Preserving for the future the — once widespread but now vanishing — knowledge on traditional pig grazing in forests and marshes (Sava-Bosut floodplain, Serbia)

Background Traditional knowledge is key for sustainability, but it is rapidly disappearing. Pig keeping in forests and marshes is an ancient, once widespread, now vanishing practice, with a major economic and ecological potential. The knowledge of pig keepers and the foraging activity of pigs are hardly documented. Methods We studied the knowledge of traditional pig keepers (svinjars) on wild plants and pig foraging on the Sava-Bosut forest-marsh complex in Serbia. We conducted picture-based interviews about 234 locally common and/or salient plant species, and participatory fieldwork (11 days) and visual observation (21 days) on pig foraging. Results 181 wild plant species were known by svinjars and 106 taxa were consumed by pigs. Svinjars knew well and could name most regularly foraged species. 98 species were reported by svinjars as foraged and 56 as not eaten. 28 species were observed by the authors as eaten regularly, while 21 were nibbled and 17 avoided. Contradictory information on foraging was rare both among svinjars (8 species) and between svinjars and researchers (7 species); several of these species were rare. Leaves of 92, fruits or seeds of 21 and ‘roots’ of 20 species were reported or observed as eaten, usually with high seasonality. Svinjars were overall observant, but knew little about some less salient species (e.g. Veronica, Circaea). The most common forages (reported and/or observed) were fruits (Quercus, fleshy fruits), grasses (Agrostis, Glyceria), herbs (Ranunculus ficaria, Circaea), nutritious ‘roots’ (Carex spp., Iris), young shrub leaves (Crataegus, Carpinus) and ‘tame’ plants growing in the sun (Persicaria dubia, Erigeron annuus). Traditional, now extinct pig breeds were reported as less selective and more ‘knowledgeable’ about plants, as they received less additional fodder. Svinjars learnt their knowledge since childhood, from community members, but long-term personal observations and everyday encounters with pigs were also important sources of knowledge. Conclusions A deeper understanding of pig foraging could contribute to using pigs in nature conservation management, resource management and organic farming, and to a better understanding of wild boar foraging. The knowledge of svinjars is a disappearing intangible cultural heritage of European importance. Knowledge holders deserve recognition, and legal and financial support to continue this tradition.

Background Extensive traditional livestock husbandry systems often produce high-quality food, create and maintain new species-rich habitats (e.g. hay meadows, wood-pastures), and manage ecosystems for biodiversity conservation [1,2]. These traditional land-use practices and the related traditional ecological knowledge, are vital for sustainability but they are rapidly disappearing [3]. Knowledge on cattle and sheep grazing is relatively well documented [4,5] but extensive traditional pig grazing is rarely studied [6].
Today the practice of keeping pigs in closed forests and marshes is almost extinct in Europe, although a revival of the practice is advocated in nature conservation management and organic farming [6,7,14,24,[29][30][31][32][33]. Allowing pigs to range free in semi-natural habitats has positive impacts on the health of the animals and the quality of their meat [34][35][36][37] is less resource-intensive (lower time, energy and feed costs; [31]), and is more environmentally friendly [38,39]. The practice can even benefit red-listed protected species, e.g. marsh plants and birds [6,40,41].
European forests and wetlands have been radically transformed over the last centuries [42], traditional landuse practices have ceased [22,33,43], and multidimensionality of use has decreased [44,45]. Prohibition of some traditional uses (e.g. forest grazing, pig grazing) has also contributed to the rapid decline and erosion of related traditional ecological knowledge [43], with severe biodiversity effects [46]. Due to the changing environment and lifestyles, innovations and novel adaptations, based on available knowledge and existing traditions, are needed for the successful continuation of extensive land-use practices (incl. pig keeping) and to further increase their benefits cf. [5,33].
Ethnobiology-among other things-documents traditional knowledge and practices, the related values and worldviews, and the ways in which traditional knowledge is generated and transmitted [47,48]. Ethnobotanical knowledge of medicinal and wild food plants is relatively well documented in Europe (e.g. [49,50]) but major gaps still exist when it comes to knowledge related to pastoral practices, e.g. about forage plants [5,51]. The practices and impacts of traditional livestock grazing on grasslands are also relatively well documented in Europe (e.g. [52][53][54]). Much less is known about extensive traditional forest and marsh grazing and the foraging behaviour of livestock in these habitats [33,43,46,55], especially about pig grazing [6,31].
Ethnobiologists understand their responsibility to document and archive traditional (folk) knowledge, and to support the continuation of traditional practices and ongoing adaptation and knowledge generation [3,56,57]. Documenting traditional knowledge can help knowledge transmission and revival programmes [58][59][60] and assist in developing culturally appropriate agricultural and conservation regulations and subsidies [61,62].
We are not aware of any recent ethnobiological scientific study on the existing ecological knowledge of traditional pig keepers rearing pigs in semi-natural forests, grasslands and marshes in Europe. All studies are ethnographical or historical, and document historical knowledge and practices. For example, Rodríguez-Estévez et al. [63] list 133 wild and cultivated forage taxa that were traditionally collected and given to dehesa pigs in Spain, while Szabadfalvi [15] reviewed traditional pig keeping in Hungary.
The domestic pig (Sus scrofa domestica L.) belongs to the same species as the wild boar, and there is a rich literature on wild boar foraging on various plant species [64,65]. Wild boar can cause serious damage, especially in crop fields, so studies focus more on agricultural areas and crops than on forests and marshes [66][67][68]. In detailed case studies conducted in semi-natural landscapes, 32-104 wild plant species were found to be foraged by wild boars [64,69], the closest to our study area [70] but identification of foraged plant species is limited by the methodologies applied (stomach and faeces analysis) [65]. Observation-based studies on behaviour and foraging (including grazing and rooting) of wild boar are scarce [71][72][73]. Observational studies on free-ranging Keywords: Knowledge generation, Cultural heritage, Conservation management, Sustainable use of natural resources, Foraging, Sus scrofa domestica, Wild boar domestic pigs are also rare and done on non-traditionally kept pigs grazing in enclosures [23,25]. Our study is the first on traditionally kept, free-ranging pigs foraging in semi-natural habitats.
The main objective of our study was to document the vanishing traditional ecological knowledge and practice of rearing pigs in closed (oak) forests and marshes in a semi-natural floodplain (the Sava-Bosut floodplain in Serbia). This knowledge was once widely possessed and applied across Europe (considering the former extent of the associated practice), but has almost completely disappeared. Now it is possessed only by 100 people at most, almost exclusively over 60 years of age, in some limited locations in Central Europe.
In this paper we studied (1) the traditional ecological knowledge of traditional forest pig keepers (called locally: svinjar) on wild plant species, specifically local folk names, local knowledge of pig-plant relationships (eat, love, avoid, toxic, medicinal etc.), other local uses and salient features of these plant species; and (2) how traditional knowledge of pig foraging behaviour and foraged plants was generated and transmitted in this community.

Study area
Although forest grazing was common even in the second half of the twentieth century in the former Yugoslavia [74], nowadays the floodplains of the Sava, Bosut and Studva rivers are among the very last places where the foraging behaviour of free ranging pigs in closed forests and marshes can be studied in Central Europe [31,32,75]. We worked in the territory of the villages Morović and Višnjićevo (marginally in Jamena) close to the administrative borders between the Republic of Serbia, Bosnia & Herzegovina and the Republic of Croatia (Fig. 1).
The study area belongs to the famous Slavonian oak forest area [76]. The Bosut forest was for centuries part of the border zone between the Austro-Hungarian Monarchy and the Ottoman Empire. The high importance of large-scale pig keeping is documented at least from the seventeenth century [77]. Forest management favouring hardwood species (Quercus, Fraxinus, Ulmus, Carpinus) dates back to the formation of the Military Border [76]. With some technological changes, Quercus robur remained the major tree species [78] and was the cornerstone of pannage. Pig, cattle and sheep grazing was the main use of the forests till the end of the nineteenth century. Forests were managed by irregular selective cutting of valuable timber. With the increased interest of the European timber market in high quality oak timber in the mid of nineteenth century, forest management intensified and the importance of forest grazing decreased [77,79]. The forests in the study area are now state owned.
The study area is dominated by forests with embedded floodplain marshes, and arable land around villages. The Bosut Forest (21,852 ha) is a local ecological hotspot, recognized as an Important Bird Area (RS007) and a core area of the National Ecological Network. The climate is subcontinental with 11 °C mean annual temperature and a yearly average precipitation of 700 mm [80]. The area was cut off from floods in the 1930s and became drier in the twentieth century [31,32,81]. Vegetation and flora are diverse [82,83]. Forests are dominated by Quercus robur and Carpinus betulus at higher elevations, and Fraxinus angustifolia subsp. pannonica at lower elevations (see auctors of species in Tables 2 and 3, based on [84]). The health status of Quercus robur stands is deteriorating. The herb layer is typical of Central European hardwood forests, but relatively poor in Fagetalia (beech forest) species. Marshes are dominated by Carex riparia, Glyceria maxima, G. fluitans, Iris pseudacorus and Salix cinerea. Grazed marshes are rich in red-listed Nanocyperion (mud preferring) species such as Marsilea quadrifolia and Ludwigia palustris.
Organized pig breeding in the vast lowland forestmarsh complex of the Sava river floodplain was integrated into traditional forest management where the beneficial effects of pig rooting on acorn germination were acknowledged by key forestry experts [85]. The pannage fees and rules were designed to be in line with forest management, with several fee rates depending on the development phase of the foraging pigs and the abundance of mast. According to local svinjars, the now extinct traditional breed Sremska Lasa was longer and larger compared to Mangulica (Mangalitza), and had a black back, white stomach and shorter hair. It farrowed more piglets than Mangulica. Pigs from the 'English Pfeifer' breed were provided to farmers by the state from 1964/1965 to replace the more extensive Sremska Lasa and Mangulica breeds. Mangulica pigs disappeared quickly, but farmers bred Sremska Lasa with Pfeifer and Duroc breeds. Wild boars are present in the area and are hunted. Boars regularly approach pig feeding places and sometimes cross-breed with domestic pigs.
Today, svinjars continue many of the traditional practices but also adapt and innovate them [86]. They keep Yorkshire, Duroc, Landrace and Piétrain breeds, mostly usually their hybrids, which the breeders found to be more resistant and better adapted to year-round outdoor living. The main goal of forest pig keeping is economic (it is cheaper), but the high quality of the meat is also widely valued by svinjars and locals alike (roasted piglets and kulen-a local meaty sausageis more tasty when prepared from forest pigs). The meat is of similar quality to the famous Iberico ham, but is much less well known. Sows are reported to be healthier and with a remarkably longer period of fertility in the forest (10-12 years) than if kept at home (5-6 years), and they also live longer. "The forest keeps pigs healthy and cures them." The number of pigs and svinjars and the area used by them are decreasing. Svinjars keep their 50-150 pigs (nowadays fewer) in an area measuring ca. 30-100 hectares (< 2000, even < 100 pigs in total in recent years). Since jackals recolonized the area, most svinjars only keep their < 10-30 sows in the forest, while piglets are brought to the village after weaning. Pig numbers kept in the forests were much higher until the 1950-1960s, reaching 30-50 thousands (not rarely hundreds per owner). Additionally cattle and sheep were kept in this area earlier in the twentieth century.
Svinjars live in the nearby villages, keep their own pigs, visit them usually every day for several hours, check their health, and provide extra fodder (mostly corn) as needed (e.g. more in drought and in snow). This regular feeding prevents pigs from wandering larger distances. Pigs are kept in the forest all year round, but shoats born in the forest are often kept at home under semi-intensive conditions on intensive fodder (partly because of the risk of predation by the golden jackal). Even modern breeds can become dangerous, protecting their piglets instinctively, so the owners take care to tame them, in some cases treating them almost as pets. The tameness of the pigs also helped our observational research (see below). During the night, pigs and piglets are kept in massive enclosures built from wood. Pigs are under veterinary control using modern medicines. African swine fever is encroaching on the region. The forest and marsh 'pastures' and masting are regulated and contracted from the local forestry office (Internal Code of forest farming, PE Vojvodinašume and PE Srbijašume). The forest is fenced towards the agricultural fields to the north and was previously also fenced to the south (towards non-grazed forests). Habitat management by pigs is part of the current and planned future nature conservation management of the local protected area [31,32].

Data collection
We deliberately used an iterative and flexible methodology, combining and alternating indoor and outdoor interviewing, participatory fieldwork, landscape walks and personal visual observation of pig foraging behaviour, in order to reveal as much of the local knowledge related to pigs and plants as possible. As our plan was to conduct exploratory research into knowledge that has hardly been studied before, no quantification was made at this stage. The seasonality of foraging behaviour and foraging on animal species were not focuses of this paper.
After three preparatory visits to the area in 2014-2016 we interviewed 7 highly knowledgeable svinjars about forest and marsh plant species in April 2017. A larger sample size was not possible because the total number of svinjars is itself small (17 persons). Pictures of 234 locally occurring forest, marsh and other plant species (based on the flora list of the area; [83] printed on A4 size sheets were shown to svinjars (plants were shown in a way traditional knowledge holders tend to recognize them: whole plants or focusing on leaves, fruits, rhizomes; see [89]). We asked about the following topics: (1) the local name of the plant; (2) its relationship with pigs (eat, love, avoid, toxic, medicinal etc.), (3) other uses and salient features of the species. The interviewed svinjars were all above 60 years of age, and had lifelong experience with pig keeping. Interviews lasted 2-4 h, and were conducted mostly at the forest huts, in Serbian, constantly translated to English or Hungarian, and transcribed later in full. Prior informed consent was obtained from all svinjars before the first interviews, adhering to the code of ethics of the International Society of Ethnobiology [57] and the GDPR of the European Union [90].
Prior to the interviews, we surveyed the vegetation of the marshes and forests along a grazing intensity gradient (methods and results in [6] and Demeter et al. ined.) to obtain a detailed understanding of the local flora and vegetation, and to help devise specific questions to ask svinjars about foraged (and non-foraged) species and pig foraging behaviour.
We also conducted participatory fieldwork. We joined svinjars during their grazing trips (8 times) and also went on landscape walks with them (12 times). During these occasions we were able to gather data on plants which were not recognized from pictures (e.g. some grasses and small herbs), and discuss pig behaviour on the spot, and observe and discuss how svinjars generated knowledge about plants and pig behaviour.
We made visual observations of pig foraging behaviour in February, March, April, June, August, October and December 2019, on altogether 21 days. We followed the herd (3-12 pigs) foraging in forests and marshes, and regularly alternated between the closely observed pig(s), every 5-20 min, depending on their movements. We observed ingestive bite selection and avoidance behaviour from morning till mid/late afternoon (see Fig. 2). Most pigs were calm, and the observed individuals were selected at random. We went as close as possible (0.4-3 m), keeping our impact on the pigs to the minimum while ensuring a good view of the plants near each animal's mouth. When plants were difficult to identify to the species level, we visited the spot just after the animal moved away. We documented how certain plant species were approached or avoided by pigs and how often and for how long certain species were eaten by them. For the less common species our data are not yet representative for a quantitative analysis, so in Table 2 we indicated the depth and reliability of our data with appropriate wording. To avoid differences among observers, most observations were done by the first author who had a full knowledge of the local flora and was experienced in similar studies [5]. Photos and videos were also made to document foraging. The area regularly visited for data collection covers ca. 750 hectares in total (Fig. 1).
During each visit to observe pig foraging we interviewed svinjars about the foraging behaviour of pigs (1) since our last visit and (2) at the moment, as well as (3) the expected foraging behaviour over the next month. We also shared our experiences collected during visual observations and asked svinjars to comment on those. We used two types of elicitation: we asked direct questions (e.g. what do you think, why were some acorns avoided by this pig?) or formulated statements and waited for svinjars' comments (e.g. we observed that pigs graze Ranunculus ficaria only for some seconds). We also made 5 additional detailed interviews about seasonal changes of foraging behaviour (from January till December). We were also keen to learn how pig keeping and pig foraging behaviour have changed since their childhood (1940s-1960s). Some of the most typical and relevant responses (in translation) are indicated in italicized text between quotation marks.
As the main objective of the research was exploratory, investigating and documenting something which was EaƟng (chewing) Carex sp. 'roots' in a marsh in February "for the sweet juice and 'vitamins' " Searching for a liƩle fresh green grass in February, "they need it aŌer eaƟng earthworms for hours" R. ficaria is the first green in spring. "They eat it as soon as it is bigger than a fingernail." Browsing on fresh Crataegus laevigata leaves in April "as a salad aŌer their morning corn" Grazed Circaea luteƟana in June (see fluid mud on the top leŌ leaf), "we do not noƟce these small plants" Grazing marsh grasses (Glyceria fluitans, G. maxima) in August, when "the forest is empty/hungry"  not considered earlier by ecologists and conservationists, during the whole study we tried to remain as open as possible regarding interview topics and field situations. In some cases our questions might have seemed slightly uncomfortable and unexpected to the svinjars, but they knew that we were keen to understand their knowledge and their culture related to forests, marshes and pigs, so they were very helpful and patient with us. They knew and agreed that the final goal of the research is to publish scientific papers and a book similar to Molnár [89] and Babai et al. [91] on their traditional ecological knowledge and the traditional pig keeping practices and to help the continuation of this still valuable but vanishing land-use practice.

Data analysis
The 7 main ethnobotanical and 5 season-specific interviews were fully transcribed (520,000 characters). A database was built where data were grouped according to plant species and main topics (foraging behaviour type, season, way of learning about plants, sayings and quotes related to worldview, historical information). All field notes taken during visual observation, participatory fieldwork and landscape walks were also transcribed and inserted into the above database.
We included a plant species in the analysis if at least two independent interview data (i.e. two svinjars) or two independent observations (2 days or two herds) were available. Plant names follow the Euro + Med Plant Base [84]. The following species occur in the study area and/ or at least one svinjar had some knowledge about them, but they still lack sufficient ethnobotanical or foraging observational data and were therefore excluded from the present analysis ( Local frequency of all studied plant species is provided in Table 2 for a general overview of the local flora. Frequency data were based on Perić [83] and the extensive fieldwork of the authors. We regarded 'roots' as what svinjars called 'koren' (i.e. roots, bulbs, rhizomes and tubers). We used the following categories of foraging: (1) eaten/loved: whole plants, or at least large parts were eaten (if consumption was often observed, it is indicated in Table 2); (2) nibbled: only smaller parts were eaten; (3) avoided, not eaten: untouched on encounter, occasionally smelled (see [5] for more details). Uncertain but probably relevant data were put into parentheses in order not to lose any already identified information in this exploratory study.
Categories of knowledge depth for the 192 analysed plant species: (1) well known: all or most (5-7) svinjars knew its local folk name(s) and could describe the species in detail; (2) moderately known: only 3-4 svinjars knew the species, and/or knowledge was less detailed; (3) little known: only 1 or 2 svinjars knew the species or if more, knowledge was not detailed (usually rare or small or 'insignificant' species); (4) not known: none of the svinjars knew the species (these species were only included in Table 2 if visual foraging observation data by the authors was available). The level of local knowledge of each species was compared to the regional level knowledge of the species (from well known to not known in other areas of the Carpathian Basin) using Molnár [89], Dénes et al. [92], Babai et al. [91], and Molnár [93].

Svinjars' knowledge of plants and observations on pig foraging behaviour
We collected ethnobotanical and/or visual observation data for 237 species, of which 192 could be analysed (i.e. had two independent data). Svinjars knew 181 of the 192 species. 102 plant species were well known by them (Table 1), 27 species were moderately known, 52 little known, and 11 species that occurred in the forests and marshes were probably not known by them (Tables 2 and  3). There were 10 species that were well or moderately known but not named by svinjars. Svinjars regretted that they could not remember any name.
Ninety-eight species were reported by svinjars as eaten by pigs and 56 as not eaten. There were 38 species (of the 192) for which svinjars could not give information about pig foraging (how much pigs like the species). 28 species were observed by the authors as eaten regularly and 21 as nibbled by pigs, while 17 species were seen as deliberately avoided. Overlaps between svinjars' and authors' observations were moderate. For 126 plant species authors were not able to observe any foraging or avoidance behaviour because of the lack of observed encounters with the species by pigs in the field (of the 126 species 41 species did not occur in the observation area). Leaves of 92 species, fruits or seeds of 21 species and 'roots' of 20 species were reported or observed as eaten.
Species that were common or moderately common in the forest but were only little known by svinjars were Circaea lutetiana, Moehringia trinervia, Veronica montana and V. hederifolia, Lysimachia nummularia, Glechoma hederacea, Brachypodium sylvaticum, Callitriche sp., Carpesium spp. and Genista tinctoria. Some species were rare in the forest but were known by svinjars from gardens (Cornus mas, Galanthus nivalis, Convallaria majalis) and svinjars knew a lot about two, regionally widely known species (Fagus sylvatica and Allium ursinum) not found recently in the area. Many other of the well known species were rare in the forest but common in the arable landscapes around the villages (e.g. Cynodon dactylon, Sonchus spp.). Svinjars knew many of the invasive alien species well.
For some 'insignificant for svinjars' species, svinjars did not know whether pigs eat them or not, though three were regularly observed by the authors as eaten (Circaea lutetiana, Rorippa amphibia, Leersia oryzoides), while others were not eaten based on our observations (Lycopus spp., Carpesium spp.). Some species were less well known by svinjars than in other parts of the Carpathian Basin (Hypericum sp., Colchicum autumnale, Achillea sp., Chelidonium majus, Lotus corniculatus), while Acer tataricum, Persicaria dubia and the 'small forest-Carex' folk taxon that includes Carex remota, C. sylvatica and C. divulsa were better known.
The most common plant forages of pigs (mentioned or observed) were fruits of trees (esp. Quercus and fleshy forest fruits), marsh grasses (esp. Agrostis and Glyceria), forest herbs (esp. Ranunculus ficaria and Circaea lutetiana), marsh plants with nutritious 'roots' (esp. Carex spp. and Iris pseudacorus), shrubs and young trees with young fresh leaves (esp. Crataegus and Carpinus), as well as 'tame' plants that grow in the sun ("pitomina", e.g. Persicaria dubia and Erigeron annuus). Some species were reported as especially loved by pigs (even if some were less available to them): Quercus robur acorns (it is the primary forage, always eaten if available), Polygonum aviculare, Amaranthus retroflexus and Sonchus spp. leaves, and Pyrus sp., Juglans regia, Prunus cerasifera and Trapa natans fruits. Svinjars spoke enthusiastically about how eagerly pigs forage on these species. Some species (e.g. Salix alba, Ulmus minor) were not eaten or only rarely because in April-June many other-"sweeter"-species were available ("nothing forces pigs to eat them, pigs are choosy in these months"). Svinjars reported that pigs graze more grass in wet years, and even eat grass from below the water. Many of the invasive alien species were not eaten by pigs, e.g. Ambrosia artemisiifolia, Amorpha fruticosa, Asclepias syriaca, Phytolacca americana, Robinia pseudoacacia, Solidago gigantea, Sorghum halepense, Vitis sp., while Erigeron annuus (little known by svinjars) was regularly eaten.
Svinjars argued that traditional pig breeds (e.g. the black coloured Sremska Lasa breed) were less selective when grazing and more 'knowledgeable' about edible forest and marsh plants, as they received less additional fodder (corn).

Other uses and salient features (e.g. impact on pigs) reported by svinjars
Trees Acer campestre L. (3), well known, klen (well known) Leaves are eaten eagerly in spring, liked more than Carpinus leaves, eaten less in summer but longer than other tree leaves, also fallen leaves in autumn, fruits are eaten in need Leaves were eaten, also fallen leaves

Other uses and salient features (e.g. impact on pigs) reported by svinjars
Typha angustifolia L., T. latifolia L. (

Other uses and salient features (e.g. impact on pigs) reported by svinjars
Stellaria media (L.) Cirillo (2)  In the first column, scientific names are followed by data on frequency in the study area (4-common, 3-frequent, 2-sporadic, 1-rare, 0-missing); how well the plant is known (well, moderately, little or not known); local folk names (followed by their meaning, if known); and finally, in parentheses, the level of folk knowledge of the species in general in the Carpathian Basin (well, moderately, little or not known-based on [89,[91][92][93] and authors' unpublished data) N.o. no observation on foraging or avoidance of the species, N.d. no data Foraging varied greatly depending on the season. Pigs grazed on marsh and forest plants and anything that remained green in winter while foraging dominantly on Quercus robur acorns and/or earthworms. Pigs loved the young fresh herb, grass and shrub leaves (especially Agrostis stolonifera, Ranunculus ficaria, Crataegus spp.) sprouting in spring, and foraged on the remaining acorns. By mid-summer leaves hardened or dried: "the forest is empty, hungry". Pigs grazed in sunny places and on grasses and waterweeds of drying and waterlogged marshes. Svinjars recalled that in the past, pigs were driven to feed on the weeds and crop residues of stubble fields in this "empty" period. In August and September acorns start falling, and forest fruits (cherry plum in July, followed later by wild pear and apple) start ripening. Pigs foraged on acorns (loved more after rains) and on fallen tree leaves (mostly Acer campestre and Fraxinus angustifolia). Foraging during the autumn and winter season depended on the availability of acorns. In snowy winters, pigs foraged more on the 'roots' of marsh plants (e.g. chewing on Carex rhizomes) and, when in severe need, on certain wild fruits (Prunus spinosa, Crataegus spp.). If there were no acorns, they searched for earthworms and ate remaining fruits and green leaves. Pigs ate'roots' (but only of non-woody species) mostly in winter, and especially in forage-poor years.
Contradictory information on foraging was rare. Svinjars disagreed on altogether 8 species, but three were rare in the forest (Viola alba, Marsilea quadrifolia, Typha spp.); in four cases (Viola reichenbachiana, Oenanthe Table 3 Plant species (30) that were asked in interviews but were probably little or not known by local svinjars (no data on pig foraging was reported or observed)

Additional information
Acer aquatica, Plantago major and Primula acaulis) one or two svinjars disagreed with the others whether it was foraged or not. Svinjars' reports contradicted our observations only for Erigeron annuus and Glechoma hederacea and for a further 5 species, but for the latter the difference was only whether the species were 'not eaten' or 'only nibbled' (see Table 2 for details).
Some plants were foraged only in a short time-window and/or in a given phenological state (e.g. young leaves of tree and shrub species mainly in the spring, Juncus 'roots' in snowy winters). Most plant species were foraged only for very short time periods during the observations (less than a minute even if available), except: acorns, marsh grasses (Agrostis, Glyceria), Carex rhizomes, Polygonum aviculare, Ranunculus ficaria, Crataegus spp. leaves, Lemna-Spirodela spp., Glechoma hederacea, Circaea lutetiana, and fallen leaves of Acer campestre and Fraxinus angustifolia.
Some species were reported as only eaten by pigs if nothing else was available (famine foods for pigs). For example, the fruits of Acer campestre, Rosa canina and Cornus sanguinea, and the 'roots' of Carex elata, Juncus effusus and Urtica dioica. Svinjars added: "They have to love them… if they are in need [food shortage]. " Some plant species were reported as being eaten differently by or having a different impact on pigs (of the same breed), depending on whether they were kept at home in the village or in the forest (Chenopodium album, Urtica dioica).
Grasses and herbs growing in sunny places at forest edges, along lanes and roadsides were called "pitomina" (from the word tame and animal feed), as opposed to the lower quality grasses and herbs growing in the shade of the forest. Persicaria dubia (P. mite) was reported and observed as being mostly eaten in sunny places. Pigs grazed 'precisely' , i.e. they were able to focus on the preferred species. For example, they grazed Rorippa amphibia from among Mentha aquatica, and Glyceria and Agrostis grasses from among Chaiturus (Leonurus) marrubiastrum.
Svinjars rarely drove pigs deliberately to forage on specific species and in specific places, and did so mostly in the past: Quercus robur and Q. cerris acorns, grassy village pastures in summer, beech mast (to mountains, but only in the past), waterweeds (lokvanj) in the rivers, and stubble fields. Driving and keeping pigs away from certain forages or sites was still practised: unharvested arable crops, reforestations (with acorns), areas designated for acorn collection, hunting areas, and to prevent damage to river dykes, railway and road verges.
Svinjars argued that "those pigs are the most 'beautiful' which can go wherever they want." "The 12-year-old sows know exactly where to look for what they want. " "Pigs know the forest six times better than me, although I also know each and every tree…" Svinjars were aware of individual and herd-level differences: "You know, some of my pigs eat mushrooms, others don't, they learn which mushrooms are edible from other herd members. " We also observed that Ranunculus ficaria was eaten often by one herd and much less often by the neighbouring herd.

Knowledge generation about plants and pig behaviour
When asked about their knowledge of the forest, plants and pigs, and life in the forest, svinjars argued that "I was born into this forest. " "As if we were 'shot' into the forest." "Here everything is clear to me. " The main source of knowledge was direct observation of pigs (see answers in Table 2 On the other hand, svinjars lacked knowledge about certain plants. "I am used to seeing these plants, [but] I do not observe them carefully. " "These [two plants: Carex remota and C. divulsa] are the same to me, but I know that they are not the same to you." Plants regarded as useless or not important were called "korov" (weed) or "paprat" (i.e. fern but meaning ca. weed/insignificant), and many Asteraceae species were called "kamilica" (chamomile) or "divlja [wild] kamilica" though svinjars knew that it was not the real chamomile.
Svinjars regularly but not often followed their free ranging pigs into the forest, even less often into marshes. "Pigs like freedom, like we do. They know where the best acorn is. " However, they did keep an eye on how far pigs went, and whether pigs entered into forbidden areas (forests closed from grazing, road verges and arable fields). Over many decades, svinjars accumulated a large amount of experience about plants and pigs.
The interviewed svinjars had decades-long and personal experience with forest pigs. "Till it goes 'through your back' , you don't know it. " (i.e. effective learning needs personal experience). "You have to see them every day from their birth to know them well. " And they do visit pigs almost every day, all year round, bringing food and checking on health and piglets. Svinjars spoke from experience, having lived through forage-rich and forage-poor years and seasons, and observed rare events (floods, severe winters) and long-term gradual changes of their environment. For example, the long-term impact of river regulation on the hydrology and species composition of marshes and forests, the impact of cattle grazing on the forests and marshes and the consequences of the abandonment of cattle and sheep grazing, the impact of pollution on waterweeds in the Bosut river, and the spread of new (invasive alien) species. Svinjars did not guess an answer to our questions, instead they said: "I don't know whether pigs eat it or not. " "I don't want to say stupid things when I simply don't know this. " Or simply closed the conversation by saying: "You know, everything is connected in the world. " "Nature is a wonder both to us and to you…".
Some svinjars were eager to learn from parents, grandparents, other old villagers and from respected foresters. Svinjars regularly recalled stories elders had told them ( Table 2, e.g. why Ajuga reptans is green in winter, the incredible endurance of Elytrigia and Cynodon rhizomes, masting in far-away mountains in the past). Other svinjars claimed: "I was never interested in what grandpa showed us. " But all agreed: "Those old guys knew more [about pigs, forests and plants] than we do. " Svinjars regularly visited their neighbour svinjars in the forest and shared information on pig movements (e.g. lost males), currently available forages, the impact of recent weather events (e.g. heat days on acorns), and changing forestry and veterinary regulations. Most local plant names documented in Table 2 were widely shared among svinjars.
All svinjars also had work experience as farmers, and most had also been employed as forest workers. In their childhood they played a lot in the forest, collected edible and medicinal plants, mushrooms, flowers, fodder for the pigs (acorn, Chenopodium, Amaranthus and Elytrigia from arable fields), and wood for firewood, huts, tools and sticks. They had therefore been familiar with many (probably most) plant species since early childhood. Svinjars remembered some practices from their childhood when they pastured pigs on weeds in stubble fields. They also acquired plant knowledge from foresters. They often recalled Dr. Josip Erdeši, former head of the Višnjićevo forestry office, who respected svinjars and supported pig grazing in the forests (Erdeši said to svinjars: "as long as the livestock is in the forest, it is beneficial to the forest, and when there is no more livestock in the forest, the forest will become ill"). TV and school were rarely mentioned as sources of plant knowledge (except in connection with the marketed wild green Allium ursinum). During the years of our research, svinjars became more interested in some plant species and the nuances of foraging behaviour. "[Last year] I told you that pigs don't eat this [Lysimachia nummularia] but now I can see they are eating it. "

Knowledge of plants and foraging pigs
Svinjars distinguished between at least 181 wild plant taxa in the studied forested floodplain, and had knowledge of 154 species foraged regularly or rarely, or avoided by pigs. This is the highest number of species documented for foraging domestic pigs in Europe (cf. [25,26,37]). The depth of ethnobotanical knowledge was comparable to other Central European traditional ecological knowledge-rich regions (e.g. Gyimes in Romania [91]; Hortobágy in Hungary [89], and also comparable to the length of lists of plant species foraged by wild boar [64,69,70]. Svinjars saw and understood many forest and marsh plant species "through the mouth" of their pigs (loved, nibbled, avoided, toxic, medicinal, cf. [51]) but were also knowledgeable about the ecological needs (e.g. habitat requirements) and human uses (medicinal, tools, wild food etc.) of the species. Svinjars had especially rich knowledge on some plant species (and groups), for example on Quercus robur and its acorns, Ranunculus ficaria, marsh grasses, Carex spp. For these species, foraging behaviour was reported in a nuanced way.
The low number of interviewed informants (however knowledgeable) is an obvious limitation of our study (cf. [94]). We were able to reach the saturation of information only for the well and moderately known species. Some plants, despite being common in our study area and even eaten by pigs, were little known by svinjars. A possible reason for uncertainties in the knowledge of svinjars may be their lack of awareness of some 'insignificant' plant species (e.g. Circaea lutetiana, Veronica montana, Glechoma hederacea, Ranunculus trichophyllus, Teucrium scordium, Stachys palustris). Such 'ignorance' was also found in other ethnobotanical studies in the region [89,91]. Furthermore, svinjars were cautious and tried to base their answers on their own observations. They all knew that we are botanists and have detailed knowledge of the local flora and already have some observational experience on pig foraging, so were careful with their answers. Some gaps in svinjars' knowledge may be due to a lack of knowledge (these plants were never known well in this area) or recent loss of knowledge (ancestors knew the species but it is not any more known) (cf. [95]).
Our own observational research also failed to produce a full picture of pig foraging. For many species we lacked visual observation data because (1) areas where these species grow were not utilized any more by pigs (stubble fields, old-fields, grassy pastures, road verges in arable land); (2) some species were rare in the forests or did not occur in the areas where the observations were done; and (3) some species are eaten only rarely and/or in a narrow seasonal window and/or under specific weather conditions (snow) or as a famine food, hence foraging was not observed.
Studies on pig foraging in open landscapes have found that herbaceous species are a staple food for pigs [23,25,26]. This was also the case in our research. The year-round main forages of pigs were similar to those of wild boar (grasses and herbs in spring, crops in summer, forest fruits and acorns in autumn, and acorns, grasses and roots in winter [64,65] and references therein) except for crops. As we also observed in the case of pigs, wild boar prefer grasses over forbs [65,96], but see the opposite for pigs in Von Flegler et al. [25].
Historical-ethnographic sources on traditional pig keeping from the region list relatively few wild plant species eaten by pigs (see [32] and Öllerer et al. ined.), probably because ethnographers and historians were interested in the extensive land-use practices in general and focused especially on the social background and social organization, and on the use of agricultural tools and buildings for livestock. We can only presume that Central European ethnographers and local historians did not use the free-listing method as a way of elicitation, as only the most salient species (e.g. Quercus spp., Typha, Phragmites) were listed in the publications. None of the ethnographic-historical sources mentioned the regularly eaten (and loved) Amaranthus retroflexus, Polygonum aviculare, Chenopodium album, Convolvulus arvensis, Trifolium spp. and Taraxacum officinale as foraged. Studies on wild food plants may also list species eaten by (given to) pigs (e.g. [97]).
Acorns were a staple forage, and svinjars kept estimating the potential autumn yield from April onwards. This was a widespread practice in the past throughout Europe [20]. Only fallen acorns were eaten in our study area, svinjars did not shake or beat down acorns with sticks in August-September, a practice known in medieval times [14] and still applied by local svinjars as recently as 30-40 years ago.
Pigs in the Bosut forest loved fleshy forest fruits. Wild boars also often feed on almost all fruits available in their territory (2-3% in volume [64]). On the Drava floodplain, Tucak [70] documented foraging by wild boar on Prunus domestica, Morus alba, Corylus avellana, Rubus sp., a similar list to ours. Most cultivated fruits were given traditionally to Iberian pigs as supplementary fodder [63]. Fruit consumption is also important ecologically as it contributes to the dispersal of these species in the forested landscape [98].
We found 20 species whose 'roots' were foraged by pigs. Pigs studied in Germany foraged on the roots of many herbaceous species (e.g. various grasses, Cirsium arvense, Taraxacum officinale and Urtica dioica [25], also observed by Stolba and Wood-Gush [23]), while dehesa pigs eat much fewer roots (< 1% in volume [28]). Ethnographic sources also often mention digging for 'roots' of wetland plants (mostly but not exclusively in winter and early spring) [12,15,99,100]. Roots of marsh plants can also compose the bulk of food for wild boars living in wetlands, although it remains a secondchoice food source [101].
Literature sources (both historical, e.g. [99,102], and recent, e.g. [23] mention that pigs forage on the roots of woody forest species, but the species are rarely documented (for wild boar, e.g. [69,70]). We tried to document this behaviour, but not once did we observe pigs foraging on the roots of woody species, and svinjars were also convinced that it did not take place ("domestic pigs do not need these roots, they never eat woody roots, they get corn from us"). Roots were only broken and torn, or if pigs did take them into their mouths, they ultimately discarded them. In the Drava floodplain, wild boars consumed, in the four seasons from spring to winter, 20%, 9%, 15% and 48% roots, respectively (in volume [70], but woody and non-woody species were not distinguished), including roots of Quercus robur, Populus spp., Salix spp., Corylus avellana, Acer spp., Robinia pseudoacacia, as well as of non-woody species such as Symphytum officinale (most commonly in winter and spring), Taraxacum officinale, Trifolium sp., Pteridium aquilinum and grasses [70]. We did not observe pigs foraging on grass roots in our study area at any time.
There is almost no data on browsing by wild boar and little by pigs. Svinjars argued that pigs prefer leaves over twigs, the opposite of red deer, and we also observed grazing much more often than browsing. Wild boars also rarely browse [65,103,104], max. up to 5% in volume [105]. Browsing by pigs is rarely mentioned in recent (Von Flegler et al. 2005, 2.6%) or historical sources (on Fagus, Carpinus, Fraxinus, Ulmus, Acer and Salix caprea [106], and Crataegus sp. [107]. In spite of domestication, the behaviour of domestic pigs greatly resembled that of wild boar (cf. [23]). Consequently, the direct visual investigation of these easily observable domestic pigs could provide a reasonable basis for formulating research hypotheses for studies on the foraging behaviour of wild boar or for a better understanding of causes shaping wild boar grazing and rooting patterns (cf. [108]).

Knowledge generation about pigs and plants
Svinjars were born and embedded into this lifestyle, were knowledgeable about plants and pig foraging, and emphasized the importance of personal, long-term, everyday experience with the animals and the forest. This understanding of how adequate knowledge of animal keeping and pastures is generated is universal among herder and pastoralist communities [89,109,110].
Though svinjars spent a lot of time with their pigs in the forest, they usually did not follow them on their foraging trips. This may be one of the reasons why some species and their consumption remained outside the knowledge of the svinjars. Inga [111] found that the knowledge of Sami reindeer herders about summer forages was significantly lower than about winter (e.g. lichen) forage, because the interviewed Sami herders usually did not follow their reindeers during summer months. By contrast, many European sheep and cattle herders still closely follow their herds during the whole grazing season, develop a deep understanding of foraging preferences and behaviour, and even use this knowledge to moderate appetite and increase forage intake [5,52].
We found similar patterns and ways of ecological knowledge generation and knowledge transmission to those documented in other traditional indigenous and local communities [112][113][114]: vertical learning (within genealogical lines), oblique knowledge transmission (between genealogical lines-e.g. older colleagues) and horizontal transmission (between members of the same generation). The dominance of shared folk plant names was an indication of the operation of knowledge transmission mechanisms and indicated frequent contact and interaction between svinjars [115]. Personal experience with nature completed knowledge transmission, while non-local sources, information gained from school or media (books, newspapers, TV, internet), were rarely mentioned, as information on pig foraging is probably rare in these sources (cf. [89]). Based on our limited number of data, personal observation seemed to be more important with regard to forage-related traditional knowledge than vertical learning from older community members (cf. [116]).
Our research definitely had an impact on knowledge generation: svinjars became more observant on some species. This is also a general phenomenon, as herders like to learn from people whom they consider, in their own judgement, to have reliable and relevant knowledge [5].
Traditional knowledge on forages and foraging was eroding in the study area, while knowledge of particular species is becoming less in-depth (cf. [95]). Similarly to other local knowledge holders worldwide, the interviewed svinjars argued that they knew less about the plants than their fathers and grandfathers did. Svinjars also emphasized the diminished dependence on pigs and local natural resources, which led to less effective knowledge generation (they accompany their pigs less often than in the past). Formal education, connections to the market economy and changing lifestyles may reduce not only the amount of local ecological knowledge but also the willingness of younger generations to continue this lifestyle (cf. [117]). The almost complete absence of the next generation of svinjars has practically ended knowledge transmission. The decreasing number of practising svinjars may also have caused a decline in the diversity of experiences. The ageing and dwindling community of svinjars is leading to the disappearance of the informal socio-cultural institutions of knowledge transmission and eliminating opportunities for imitation and improvisation as important mechanisms in knowledge acquisition [114,118,119].
Svinjars were eager to teach us their knowledge and hoped that science could help archive their knowledge, and even improve knowledge transmission by raising interest among the younger generation to learn more about traditional pig keeping [a new local association of pig keepers was recently formed to preserve traditional farming practices (svinjars' pers. comm.)]. However, economic drivers, the worsening of the local and the national regulatory environment and the spread of modern lifestyles still threaten the continuation of this traditional forest-marsh pig keeping practice [31,32].

Future potential of this almost lost traditional practice
The historical practice of extensive pig grazing is regularly advocated for use in land management, for example, in forestry [34,102,120,121], nature conservation management [6,21,40,41], and organic/environmentally friendly farming [34,38,39].
The possibility of using pig grazing and rooting in nature conservation management is, however, hardly understood [122], and may also be misunderstood and undervalued [41]. Evidence shows that extensive pig keeping is an efficient way of managing some mud specialist species and of restoring formerly grazed marshes [6,40]. For example, several red-listed marsh plant species (Marsilea quadrifolia, Ludwigia palustris, Elatine spp., Hottonia plaustrisi Lindernia procumbens etc. [6]) benefit from pig grazing, as do some birds (waders, ducks and geese [41]). Pigs were also suggested for managing wood-pastures and open forests, based on their interwoven history [14] and on the experience of the ongoing practice in the dehesas (e.g. [27]).
The carefully controlled application of livestock grazing in certain forest management situations is recommended by Öllerer et al. [46], which is in line with the knowledge and view of some forestry experts (e.g. [84,[123][124][125]). Data on the forestry use of pig grazing are, however, also scarce. Historical data indicate that pigs were used before the planting of tree seedlings to eat up wormy, unhealthy acorns and to loosen up the soil, mixing it with leaf litter in order to achieve better conditions for the germination of healthy acorns that fell subsequently [85], but also to provide protection against Rubus and weed invasion and against pests (insects, small rodents) [120,121,[124][125][126]. However, more research is needed to understand the potential benefits of pig grazing in present-day forestry systems, as habitats (their hydrology, abundance of invasive species) may have changed considerably since the nineteenth century [127]. The use of free-ranging pigs in modern agroforestry systems (e.g. with fast growing tree crops [38,39] is a novel practice of raising pigs in an environmentally friendly way. The use of pigs to open up dense encroached secondary forests to more intensive use and to loosen and clear their soil for agriculture is being tested on US and UK farms [128,129]. Knowing the often detrimental impact of wild boars on semi-natural habitats [130] one may ask: are domestic pigs better than wild boars? We cannot answer this question for certain at present (there are no comparative studies in which spatial and temporal constraints etc. are controlled for), but we can be sure that domestic pigs seem to forage somewhat differently, while their impact can be managed more easily in space and time (to protect sensitive semi-natural habitats and crop fields). Historical data show that domestic pigs were not listed among the livestock types harmful for forests and the pig was the last livestock type excluded from the forests in the region [131].
We made our study in a high conservation value floodplain area that desperately needs an improved conservation management system. Floodplains are dynamic systems, where extreme events (especially unpredictable major floods) challenge conservation and the utilization of natural resources [29]. Innovations and creativity are needed for sustainable management and conservation. The studies of Kiš et al. [31,32] show that extensive pig grazing has a place and role in this, especially with regard to maintaining open habitats and increasing water infiltration. Local pig keeping methods develop in order to adapt to market changes and new forestry and veterinary regulations, while still preserving many ancient elements. The adaptive capacity of the system is, however, limited, and needs strengthening.
We do not know what knowledge humanity will need in the coming decades and centuries in order to develop and survive. Scientists warn that ancient knowledge needs recognition and support at multiple levels in order to continue adapting [2,3]. Consequently, national and local governments, veterinarians, forestry companies and nature conservation institutions all have a share of responsibility in maintaining this special pig keeping practice for the future.

Conclusions and outlook
Traditional pig keepers (svinjars) in Serbia developed their deep ecological knowledge of plants and foraging behaviour of pigs by maintaining a close relationship with the forest, the pigs, plants and wild animals, and the knowledge of their ancestors. We started to document this huge body of knowledge, which is vanishing rapidly because of internal (e.g. livelihood changes) and external (e.g. markets, regulations) drivers.
A much deeper scientific understanding is needed of this traditional pig grazing practice to efficiently harness its potential for conservation management in protected areas and for organic farming (and forestry), and to understand its past impacts on the ecology of European deciduous forests. Knowledge co-production with further scientific disciplines and professions (e.g. ethnozoology, animal behaviour science, ecology, veterinary and food science) is needed to cover the wide spectrum of svinjars' knowledge. Svinjars with their pigs convert inedible (i.e. not consumed by humans nowadays) biomass, such as forest and marsh grasses and herbs, acorns, roots and earthworms, into high quality meat, using little modern technology. This knowledge is an invaluable intangible cultural heritage of Serbia.
Only a few svinjars remain in the Bosut floodplain, and hardly any of them is under the age of 60. If external drivers are not managed properly, this practice may disappear very soon. Decision makers, in particular forestry, nature conservation, hydroengineering and veterinary experts and officials, need to recognize svinjars' knowledge and practices, support ongoing and future development and adaptation of this traditional practice, and support bottom-up initiatives to develop and promote local products and tourism. We hope that our study will encourage others to delve deeper into the knowledge and methods of traditional pig keeping in forests and marshes, and help the survival of this ancient practice.