald et al. 2011;Yirga et al. 2012) show that anthropogenic food waste plays a critical role in their diet. In the paleo-synanthropic niche, it was the same, except that Pleistocene food waste was markedly different from our modern food waste. The main prey of Late Pleistocene AMHs and Neanderthals were large herbivores such as mammoths, horses, and reindeer (Niven 2006(Niven , 2007Bocherens 2009;Sheppard et al. 2018;Meadows et al. 2019;Wißing et al. 2019;Drucker et al. 2020); additionally, they consumed small game, aquatic resources and plants (Stiner et al. 2000;Richards et al. 2001;Madella et al. 2002;Stringer et al. 2008;Brown et al. 2011;Hardy and Moncel 2011;Henry et al. 2011;Hardy et al. 2012;Conard et al. 2013;Wood 2019;Wroth et al. 2019). In the rocky regions of southwestern Europe, ibex and chamois were common Neanderthal prey (Yravedra and Cobo-Sánchez 2015; Daujeard et al. 2016). Each archaeological site is unique and, therefore, has its own range of food waste. Through zooarchaeological analysis, it is possible to get an idea of the possible food waste left behind by Late Pleistocene humans. Whether the site is a cave site or an open-air site plays an important role. People did not always hunt where they lived. Kill sites, i.e., archaeological sites where animals were hunted and butchered, are almost always open-air sites. There, (temporary) camps were built to process the prey. This is the case, for example, at the Moravian sites Svoboda et al. 2019). Here, mammoth was the main prey in the respective paleo-synanthropic niche, because these animals were hunted by humans and the butchering of the animals produced a lot of food waste. Large prey, such as mammoth, was butchered at the kill sites, and only the meat and certain bones or ivory were transported, e.g., to the cave sites (Niven 2007). Reconstructions of the diets of humans found in caves indicate that they also consumed mammoth as their main protein diet (Wißing et al. , 2019. Nevertheless, the remains of medium-sized herbivores (such as reindeer) are often found in cave sites (Krönneck et al. 2004;Niven 2006;Kitagawa 2014;Bertacchi 2017;Münzel et al. 2019). These animals were brought into the cave in their entirety and butchered and processed in or near the cave. As a result, they make up most of the food waste, as in the cave sites of the Swabian Jura (Baumann 2020;Baumann et al. 2020a, b, c). At other sites, of course, Fig. 2 A Concept of the paleosynanthropic niche. This niche was associated with human (hunting) behavior and was siteindependent. Late Pleistocene humans created a stable food resource in a microenvironment that was safe from large predators and accessible to small, opportunistic carnivores for a long time. B The paleosynanthropic niche moved with the migrations of human groups. The niche may have evolved independently in different human groups and allowed the occupants to move between these groups. Thus, the paleosynanthropic niche would have been available for a very long time as long as Late Pleistocene humans had the same impact on nature by providing enough food waste and a predator-free micro-environment the food waste may be different, such as at Buran-Kaya-III, a rock shelter in southern Crimea, where the main food waste was saiga upon which red foxes had fed (Péan et al. 2013;Baumann et al. 2020a, b, c).
The next important key feature is the security provided by a synanthropic niche (Shochat et al. 2006). For the paleosynanthropic niche, this means safety from large predators that were also dangerous to humans ( Fig. 2A). It was important for Late Pleistocene humans to create a safe area for themselves, and as a side effect, this area was also safe for paleo-synanthropes. To determine whether a niche is safe from large predators, niche overlap in C-N-isospace is a good indicator. By analyzing δ 13 C and δ 15 N from preserved bone collagen, not only the diet but also the trophic niches can be determined (Baumann et al. 2020a, b, c;Krajcarz et al. 2020;Baumann et al. 2022). If the niches of paleosynanthropic animals and large predators overlap, these animals may have interacted with each other and with the same food resource. Such interactions could, for example, be competitive or commensal. The latter refer to the real ecological sense, e.g., commensal behavior of foxes to wolves or vultures to lions. The paleo-synanthropic niche should therefore not overlap with the niches of large predators; otherwise, these predators would have had access to the same food resource, the anthropogenic food waste.
In order to adapt to a new niche, the animal needs to have access to its resources over a long period of time. This long-term availability is therefore another key feature of the paleo-synanthropic niche. When stable isotopes are used to define trophic niches, this is often done with δ 13 C and δ 15 N from preserved bone collagen. The regular renewal of the bone during life and the resulting turn-over of the stable isotopes in the collagen (Huja et al. 2006;Hedges et al. 2007;Huja and Beck 2008) ensure that the measured values can only be understood as average over several years (Bocherens and Drucker 2013). This is well demonstrated in the study of Moravian ravens (Baumann et al. 2022): ravens with a local sulfur signal had already adapted to the local paleosynanthropic niche, whereas ravens with high δ 34 S values had not yet adapted their δ 13 C and δ 15 N values. This means that when an animal's isotopic values had match the paleosynanthropic niche, this animal must have occupied that niche for an extended period of time. Conversely, this means that the paleo-synanthropic niche must have been available for a long time.

Geographical flexibility and long-term availability
Pleistocene hunter-gatherer societies were generally not sedentary but highly mobile (Kretschmer 2015;Wißing et al. 2019). Thus, it is not prudent to apply the same understanding of long-term availability to these societies as we do to sedentary societies from the Neolithic onward. The main argument against synanthropic behavior before sedentarization is the lack of continuity in human occupation and the absence of built structures such as houses and permanent settlements (Tangri and Wyncoll 1989;Frynta et al. 2005;Zeder 2012a, b;Hu et al. 2014;Zeder 2015). For paleo-synanthropic niches precisely this assumption of a geographically stable long-term availability does not work. Rather, paleo-synanthropic niches must have been as geographically flexible as Late Pleistocene humans themselves. Human behavior is the anchor point of this niche, as opposed to a specific location (Fig. 2B). If a human group is large enough to produce a significant amount of food waste and to protect itself from large predators within a certain radius, then the paleo-synanthropic niche is also stable in time. When human groups migrate, the paleo-synanthropic animals migrate with them. Thus, paleosynanthropic behavior may have evolved independently in different human groups once the above conditions were met. It is also conceivable that the paleo-synanthropes changed their host human groups and thus remained in this niche even longer. This is a plausible hypothesis to explain the measured isotope values and the constructed trophic niches of the paleo-synanthropes, but further research, especially paleogenetic in combination with stable isotope analysis, is needed to investigate the long-term effects of this behavior.

Requirements for paleo-synanthropic candidates
Not every animal species had the opportunity to occupy the paleo-synanthropic niche, just as not every species exhibits synanthropic behavior today. Each niche has certain requirements that potential candidates must meet (Higashi et al. 1992;Svanbäck and Persson 2004;Kearney 2006). For the paleo-synanthropic niche, there are three essential requirements: a carnivorous or omnivorous diet, an opportunistic generalist feeding habit, and small body size. Diet is most obvious because the food resource is anthropogenic food waste, which in the Late Pleistocene consisted mainly of large mammal remains. The dietary type (e.g., herbivorous, omnivorous, or carnivorous) of Pleistocene animals can be determined either morphologically from the dentition (i.e., in mammals) or by behavioral analogies with present-day representatives (e.g., in birds). Stable isotope analysis, especially δ 15 N of individual amino acids in bone collagen, can also provide a reliable indication of the dietary type O'Connell 2017;Jaouen et al. 2019;Pollierer et al. 2019). On the one hand, a generalist feeding habit is necessary to ensure that the food offered (i.e., food waste) is part of the dietary spectrum of the species. On the other hand, individuals or groups of individuals of a generalist species may tend to specialize on certain resources (Araujo et al. 2011;Sheppard et al. 2018;Scholz et al. 2020). This means that generalists can more easily accept new dietary resources and thus occupy new niches, such as the paleo-synanthropic niche. Generalists have broad species niches, which can be determined either by behavioral analogies with present-day representatives of the species or by analysis of δ 13 C and δ 15 N from bone collagen of Pleistocene fossils. For the latter method, the niche width in the C-N-isospace is visually apparent and can be studied using niche metrics (Layman et al. 2007;Jackson et al. 2011;Baumann et al. 2020a, b, c;Krajcarz et al. 2020;Baumann et al. 2022). Individually specialized groups can be identified using cluster analysis. Detailed information can be found in Baumann et al. 2020a, b, c. The opportunistic behavior is related to the generalist feeding habit and can probably only be defined by analogy with the behavior of modern representatives of the species. However, this behavior is crucial for individual animals to take advantage of the opportunity to occupy a new niche. The final requirement for potential candidates is the right body size. Today, living synanthropic animals in our immediate environment (e.g., in cities) are small to fox/badger/raccoon size (Klegarth 2016). These animals are inconspicuous and are perceived as harmless to humans and are therefore usually tolerated. It may have been similar in Late Pleistocene hunter-gatherer societies. As described above, these people had created an environment free of large carnivores, which provided security for themselves. However, small scavengers, such as foxes and ravens, had the opportunity to invade this safe territory because they may have been considered harmless and therefore were tolerated. The body size of Pleistocene animals can be reconstructed, for example, from bone measurements (Weinstock 1997(Weinstock , 2002Grayson 2014).
To find potential candidates for the paleo-synanthropic niche, it is therefore advisable to look at present-day synanthropic vertebrates and compare them with the taxon lists of Late Pleistocene sites. Which animals already existed in archaeological sites and what were their feeding habits and body sizes? The most promising candidates therefore come from the canid family (Canidae, e.g., foxes (Yeshurun et al. 2009;Maher et al. 2011;Baumann et al. 2020a, b, c;Baumann et al. 2020b, c, a)), the mustelid family (Mustelidae, e.g., martens (Jędrzejewski et al. 1993;Russell and Storch 2004) and the European badger (Hewson and Kolb 1976;Young et al. 2015a;Young et al. 2015b)), the corvid family (Corvidae, e.g., the common raven (Baumann et al. 2022) and crows (Hewson 1981)), and the gull family (Laridae, e.g., the herring gull (Schwartz et al. 2018)).

Benefit and harm of paleo-synanthropism
As described at the beginning, the paleo-synanthropic behavior is not easy to equate with commensal behavior because, on the one hand, the safety aspect that the proximity to humans provides plays a role, and on the other hand the relationship between animal and human is not a + /0 relationship. The benefit that the paleo-synanthropic niche provided to the individuals that occupied it is similar to the benefit that synanthropic animals have today: a secure food source in a secure environment, which leads to increased reproduction, decreased home range, better survival advantages, and consequently higher population density (Gehrt et al. 2011;Hulme-Beaman et al. 2016). The higher the number of paleo-synanthropic animals at human camp sites, the easier it was for humans to hunt these animals (e.g., with traps) (Baumann 2020;Baumann et al. 2020a, b, c). Especially at the beginning of the Upper Paleolithic, smaller prey animals are increasingly found at sites, which were hunted by AMHs in addition to large game (Stiner et al. 2000;Starkovich 2012;Conard et al. 2013). This possibility could be related to the increased occupation of the paleo-synanthropic niche. The higher population density of paleo-synanthropic specimens compared to non-synanthropic representatives of the same species, as well as the relatively easy huntability in the immediate vicinity of human camps, resulted in a high quantity of remains of these animals in archaeological sites (Baumann 2020;Baumann et al. 2020a, b, c). Moreover, the overrepresentation of small scavengers may be an indication of paleo-synanthropic behavior, as already suggested by O'Connor (2013). In addition to the relative easy accessibility of meat, fur, and feathers, the utility of paleo-synanthropic animals for waste disposal (as known from later synanthropes (Dixon 1989;Moleon et al. 2014)) is another reason why humans may have tolerated them. When harmless small scavengers such as foxes and ravens removed people's food waste, it reduced the attraction of large, dangerous predators. Thus, paleo-synanthropic animals themselves may have helped to keep the environment safe from large predators.
However, paleo-synanthropic animals may have had one major disadvantage for humans. Because of the constant proximity of humans to these animals, diseases could have spread across species (Okulewicz et al. 2005;Klegarth 2016;Keck and Lynteris 2018). Scavengers, in particular, have a high potential for disease transmission due to their feeding habits (Murrell and Pozio 2000;Vicente and Vercauteren 2019). What, if any, infectious diseases may have played a role in the Late Pleistocene is still unexplored. Nonetheless, it is known from later synanthropic animals that their function as disease vectors poses a serious threat to humans (Keck and Lynteris 2018; Vicente and Vercauteren 2019).

Conclusion and perspectives for the studying of paleo-synanthropism
In summary, paleo-synanthropic behavior is an adaptation to the micro-environment created by Late Pleistocene hunter-gatherer societies. The resulting niche allowed small to medium-sized opportunistic generalist carnivores or omnivores to gain access to a long-term food resource, i.e., anthropogenic food waste. In addition, this niche provided security against large carnivores, which was ensured by their displacement by humans. As a result, the inhabitants of this new niche experienced population growth and a reduced home range. The paleo-synanthropic niche is not linked to specific geographic regions but is tied to human behavior and is therefore regionally flexible. This enabled paleo-synanthropes to exploit their niche for a long time. The high population density of these animals and their proximity to humans also allowed humans to use them as a resource for meat, fur, or feathers. On the downside, diseases such as zoonoses may have spread more easily.
Research on paleo-synanthropic behavior is still in its early stages, but recent results suggest that paleo-synanthropes may be useful as proxies for humans (Baumann 2020;Baumann et al. 2020a, b, c). Thus, the study of these animals can provide new insights into the resource use of past hunter-gatherer societies and consequently their ecological impact. Analysis of the trace isotopes sulfur and strontium can also provide new insights into human migration patterns and the occupation of archaeological sites. These insights can be gained by analyzing paleo-synanthropic animals without sampling precious human fossils. However, insights can also be gained into the process of the evolution of synanthropic behavior by answering the question of what mechanisms enabled certain animals to adapt to humans. This has implications for the crucial question of how biodiversity will evolve in the future. Which species have the potential to develop new niches in a human-made environment? Which species cannot and therefore need special protection? To answer these and many other questions, further paleogenetic, stable isotope, and zooarchaeological research on paleo-synanthropic animals is needed, as well as a link between this research and the field of urban evolution.
Acknowledgements I want to thank my colleagues Ruth Rey, Anja Furtwängler, Ella Reiter, and William Daniel Snyder for proofreading and helpful comments.
Author contributions C.B. did the conceptualization, manuscript writing, and visualization.
Funding Open Access funding enabled and organized by Projekt DEAL.
Data Availability All of the data on which this study is based can be found in the publications cited or have been published together with this study.

Competing interests The author declares no competing interests.
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