An Ecological and Evolutionary Framework for Commensalism in Anthropogenic Environments
Authors: Ardern Hulme-Beaman, Keith Dobney, Thomas Cucchi, Jeremy B. Searle
Journal: Trends in Ecology & Evolution (2016)

The definition of commensal taxa should be based on dependency. The concept of anthrodependent taxa leads to new species categorised as commensal. Commensals give insight into selective pressures in rapidly changing environments. Commensalism within anthropogenic environments has not been extensively discussed, despite its impact on humans, and there is no formal framework for assessing this ecological relationship in its varied forms. Here, we examine commensalism in anthropogenic environments in detail, considering both ecological and evolutionary drivers. The many assumptions about commensalism and the nature of anthropogenic environments are discussed and we highlight dependency as a key attribute of anthropogenic commensals (anthrodependent taxa). We primarily focus on mammalian species in the anthropogenic-commensal niche, but the traits described and selective pressures presented are likely fundamental to many species engaged in intense commensal relationships with humans. Furthermore, we demonstrate that this largely understudied interaction represents an important opportunity to investigate evolutionary processes in rapidly changing environments. Commensalism within anthropogenic environments has not been extensively discussed, despite its impact on humans, and there is no formal framework for assessing this ecological relationship in its varied forms. Here, we examine commensalism in anthropogenic environments in detail, considering both ecological and evolutionary drivers. The many assumptions about commensalism and the nature of anthropogenic environments are discussed and we highlight dependency as a key attribute of anthropogenic commensals (anthrodependent taxa). We primarily focus on mammalian species in the anthropogenic-commensal niche, but the traits described and selective pressures presented are likely fundamental to many species engaged in intense commensal relationships with humans. Furthermore, we demonstrate that this largely understudied interaction represents an important opportunity to investigate evolutionary processes in rapidly changing environments. Commensal taxa (see Glossary) have been major contributors to the human-mediated biodiversity crisis [1Stenseth N.C. et al.Mice, rats, and people: the bio-economics of agricultural rodent pests.Front. Ecol. Environ. 2003; 1: 367-375Crossref Scopus (213) Google Scholar, 2Martin L.J. et al.Evolution of the indoor biome.Trends Ecol. Evol. 2015; 30: 223-232Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar], are implicated in zoonoses [3Capizzi D. et al.Rating the rat: global patterns and research priorities in impacts and management of rodent pests.Mamm. Rev. 2014; 44: 148-162Crossref Scopus (102) Google Scholar], and were possible sources of domesticates [4Larson G. Fuller D.Q. The evolution of animal domestication.Annu. Rev. Ecol. Evol. Syst. 2014; 45: 271-295Crossref Scopus (300) Google Scholar, 5Zeder M.A. Pathways to animal domestication.in: Gepts P. Biodiversity in Agriculture: Domestication, Evolution and Sustainability. Cambridge University Press, 2012: 227-259Crossref Scopus (193) Google Scholar]; thus, they have a profound impact on life on Earth. However, many aspects of this relationship, and the ecological and evolutionary processes associated with commensal taxa, are surprisingly poorly understood. This stems from the seemingly simple definition of commensalism: a relationship where one species benefits (+) and the other species experiences no impact (0) [6Johnson N.C. et al.Functioning of mycorrhizal associations along the mutualism-parasitism continuum.New Phytol. 1997; 135: 575-585Crossref Scopus (1356) Google Scholar]. Observations of animals benefitting from anthropogenic environments often lead to the conclusion that these animals are undergoing commensal-specific evolution. There are many assumptions associated with this evolutionary process, which we break down in this paper. We contend that anthropogenic environments are highly unstable and do not entirely buffer commensal species from seasonality. Therefore, the evolution of commensal taxa is defined by their ability to become dependent on these unstable anthropogenic environments. Observations of commensal interactions have led researchers in fields such as archaeology and studies of domestication to categorise species as commensally dependent, but without providing a wider ecological context. For example, because there is currently no adequate ecology-based framework for commensal evolution, the view that feral dog populations are reasonable models for 'proto-domestic' forms (e.g., [4Larson G. Fuller D.Q. The evolution of animal domestication.Annu. Rev. Ecol. Evol. Syst. 2014; 45: 271-295Crossref Scopus (300) Google Scholar, 7Coppinger R. Coppinger L. A Startling New Understanding of Canine Origins, Behaviour and Evolution. Scribner, 2001Google Scholar]) has not been assessed with sufficient rigour. The assumption that domestic dogs derive ultimately from commensal wolves, therefore remains a premise underpinning much of the research into dog domestication (e.g., [8Axelsson E. et al.The genomic signature of dog domestication reveals adaptation to a starch-rich diet.Nature. 2013; 495: 360-364Crossref PubMed Scopus (599) Google Scholar]). Although wolves do engage in commensal relationships with humans as facultative scavengers (e.g., [9Boitani L. Ciucci P. Comparative social ecology of feral dogs and wolves.Ethol. Ecol. Evol. 1995; 7: 49-72Crossref Scopus (122) Google Scholar]), whether it is possible for a subgroup of wolves, prior to direct human interaction, truly to become a commensal taxon in the evolutionary sense remains untested. A review of the current usage of the term 'commensalism' in anthropogenic environments highlights how dependence is central to the understanding of commensal taxa. Here, we identify the unpredictability and instability of the anthropogenic niche as the primary selective pressures involved in the evolution of commensal species. Furthermore, we demonstrate that an ecological framework is required to: (i) identify the intensity of the relationship in order to define a taxon as commensal; (ii) assess the complexities of ecological systems associated with commensals; and (iii) assess selective pressures on commensal taxa that drive their evolution. We propose a new term for organisms that might represent the typical commensal condition in an anthropogenic environment: 'anthrodependent taxa' or 'anthrodependants' (Box 1).Box 1Previously Used Definitions of Species in Anthropogenic Environments (with Our Proposed New Category*)Commensal taxa (com-: sharing and -mensa: table)Commensalism in its original and simplest form specifies a relationship, represented as +/0, whereby one organism benefits (+) from another without causing deleterious effects to the other (0) [6Johnson N.C. et al.Functioning of mycorrhizal associations along the mutualism-parasitism continuum.New Phytol. 1997; 135: 575-585Crossref Scopus (1356) Google Scholar]. As applied to taxa, rather than relationships, it often refers to species living within houses [10Aplin K.P. et al.Evolutionary biology of the genus Rattus: profile of an archetypal rodent pest.in: Singleton G.R. Rats, Mice and People: Rodent Biology Management. ACIAR, 2003: 487-498Google Scholar, 12Pocock M.J.O. et al.Adaptations of animals to commensal habitats: population dynamics of house mice Mus musculus domesticus on farms.J. Anim. Ecol. 2004; 73: 878-888Crossref Scopus (99) Google Scholar]. More precisely, 'true commensals' live entirely in houses, 'occasional commensals' move between houses and outdoor habitats, and 'obligate commensals' are those that can only survive in an area because of their ability to occupy houses (e.g., black rats in northern Europe).Synanthropic and synurban taxa (syn-: together)These terms are used in studies of species that continue to live in areas that are occupied and altered by humans [83Baker P.J. Harris S. Urban mammals: what does the future hold? An analysis of the factors affecting patterns of use of residential gardens in Great Britain.Mamm. Rev. 2007; 37: 297-315Google Scholar, 84Granjon L. Duplantier J-M. Social structure in synanthropic populations of a murid rodent Mastomys natalensis in Sénégal.Acta Theriol. 1993; 38: 39-47Crossref Scopus (18) Google Scholar]. They can include both obligate-commensal species and also species not dependent on, or even particularly benefiting from, humans [83Baker P.J. Harris S. Urban mammals: what does the future hold? An analysis of the factors affecting patterns of use of residential gardens in Great Britain.Mamm. Rev. 2007; 37: 297-315Google Scholar]. These synanthropic species show a changed lifestyle associated with living in close proximity with humans (affecting breeding cycles, territorial behaviour, foraging behaviour, diet, etc. [85Gliwicz J. et al.Characteristic features of animal populations under synurbization - the case of the blackbird and of the striped field mouse.Memorab. Zool. 1994; 49: 137-244Google Scholar, 86Luniak M. Synurbization - adaptation of animal wildlife to urban development.in: Shaw W. Proceedings of the 4th International Urban Wildlife Symposium. University of Arizona Press, 2004: 50-55Google Scholar]). Synanthropic is also used to describe proto-commensal species in archaeology (e.g., [87Cucchi T. et al.On the origin of the house mouse synanthropy and dispersal in the Near East and Europe: zooarchaeological review and perspectives.in: Macholán M. Evolution of the House mouse. Cambridge University Press, 2012: 65-93Crossref Scopus (50) Google Scholar]).Anthropophilic taxa (-phile: lover of)Traditionally applied to ectoparasites, this term has increasingly been used in archaeology to indicate species attracted to human environments and activities [4Larson G. Fuller D.Q. The evolution of animal domestication.Annu. Rev. Ecol. Evol. Syst. 2014; 45: 271-295Crossref Scopus (300) Google Scholar, 88Cheke A. The timing of arrival of humans and their commensal animals on Western Indian Ocean oceanic islands.Phelsuma. 2010; 18: 38-69Google Scholar, 89Cucchi T. et al.First occurrence of the house mouse (Mus musculus domesticus Schwarz & Schwarz, 1943) in the Western Mediterranean: a zooarchaeological revision of subfossil occurrences.Biol. J. Linn. Soc. 2005; 84: 429-445Crossref Scopus (186) Google Scholar]. This can include not only commensal species, but also species opportunistically benefitting from humans, without dependence [90Vigne J.D. The origins of animal domestication and husbandry: a major change in the history of humanity and the biosphere.Comptes Rendus - Biol. 2011; 334: 171-181Crossref PubMed Scopus (228) Google Scholar].Domestic taxa (domus-: house)Humans actively buffer these taxa from external selective pressures, enhancing survival, and breeding. Therefore, humans reduce and sometimes remove impacts of natural evolutionary processes on these taxa and, at more advanced stages of domestication, humans directly control the selective pressures.*Anthrodependent taxaWe propose this term to refer to those taxa that might have had, and continue to have, a commensal relationship (+/0), but are defined by their dependence on anthropogenic resources, which in some cases can have a negative impact on their once commensal partner. Commensal taxa (com-: sharing and -mensa: table) Commensalism in its original and simplest form specifies a relationship, represented as +/0, whereby one organism benefits (+) from another without causing deleterious effects to the other (0) [6Johnson N.C. et al.Functioning of mycorrhizal associations along the mutualism-parasitism continuum.New Phytol. 1997; 135: 575-585Crossref Scopus (1356) Google Scholar]. As applied to taxa, rather than relationships, it often refers to species living within houses [10Aplin K.P. et al.Evolutionary biology of the genus Rattus: profile of an archetypal rodent pest.in: Singleton G.R. Rats, Mice and People: Rodent Biology Management. ACIAR, 2003: 487-498Google Scholar, 12Pocock M.J.O. et al.Adaptations of animals to commensal habitats: population dynamics of house mice Mus musculus domesticus on farms.J. Anim. Ecol. 2004; 73: 878-888Crossref Scopus (99) Google Scholar]. More precisely, 'true commensals' live entirely in houses, 'occasional commensals' move between houses and outdoor habitats, and 'obligate commensals' are those that can only survive in an area because of their ability to occupy houses (e.g., black rats in northern Europe). Synanthropic and synurban taxa (syn-: together) These terms are used in studies of species that continue to live in areas that are occupied and altered by humans [83Baker P.J. Harris S. Urban mammals: what does the future hold? An analysis of the factors affecting patterns of use of residential gardens in Great Britain.Mamm. Rev. 2007; 37: 297-315Google Scholar, 84Granjon L. Duplantier J-M. Social structure in synanthropic populations of a murid rodent Mastomys natalensis in Sénégal.Acta Theriol. 1993; 38: 39-47Crossref Scopus (18) Google Scholar]. They can include both obligate-commensal species and also species not dependent on, or even particularly benefiting from, humans [83Baker P.J. Harris S. Urban mammals: what does the future hold? An analysis of the factors affecting patterns of use of residential gardens in Great Britain.Mamm. Rev. 2007; 37: 297-315Google Scholar]. These synanthropic species show a changed lifestyle associated with living in close proximity with humans (affecting breeding cycles, territorial behaviour, foraging behaviour, diet, etc. [85Gliwicz J. et al.Characteristic features of animal populations under synurbization - the case of the blackbird and of the striped field mouse.Memorab. Zool. 1994; 49: 137-244Google Scholar, 86Luniak M. Synurbization - adaptation of animal wildlife to urban development.in: Shaw W. Proceedings of the 4th International Urban Wildlife Symposium. University of Arizona Press, 2004: 50-55Google Scholar]). Synanthropic is also used to describe proto-commensal species in archaeology (e.g., [87Cucchi T. et al.On the origin of the house mouse synanthropy and dispersal in the Near East and Europe: zooarchaeological review and perspectives.in: Macholán M. Evolution of the House mouse. Cambridge University Press, 2012: 65-93Crossref Scopus (50) Google Scholar]). Anthropophilic taxa (-phile: lover of) Traditionally applied to ectoparasites, this term has increasingly been used in archaeology to indicate species attracted to human environments and activities [4Larson G. Fuller D.Q. The evolution of animal domestication.Annu. Rev. Ecol. Evol. Syst. 2014; 45: 271-295Crossref Scopus (300) Google Scholar, 88Cheke A. The timing of arrival of humans and their commensal animals on Western Indian Ocean oceanic islands.Phelsuma. 2010; 18: 38-69Google Scholar, 89Cucchi T. et al.First occurrence of the house mouse (Mus musculus domesticus Schwarz & Schwarz, 1943) in the Western Mediterranean: a zooarchaeological revision of subfossil occurrences.Biol. J. Linn. Soc. 2005; 84: 429-445Crossref Scopus (186) Google Scholar]. This can include not only commensal species, but also species opportunistically benefitting from humans, without dependence [90Vigne J.D. The origins of animal domestication and husbandry: a major change in the history of humanity and the biosphere.Comptes Rendus - Biol. 2011; 334: 171-181Crossref PubMed Scopus (228) Google Scholar]. Domestic taxa (domus-: house) Humans actively buffer these taxa from external selective pressures, enhancing survival, and breeding. Therefore, humans reduce and sometimes remove impacts of natural evolutionary processes on these taxa and, at more advanced stages of domestication, humans directly control the selective pressures. *Anthrodependent taxa We propose this term to refer to those taxa that might have had, and continue to have, a commensal relationship (+/0), but are defined by their dependence on anthropogenic resources, which in some cases can have a negative impact on their once commensal partner. The term 'commensal', denoting a relationship, has a straightforward meaning [6Johnson N.C. et al.Functioning of mycorrhizal associations along the mutualism-parasitism continuum.New Phytol. 1997; 135: 575-585Crossref Scopus (1356) Google Scholar]. However, when the term is applied to a taxon (e.g., to rodents [10Aplin K.P. et al.Evolutionary biology of the genus Rattus: profile of an archetypal rodent pest.in: Singleton G.R. Rats, Mice and People: Rodent Biology Management. ACIAR, 2003: 487-498Google Scholar] or to a so-called 'proto-domestic' species [4Larson G. Fuller D.Q. The evolution of animal domestication.Annu. Rev. Ecol. Evol. Syst. 2014; 45: 271-295Crossref Scopus (300) Google Scholar]), it often means something more complex, reflecting both the acquisition of a new capability to exploit the anthropogenic environment and an associated reliance upon it. Furthermore, as the relationship becomes more intense (one of dependence), these 'commensal taxa' rarely have a neutral impact on humans; for example, they consume crops, steal food, and/or harbour disease [3Capizzi D. et al.Rating the rat: global patterns and research priorities in impacts and management of rodent pests.Mamm. Rev. 2014; 44: 148-162Crossref Scopus (102) Google Scholar, 11Morand S. et al.Global parasite and Rattus rodent invasions: the consequences for rodent-borne diseases.Integr. Zool. 2013; 10: 409-423Crossref Scopus (63) Google Scholar] (Box 2).Box 2The Nature of a Relationship: 'Commensal Neutrality'Many so-called 'commensal' species have anything but a neutral relationship with humans [3Capizzi D. et al.Rating the rat: global patterns and research priorities in impacts and management of rodent pests.Mamm. Rev. 2014; 44: 148-162Crossref Scopus (102) Google Scholar]. Their role as disease vectors might be considered a key negative impact [11Morand S. et al.Global parasite and Rattus rodent invasions: the consequences for rodent-borne diseases.Integr. Zool. 2013; 10: 409-423Crossref Scopus (63) Google Scholar], but it is entirely indirect, given that essentially no energy is expended to spread the disease (although parasite-induced behaviours exist [91Webster J.P. Rats, cats, people and parasites: the impact of latent toxoplasmosis on behaviour.Microbes Infect. 2001; 3: 1037-1045Crossref PubMed Scopus (165) Google Scholar]). Competition for resources is understood to be a–/–relationship [6Johnson N.C. et al.Functioning of mycorrhizal associations along the mutualism-parasitism continuum.New Phytol. 1997; 135: 575-585Crossref Scopus (1356) Google Scholar], with more obvious direct interaction. However, the comparison between the impact of disease and the damage from competition is difficult to quantify and might suggest that, when present, the indirect pathogen impact has a greater and more acute effect than the direct competition resulting from crop consumption [3Capizzi D. et al.Rating the rat: global patterns and research priorities in impacts and management of rodent pests.Mamm. Rev. 2014; 44: 148-162Crossref Scopus (102) Google Scholar]. Also, should we take into account the ability of humans to mitigate such impacts when considering whether a relationship is largely neutral or otherwise? Another question that is raised by the quantification of interactions is whether negative interactions are overriding and additive or whether they can be balanced by beneficial activities. Feral cats are a possible commensal species; they carry toxoplasmosis, a disease known to affect humans [91Webster J.P. Rats, cats, people and parasites: the impact of latent toxoplasmosis on behaviour.Microbes Infect. 2001; 3: 1037-1045Crossref PubMed Scopus (165) Google Scholar], but they also consume and regulate populations of other species known to carry many more diseases [11Morand S. et al.Global parasite and Rattus rodent invasions: the consequences for rodent-borne diseases.Integr. Zool. 2013; 10: 409-423Crossref Scopus (63) Google Scholar]. Feral cats consume competitors of humans (rodents), so if disease is not considered, does that then make the relationship mutual? Does the perceived beneficial behaviour of feral cats mitigate the negative impact they can have on humans? Ultimately, this extremely complex set of impacts affects the evolution of all organisms involved but, on an evolutionary ecology scale, it is currently unquantifiable. As a result, it might be best to simply categorise these interactions as fluctuating and complex rather than trying to define them as negative or neutral. Many so-called 'commensal' species have anything but a neutral relationship with humans [3Capizzi D. et al.Rating the rat: global patterns and research priorities in impacts and management of rodent pests.Mamm. Rev. 2014; 44: 148-162Crossref Scopus (102) Google Scholar]. Their role as disease vectors might be considered a key negative impact [11Morand S. et al.Global parasite and Rattus rodent invasions: the consequences for rodent-borne diseases.Integr. Zool. 2013; 10: 409-423Crossref Scopus (63) Google Scholar], but it is entirely indirect, given that essentially no energy is expended to spread the disease (although parasite-induced behaviours exist [91Webster J.P. Rats, cats, people and parasites: the impact of latent toxoplasmosis on behaviour.Microbes Infect. 2001; 3: 1037-1045Crossref PubMed Scopus (165) Google Scholar]). Competition for resources is understood to be a–/–relationship [6Johnson N.C. et al.Functioning of mycorrhizal associations along the mutualism-parasitism continuum.New Phytol. 1997; 135: 575-585Crossref Scopus (1356) Google Scholar], with more obvious direct interaction. However, the comparison between the impact of disease and the damage from competition is difficult to quantify and might suggest that, when present, the indirect pathogen impact has a greater and more acute effect than the direct competition resulting from crop consumption [3Capizzi D. et al.Rating the rat: global patterns and research priorities in impacts and management of rodent pests.Mamm. Rev. 2014; 44: 148-162Crossref Scopus (102) Google Scholar]. Also, should we take into account the ability of humans to mitigate such impacts when considering whether a relationship is largely neutral or otherwise? Another question that is raised by the quantification of interactions is whether negative interactions are overriding and additive or whether they can be balanced by beneficial activities. Feral cats are a possible commensal species; they carry toxoplasmosis, a disease known to affect humans [91Webster J.P. Rats, cats, people and parasites: the impact of latent toxoplasmosis on behaviour.Microbes Infect. 2001; 3: 1037-1045Crossref PubMed Scopus (165) Google Scholar], but they also consume and regulate populations of other species known to carry many more diseases [11Morand S. et al.Global parasite and Rattus rodent invasions: the consequences for rodent-borne diseases.Integr. Zool. 2013; 10: 409-423Crossref Scopus (63) Google Scholar]. Feral cats consume competitors of humans (rodents), so if disease is not considered, does that then make the relationship mutual? Does the perceived beneficial behaviour of feral cats mitigate the negative impact they can have on humans? Ultimately, this extremely complex set of impacts affects the evolution of all organisms involved but, on an evolutionary ecology scale, it is currently unquantifiable. As a result, it might be best to simply categorise these interactions as fluctuating and complex rather than trying to define them as negative or neutral. Taxa categorised as 'commensal' are reasonably assumed to be exposed to selective pressures relating to their exploitation of the anthropogenic niche [12Pocock M.J.O. et al.Adaptations of animals to commensal habitats: population dynamics of house mice Mus musculus domesticus on farms.J. Anim. Ecol. 2004; 73: 878-888Crossref Scopus (99) Google Scholar]. However, just what the main characteristics of the anthropogenic niche are (beyond a simplistic view that it represents a concentration of resources associated in some way with humans) remains unexplored, and this has led to further broad assumptions being made about evolutionary pressures on proposed commensal taxa (e.g. [8Axelsson E. et al.The genomic signature of dog domestication reveals adaptation to a starch-rich diet.Nature. 2013; 495: 360-364Crossref PubMed Scopus (599) Google Scholar]). Beyond the obvious settings of human-made structures and urban environments, it must also be recognised that agricultural environments are also subject to unpredictable and unseasonal human activity, resulting in 'unnatural' availability of resources. These too have a range of associated dependent taxa, often considered 'pests'. At low densities, such taxa might be considered to hold commensal relationships, but, as densities become higher, they might be viewed as competitors, although these categories are largely subjective. From an ecological and evolutionary perspective, these competing taxa inhabit an unpredictable and fluctuating anthropogenic environment similar to that experienced by many commensals and, therefore, can be considered anthrodependent with a commensal origin. Other discrepancies in definitions stem from a research bias towards small-bodied animals living in human-made structures [10Aplin K.P. et al.Evolutionary biology of the genus Rattus: profile of an archetypal rodent pest.in: Singleton G.R. Rats, Mice and People: Rodent Biology Management. ACIAR, 2003: 487-498Google Scholar, 12Pocock M.J.O. et al.Adaptations of animals to commensal habitats: population dynamics of house mice Mus musculus domesticus on farms.J. Anim. Ecol. 2004; 73: 878-888Crossref Scopus (99) Google Scholar]. Occupation of such structures sets an arbitrary definition of 'commensal taxa' with the result that comparisons of similar evolutionary processes experienced by large-bodied species are overlooked. In this way, a unifying definition and understanding is needed of 'commensal taxa' that allows an evolutionary understanding and is transferable between taxa (with varying body and home-range sizes) that occupy different anthropogenic environments and exploit diverse anthropogenic resources. Far more useful is to consider commensalism (and other related types of species interaction) in terms of the degree of dependence upon an anthropogenic niche (Figure 1). Dependency comes with both benefits and costs, which in turn drive the evolution of anthrodependent taxa [5Zeder M.A. Pathways to animal domestication.in: Gepts P. Biodiversity in Agriculture: Domestication, Evolution and Sustainability. Cambridge University Press, 2012: 227-259Crossref Scopus (193) Google Scholar]. Examining the selective pressures stemming from both benefits and costs provides a more appropriate framework that allows detailed insight into evolutionary processes specifically associated with past, present, and future anthropogenic environments. Only by interrogating the archaeological record will we gain a fuller understanding of the way such taxa evolve and adapt [13Weissbrod L. Biological indicators of occupation intensity: an environmental ethnoarchaeology of Maasai ettlements.in: Dean R.M. The Archaeology of Anthropogenic Environments. Illinois University Press, 2010: 295-320Google Scholar]. Humans have been actively altering the environment around them for thousands of years in what can be considered a form of niche construction [14Zeder M.A. Domestication as a model system for niche construction theory.Evol. Ecol. 2016; 30: 325-348Crossref Scopus (98) Google Scholar]. It is this constructed environment, and the abundance of resources within it, to which anthrodependent taxa have adapted and thrived. The extent of anthropogenic environments has increased through time [14Zeder M.A. Domestication as a model system for niche construction theory.Evol. Ecol. 2016; 30: 325-348Crossref Scopus (98) Google Scholar], as has the prevalence of commensal taxa [15Cucchi T. et al.On the trail of Neolithic mice and men towards Transcaucasia: zooarchaeological clues from Nakhchivan (Azerbaijan).Biol. J. Linn. Soc. 2013; 108: 917-928Crossref Scopus (31) Google Scholar]. The ability of a taxon to maintain access to anthropogenic environments dictates its success and evolution as an anthrodependant. In particular, this environment is often characterised by extreme and unpredictable fluctuations of resource availability and nest sites [12Pocock M.J.O. et al.Adaptations of animals to commensal habitats: population dynamics of house mice Mus musculus domesticus on farms.J. Anim. Ecol. 2004; 73: 878-888Crossref Scopus (99) Google Scholar, 16Newsome A.E. A population study of house-mice temporarily inhabiting a South Australian wheatfield.J. Anim. Ecol. 1969; 38: 341-359Crossref Google Scholar], characteristics that are frequently overlooked. It is often assumed that anthropogenic environments are more stable in terms of resource availability than are natural environments [17Anderies J.M. et al.Living in the city: resource availability, predation, and bird population dynamics in urban areas.J. Theor. Biol. 2007; 247: 36-49Crossref PubMed Scopus (102) Google Scholar]. This is not always the case [12Pocock M.J.O. et al.Adaptations of animals to commensal habitats: population dynamics of house mice Mus musculus domesticus on farms.J. Anim. Ecol. 2004; 73: 878-888Crossref Scopus (99) Google Scholar]; neither was it likely to have been so throughout the past, when wild organisms first entered the anthropogenic niche [13Weissbrod L. Biological indicators of occupation intensity: an environmental ethnoarchaeology of Maasai ettlements.in: Dean R.M. The Archaeology of Anthropogenic Environments. Illinois Universit