The domestication of cargo-bearing animals contributed several important elements that transformed the nature of regional exchange relationships. First, there are some cross-cultural commonalities in the structure of contemporary societies practicing pastoralism, and it is probable that these factors had some role in prehistoric pastoral societies as well. Second, cargo-bearing animals transform the costs of transport and, consequently, the nature of long-distance interaction. Finally, the structure of wealth in animal herds conveys particular scalar advantages to powerful kin-groups that possess large herds. These factors condition long-term transformations such as sedentism, the nature of food production, and the institutionalization of social inequalities. These issues will be explored in three sections: household level articulation with agriculturalists, wealth accumulation among herders that is limited by risk and pasture, and caravans and the organization of pastoral labor.
An important structuring principle to pastoralist exchange is that herding systems are not economically independent because humans must consume a sufficient diversity of major food groups for nutritional reasons; a condition known as non-autarkic (Khazanov 1984;Nielsen 2000). Depending on available wild plants, herders may acquire a portion of their non-animal products from gathering activities but the more common solutions involve a mixed agro-pastoral strategy or articulation with agriculturalists. Furthermore, herders with animals capable of bearing loads are the natural agents for facilitating this articulation with agriculturalists (Browman 1990;Flores Ochoa 1968). Thus, exchange relations are a basic necessity for dedicated herders. For herders with cargo animals, the transport of exchange goods in some capacity was likely a regular feature of pastoral household economies, and to become more common and less laborious in terms of quantity of goods exchanged due to assistance of cargo animals. As pastoralist households are not autarkic, due to the need for non-animal foods on a regular basis, households usually have cargo animals as part of their herd and relatively brisk exchange networks are likely to develop between households without the need for elites, administrative oversight or investment from super-family organization.
Inequalities are evident in most pastoral societies as owners with large herds are better able to maximize by grazing all available lands when conditions allow and hiring additional help with herding tasks like shearing and butchering. Furthermore, large herds can reproduce more quickly, are better able to survive hardship, and better maintain a minimum herd size threshold for viability. Nevertheless, pastoral wealth is widely recognized as unstable due to the overall vulnerability to drought, disease, parasites, predators, theft, and accidents that can cause declines of over 50% in a given year (Khazanov 1984: 156;Kuznar 1995;Nielsen 2000: 42;Salzman 1999). Herders mitigate this risk by diversifying production, maintaining extensive exchange networks, holding access to grazing land in common, and utilizing other institutional means of risk reduction, such as the redistribution mechanism of suñay among Andean pastoralists (Flannery, et al. 1989).
A significant pastoral institution that appears to function as a leveling mechanism is the corporate ownership of pasture. While herds are typically held by individuals or kin-groups, herding is spatially extensive rather than intensive, and access to pasture in herding societies almost universally requires community negotiation (Ingold 1980;Khazanov 1984;Nielsen 2000: 46-51). Among pastoral societies that do not store fodder the carrying capacity of the land, and therefore the intensifiability of production, is limited by the season with the lowest productivity (Nielsen 2000: 43). Finally, in some regions of the world, such as the Andes, modern herds are bilaterally inherited which serves to prevent accumulation in specific descent groups (Lambert 1977;Webster 1973: 123). Thus, herding does not organizationally contain the seeds for social inequality, but intensified pastoral wealth in the form of very large herds has been documented as one principal form of investment in ethnohistorically known hierarchical societies in herding regions.
Pastoralist societies that organize into seasonal trade caravans share structural characteristics; some of these characteristics tend towards promoting social inequalities and others that counter-act the tendency (Browman 1990). The organizing of a trade caravan is often simplified among dedicated pastoralists because pastoralism is relatively efficient in the use of labor, and the herding and caravanning schedules can be prioritized (Nielsen 2000: 44-45). A single herder can monitor hundreds of animals on a typical, uneventful day of pasturing without a great deal of effort, and as physical labor is low as compared with agricultural tasks, children and the elderly often contribute and broaden the herding labor pool. As a consequence, during caravan season the loss of several capable family members (usually adult men) to the caravan journey for weeks or months during a single year may not unduly hamper the productivity of a household of dedicated pastoralists.
As mentioned above, all pastoral households must acquire non-pastoral products through diversification or exchange, however the ability to organize a caravan inherently favors the wealthier herders for several reasons.
(1) Herders with large herds are more likely to have a sufficient number of hearty animals capable of enduring long journeys with cargo.
(2) Caravan animals provide the mechanism of transport. Therefore, for direct exchange consisting of spot transactions, pastoralists must initiate the trade opportunity by traveling to their trade partners with a sufficient surplus of goods to acquire goods in exchange.
(3) The rewards of such trade caravans accrue differentially, allowing those who regularly participate in such ventures to acquire access to non-pastoral resources, a more extensive social network and perhaps fictive kin among distant trade partners, and enhanced prestige among their community.
While some elements of dedicated herding societies favor differential accumulation of wealth in the form of large herds, diversification of the resource base, and extensive trade networks, the realities of high risk to pastoral wealth and low intensity of land use affect all herding households equally to the extent that they dedicate themselves to pastoralism. Thus, while redistributive mechanisms and corporate tenure of pasture serve to stabilize pastoral systems, the structure of herding systems also provides a few opportunities for strategic advantage to more opportunistic and aggrandizing elements of prehistoric society. In addition, despite the risk in herding systems the ownership of a large herd may directly confer prestige on pastoralists (Aldenderfer 2006;Hayden 1998;Kuznar 1995: 45), and inequalities in pastoral wealth can be channeled into more enduring and intensifiable avenues such as increased exchange (ultimately resulting in large trade caravans) or a mixed agro-pastoral strategy. As a pastoral economy with cargo transport capabilities first takes hold in a region, increased social differentiation may reflect a co-evolution between more extensive exchange relationships, greater sedentism, population increases, and larger population centers.