I use the terms traditional knowledge and traditional knowledge systems to refer to the knowledge of resource and ecosystem dynamics and associated management practices existing among people of communities that, on a daily basis and over long periods of time, interact for their benefit and livelihood with ecosystems [
19,
20]. The term does not merely refer to information about human uses of plants and animals [
20]. Rather, it includes a system of classifications, a set of empirical observations about the local environment, and a system of resource use and management. It also includes believes in non-human beings (i.e., spirits, ancestors, ghosts, gods) and on how they relate to society. The study of TKS parallels ethnopharmacology in that both fields of research initially emphasized descriptive accounts, but they are now moving towards a more hypotheses-driven research. Here I will focus on three theoretical contributions from research on TKS, highlighting their relation to ethnopharmacological research.
TK as a holistic system of knowledge
The first theoretical contribution relates to the holistic nature of traditional knowledge systems. As mentioned, TK, rather than a compilation of information about plants and animals, is a way to understand the world, or what we understand as "culture". Anthropologists state that culture patterns human behavior and -through it- affects human health and well-being. In traditional societies, an essential function of culture has been to establish and transmit a body of knowledge, practices, and believes regarding the use of locally available natural resources to improve health and nutritional status. Quantitative research on the topic highlights the effects of locally developed traditional knowledge on adult and infant health and nutritional status. For example, in my collaborative research among the Tsimane', a hunter-horticulturalist society in the Bolivian Amazon, we have found that the level to which an individual shares the knowledge of the group is associated to own nutritional status [
21] and offspring's health [
22]. That is, people who share larger amounts of the traditional knowledge developed by the group display better health -measured through objective and subjective indicators - than people who do not share as much knowledge.
Ethnopharmacology can draw two important theoretical conclusions from those research findings. First, notice that those findings are based in a broad measure of traditional knowledge, not on the targeted study of a plant or a group of plants with active compounds. That is, we did not conduct a pharmacological study of local medicinal plants and then include those with active compounds in our questionnaire. Furthermore, our measure of TK was not limited to medicinal plants. Rather, our measure included questions on a wide range of useful plants (medicinal, but also edible, construction, dyes, and plants with other uses). We interpret the positive association between our broad measure of TK and objective and subjective indicators of health as indications that medicinal knowledge systems are not built of isolated pieces of information, but rather constitute a complex body of knowledge linked to a larger coherent ensemble. The implication is then that identifying active compounds in a plant might be of good use for the pharmacological industry, but it might be of limited use for knowledge holders, because it is possible that for a given medicine to be effective in the local context, it requires the accompanying practices and beliefs that provide the medicinal "meaning" to the plant (
sensu Moerman, see bellow). The first point I want to stress here, then, is that, while it is evident that many plants used in indigenous pharmacopoeias do have active compounds, it is also likely that those active compounds do not act alone in indigenous healing systems, but they partially act because they have a shared medicinal cultural meaning [
23]. And, as it has been highlighted by previous researchers [
10,
23], the efficacy of a medicinal plant should be measured in a culturally appropriated way, and the failure to consider the cultural context within which plants are used can result in misunderstandings of a plant's efficacy. So, it is the complex system, rather than the intake of particular plants with active compounds, that might shape the health and well-being of TK holders.
The second related lesson to be drawn from the example above relates to the indigenous understanding of health. Indigenous peoples have sophisticated ideas of health and well-being. As also recognized for the World Health Organization, for many indigenous peoples, health is not merely absence of disease [
24]. Health is a state of spiritual, communal, and ecosystem equilibrium and wellbeing [
25], which probably explains why traditional pharmacopeias include remedies both to cure physical ailments (whether caused by spiritual or magical beings, or by the physical world) and to improve one's well-being (i.e., to protect infants from witches or evil spirits or to enhance hunting abilities). Furthermore, among indigenous peoples, the choice of a medical treatment is often explained by this complex understanding of health and the perceived causes of illness. For example, the Tsimane' choice of medical treatment is often related to the perceived cause of the illness. Common illnesses, caused by the natural world, can be cured by medicinal plants or drugs, whereas illnesses caused by spiritual beings can only be cured by the intervention of a traditional healer [
26]. When a person gets sick, she is often first treated as if she suffered from a common illness. Plants (or pharmaceutical) remedies are administered sequentially or simultaneously, often without consultation from any expert. If the condition persists, the Tsimane' start being suspicious that the illness is caused by witchcraft, in which case, they seek the help of a traditional healer. So, physical symptoms are only one of the clues to be used when selecting a treatment and the perceived (natural or spiritual) causes of the illness might be more relevant in the selection of the treatment. In that sense, as Moerman and Jonas have highlighted [
23,
27,
28], even plants without active compounds can have healing effects, in the same way that placebo medicines have healing effects in Western culture. Plants and medicines might be effective, not because of their pharmacology, but because of the cultural "meaning" (
sensu Moerman 2007) assigned to them. To put it in Moerman's [
23] words:
However, the effectiveness of these plants as medicines is not simply a consequence of their pharmacology; they are not pills disguised as herbs. Botanical medicinal effectiveness is inevitably some varying combination of pharmacology and meaning. Neglecting either aspect of this effectiveness is to provide only a partial, and thereby an erroneous, view of the subject (pg. 459).
In sum, research on TKS and its relation to the health of indigenous people suggests that the medicinal uses of plants, animals, fungi, and minerals are better understood if studied as a domain of knowledge embedded in the large body of cultural knowledge, practices, and beliefs of a group. The focus on testing the active compounds of indigenous pharmacopoeias conveys the idea that local medicines become meaningful only when pharmacologically validated, and thus diminishes traditional knowledge systems and indigenous explanations of the world. Thus, an important task ahead for ethnopharmacology is to contextualize uses and cultural perceptions of plants as a way to acknowledge that the intangible attributes of a species may be as important criteria for inclusion in indigenous pharmacopeias as its tangible attributes.
The distribution of Traditional Knowledge
The second theoretical contribution from research on TK that can help in the ethnopharmacological enterprise relates to the distribution of knowledge within a group. Recently, Heinrich and colleagues [
29] claimed that
"minimally, any [ethnopharmacological] field study should examine how plant knowledge is distributed in a society, and include some sort of consensus analysis to highlight the difference between common and specialist knowledge" (pg. 9). The legitimate question is "why?"
From research initiated in the 1970s and continued to this day, we know that there are differences in the amount of cultural knowledge that individuals' hold [
30‐
34]. For instance, in a study in the Brazilian Amazon, Wayland [
35] shows that knowledge and use of medicinal plants is concentrated among women because of their role as managers of household health. Some other variables that have been shown to correlate with intra-cultural variation of TK include market integration [
36,
37], kinship affiliation [
38], age [
39], schooling [
40], positions in a social network [
41], and -of course- level of specialization on the domain of knowledge [
42‐
45]. For example, in a now classic study in a Tarascan community in Mexico, Garro [
42] found important differences in the level of medical knowledge of curers and laypeople. Overall curers and laypeople shared a single system of beliefs, however, curers showed higher agreement among themselves in expressing this system than non-curers.
The implications of intra-cultural differences on how laypeople and specialists understand the causes, symptoms, and treatments of illnesses have been addressed in medical [
46], but not so much in ethnopharmacological research. Three decades ago, Kleinman and colleagues [
46] suggested that the models of sickness held by laypersons and specialists may differ in terms of perceptions of what caused the ailment, why it started, when it did, what it did to the person, how severe it was, what were the treatment options, what results were expected from treatment, and what were the fears about the illness. They stressed the critical importance of understanding potential differences between laypersons and specialists for the successful resolution of health problems. As they argued, the different understanding of illness between patients and specialists may be at the root of medical problems, particularly because different understanding of illnesses might result in patient lack of adherence to medical regimens.
Folk healers (i.e., herbalists, curers, shamans, and the like) have been the typical focus of ethnopharmacological research. Ethnopharmacologists have focused on folk healers under the assumption that they concentrate most ethnopharmacological knowledge. However, specialists have often been studied in isolation, giving little attention to how specialists relate, interact, and contrast with non-specialists. But if -as we have learned from research on the distribution of TK- specialists and non-specialists do not necessarily share the same body of knowledge, nor the same understanding on how to cure diseases, then the focus on specialists knowledge necessarily biases the type of information being collected in ethnopharmacological studies. Furthermore, this focus on specialists limits the possibility of understanding how the patterned distribution of ethnopharmacological knowledge within a society affects the health of the group.
Thus, the patterned distribution of TK has two important implications for ethnopharmacological research. The first implication relates, of course, to the selection of informants. If TK is unequally distributed, the amount and quantity of information one can obtain clearly depends on how much and what type of knowledge is held by the selected informants. Researchers have highlighted differences between laypersons and specialists, but -as in other domains of traditional knowledge- most likely other patterned differences exist. For example, men can give different explanations to illnesses symptoms and treatments than women, or young people might use different treatments than elders. Thus, minimally understanding how knowledge is distributed in a community should be an important consideration in ethnopharmacological research, which so heavily relies on locally provided information.
The second implication of the patterned distribution of knowledge for ethnopharmacological research is more theoretical. If ethnopharmacological knowledge is unevenly distributed, and if this uneven distribution is patterned, then one should expect that people in certain characteristics should benefit more from the ethnopharmacological knowledge of the group than people without those characteristics. It also implies that similarities and differences in the belief systems of specialists and non-specialists are likely to affect how treatment alternatives are perceived and utilized. All important issues that ethnopharmacology could potentially address.
Transmission of Traditional Knowledge
A third theoretical contribution from research on TK to ethnopharmacological research relates to the study of the social relations that enable the generation, maintenance, spread, and devolution of cultural traits and innovations, including ethnopharmacological knowledge. Researchers have hypothesized that, unlike biological traits, largely transmitted by a vertical path through genes, cultural information can be transmitted through at least three distinct -but not mutually exclusive- paths: 1) from parent-to-child (vertical transmission), 2) between any two individuals of the same generation (horizontal transmission), and 3) from non-parental individuals of the parental generation to members of the filial generation (oblique transmission) [
47]. Oblique transmission can take the form of (a) one-to-many, when one person (e.g., a teacher) transmits information to many people of a younger generation or (b) many-to-one, when the person learns from older adults other than the parents [
47].
So the question is "how is ethnopharmacological knowledge transmitted?" Some anthropologists have stated that folk biological knowledge, including knowledge about what constitutes an illness and how to cure it, is mainly transmitted by parents to offspring [
48,
49]. For example, in a study of a rural population in Argentina, Lozada and colleagues [
50] analyzed the transmission of knowledge of medicinal and edible plants and concluded that family members (especially mothers) were the most important source of medicinal knowledge. Other researchers have argued that parent-child transmission might not be the dominant mode of cultural learning, at least when a person's total lifespan is considered [
51]. Quantitative studies on oblique transmission of ethnobotanical knowledge are scarce and focus on the transmission of knowledge from one-to-many. For example, Lozada and colleagues [
50] found that experienced traditional healers outside the family are the second important source for the acquisition of knowledge of medicinal plants. Last, several authors have argued that there are also social and evolutionary reasons to expect intra-generational transmission of some types of cultural knowledge [
52,
53]. Observational studies suggest that, in some domains, children learn a considerable amount from age-peers [
48,
54]. For example, children regularly teach each other tasks and skills during the course of their daily play [
48]. In a study in Mexico [
54], Zarger showed that siblings pass along extensive information to one another about plants, including where to find them, their uses, or how to harvest or cultivate them. In my own fieldwork, I have often observed children using plants for medicinal purposes, both for themselves and for they playmates, which would suggest that children also pass to each other information on curative plants. Research also suggests that, later in life, young adults turn to age-peers rather than parents for information. Specifically in situations of cultural change, age-peers -not elders- are most likely to have tracked changes and should provide the best information to navigate in the new context; information that sometimes updates or replaces information previously acquired from parents [
47,
51]. In sum, although previous empirical research has outlined the importance of the vertical path in the transmission of TK, theoretical models and empirical evidence from fields other than anthropology suggest that the importance of vertical transmission may be overstated [
51], and that neither vertical nor oblique transmission should be expected to dominate across all domains [
55,
56].
The studies cited here also highlight that the selection of one type of transmission over another might depend both on the cultural group and the domain of knowledge examined. For example, medicines to cure illnesses from the natural world might be transmitted by a different channel than medicines to cure illnesses caused by spirits. Understanding the strategy selected by a society for the transmission of ethnopharmacological knowledge is important because each of those transmission pathways -or the way they are combined- affect differently the distribution, spread, and therefore maintenance of knowledge. For example, as is the case for other cultural traits [
47], ethnopharmacological information vertically transmitted (i.e., from the parent to the child, or from one selected adult in the parent generation to one selected young, as many iniciatic systems) would be highly conservative. That is, because it is less shared, information vertically transmitted may maintain individual variation across generations. Furthermore, innovations and new information would experience slower rates of diffusion in a population when compared with horizontal or oblique transmission. By contrast, horizontal transmission might lead to fast diffusion of new information or innovations if contact with transmitters is frequent. Furthermore, vertical transmission is based in two models, whereas oblique and horizontal transmissions are based on larger samples, and larger samples might provide more accurate (less biased) information [
57]. The combination of horizontal and oblique transmission involving many transmitters to one receiver would generate the highest uniformity in ethnopharmacological knowledge within a social group, while allowing for generational cultural change.
It is also possible that the strategies to transmit TK change over time. Theoretical modeling suggests that changing social contexts, as the ones that experience many indigenous societies nowadays with globalization and market integration, favor reliance on oblique rather than on vertical transmission [
55]. For example, with increasing exposure to market economy and commercial drugs, ethnopharmacological knowledge might need to be used in new situations or in interaction with new products. To navigate cultural shifts, individuals might opt to select information that has been effective from a wider subset of the population (like non-parental adults). This shift might help ethnopharmacologists understand why indigenous pharmacopoeias heavily reliant on vertical transmission are threatened by modernization in a much deeper way that indigenous pharmacopoeias that have traditionally been transmitted through other pathways.
Last, research on the transmission of TK can also help ethnopharmacologists understand the different paths through which different types of knowledge are transmitted. For example, research among the Tsimane' suggests that ethnobotanical knowledge (such as names or traits used for plant recognition) and skills (or how to put this knowledge into practice) are not transmitted through the same paths [
56]. Ethnobotanical knowledge might be easier to acquire than ethnobotanical skills and is mainly acquired during childhood. The acquisition of knowledge relies on cumulative memory and individuals can learn quickly and effectively through relatively few interactions; therefore, individuals can acquire ethnobotanical knowledge from many sources. The acquisition of skills might require higher investments by the learner. Acquiring skills is more costly in time and might require a number of direct observations and repetition within a particular context. So, individuals might be more conservative in selecting models for the transmission of skills and place more weight on information acquired from older or more experienced informants.
To sum up, a focus on understanding how ethnopharmacological knowledge is transmitted would open new research possibilities in ethnopharmacology. Specifically, quantitative data on the mechanisms of transmission of cultural traits could be useful in predicting within-group variability and stability of traditional pharmacopeias over time and space.
I now move to discuss how methodological contributions in the study of TKS can help in ethnopharmacological research.