South Arabia’s prehistoric monument landscape shows social resilience to climate change

PLOS One | https://doi.org/10.1371/journal.pone.0323544 May 28, 2025 15 / 24 Social resilience emerges from coping, adaptive, and transformative capacities to “tolerate, absorb or adjust to environ- mental and social threats” [97, p.8]. These are capacities all linked at the community level through network densities that “generate and produce assets that advance shared objectives” [98, p.3]. We consider monuments as assets that facili- tated social belonging and were technically adjustable to sustain social cohesion for desert pastoralists who did not aban- don herding with environmental changes. Such changes included reduced graze, drying springs, changes in frequency and distributions of seeps, and in some eras, amelioration of rangeland. In contrast to people occupying the vegetated plateau and escarpment, populations in the arid zone relied on monu- ment construction and subsequent visits for social contacts that defined their socioeconomic and cultural networks. We attribute monuments’ distribution to their roles in communicating socio-cultural messages to people often absent. Mon- uments communicate a culturally specific, shared meaning from builders to contemporaries and facilitate exchange and social relationships [99]. Builders may also have intended to signal subsequent generations or to reify builders’ attach- ments to landscapes and places already established by prior generations. The placement of monuments could communicate environmental information like grazing or routes or could be reactive

to and built alongside an established route. Monument placement close to water and streambeds, where vegetation is highest, reflects that these are the places desert people frequent. These low-lying areas offer graze and browse, shade under trees, rockshelters, and travel routes of least effort. With consistently higher biomass in the escarpment through time, populations there were probably denser than in the arid interior. Dhofar’s mountain people could rely on direct contact to build social networks for resilient communities, rather Fig 7. Boxplots comparing total stone volumes across several monument groupings. Groupings include the five monument types, the two cate- gories of technological requirement, and whether or not one or more large stones (>0.75 m) were present. We also compare the latter for all monuments, within monument types, and the two categories of technological requirement. The letters show the results of Tukey HSD post hoc pairwise comparisons following ANOVA tests. Monument types with letters in common indicate no statistically significant differences, while different letters indicate statistically significant differences. https://doi.org/10.1371/journal.pone.0323544.g007

PLOS One | https://doi.org/10.1371/journal.pone.0323544 May 28, 2025 16 / 24 than rely on the technologies of communication involving monuments with culturally specific construction and mnemon- ics[42]. Moreover, cloud forests and tall grasses on the escarpment limited visibility, thereby limiting the utility of a monu- ment. (Ancient houses, corrals, and hearths are visible, so preservation and sampling do not prevent archaeologists from detecting monuments). Large differences exist in the numbers of examples we recorded for each monument type. Prior experience in a compa- rable landscape (Hadramawt, southern Yemen) demonstrated that these numbers are not significantly explained by pres- ervation conditions of different landforms. Moreover, platforms are significantly associated with lower elevations and HCT in higher elevations [58], both captured in our viewshed boundary in this research so that our monument counts would be relatively unaffected by terrain differences. Therefore other factors—such as period, cultural conventions, available labor—account for differences in the numbers of monuments of each type. Results of collocation and environmental analyses suggest that historic place attachments alone cannot explain spatial distributions of different monument types. A shift from earlier platforms close to springs to HCT built further from springs coincides with major aridification. Perhaps desert people no longer clustered as close to reliable water (and there is no

evidence they adopted oasis farming in Dhofar). Instead, the shift reflects adaptive resilience through major subsistence change during an era when goats replaced cattle in the arid interior [100, p. 228–9]. Goats can go longer without water and browse a wider range of forage [101]. HCT are also located on elevated sites compatible with the steep terrain of goats. The northern face of a slope would have less vegetation, given that prevailing moisture comes as fog from the south, so HCT would have higher prominence on bare terrain, perhaps consonant with the territorial signaling archaeologists have suggested as their function [8,102]. HCTs and triliths occurring across a broader geographical range are consistent with populations unable to support clustered herds on the sparse graze available near springs and adapting through dispersal. This adjustment in pastoralism (while eschewing farming) raises the role of monuments in the adaptive social resilience of a dispersed, thinned, Middle Holocene Dhofari population responding to climate change. Harrower’s analysis of water management in Wadi Sana, Hadramawt, noted the contemporaneity of HCTs and water management in a drying land- scape, speculating that “death and ancestry-infused landscape markers maintained deep social importance” [102, p. 505]. To other archaeologists also, the appearance of Bronze Age tombs suggests that resilient people now forged social identities and mediated access to resources through kinship with memorialized dead [8,34]. Our analysis

scrutinizes and quantifies these purported relationships, explaining why monuments of different eras do not appear to cluster as historic attachment to place. People’s choice to build different styles in different places signals or reflects a changing social landscape in differ- ent eras. Collocation analysis and the significantly different local environments of different monument types support the chronological framework developed from a dated sample of monuments. While triliths extend beyond our study area and may mark the once-expanded range of a particular ethnic group [74,103, p.73-4], the distribution of boat graves is restricted to eastern Dhofar, with possible parallels elsewhere [39]. Anecdotally, our team noted very few instances in which early monuments were quarried in antiquity to erect later monuments. If people did respond to prior monument placement at all, it was to place some later monuments apart from their antecedents, not to rebuild close by. We suggest that the technology of monuments communicating culturally specific social identities was a persistent feature of pastoral groups, even as the messages that monuments communicated varied through time and space. Changes in the volume and size of stones used in construction point to variable labor investments. Containing larger stones, Neolithic platforms are the largest and earliest single-episode monuments. To assemble such a platform requires balancing uprights with interior chock stones until all are held in place by a rapidly accumulated rubble fill. The construc-

tion sequence suggests that simultaneous cooperation among multiple herding households provided labor for these significantly larger monuments. At minimum, the group size could be several families, with seven strong adults to lift the largest stones. At one Neolithic platform in Yemen, rare preservation conditions indicate gatherings of up to 5000 peo- ple [29,53,71]. Ethnographic case studies document episodic monument construction events that aim to showcase an

PLOS One | https://doi.org/10.1371/journal.pone.0323544 May 28, 2025 17 / 24 especially large amassing of labor [104–106]. Construction of platforms commemorating such gatherings ended around 6200 cal BP, as the Southwest monsoon influence began to wane and with it, vegetation, animal life, and human popula- tion densities in the Arabian interior [26,107]. Perhaps it became more difficult to gather and feed a large, simultaneous labor force. Thereafter, building HCT required shifting less stone, which we suggest reflects smaller groups in thinned, still mobile populations responding to aridity. The new practice of burying the dead in monuments--now tombs--reflects a resilient strategy emphasizing people’s social connections through genealogy and the transfer of more complex skills in construc- tion. By memorializing the dead, people created touchstones for living lineages, even as people were less likely to gather in large social groups and clung to regions of fog density and seasonal graze [8,24,55,102]. This threshold in social consti- tution archaeologically anchors the beginnings of genealogical time-depths, consequential to this day across arid regions of Arabia, Africa, and Central Asia. Many label this social framework, “tribes” [108–111], but ethnographic examples are, at best, rudimentary analogs for social configurations of the distant archaeological past. Higher frequency and smaller stone volume of accretive triliths with few heavy stones is consistent with monuments

built incrementally by smaller, dispersed groups from a reduced population and persistent in an era of hyper-aridity. In the deserts, scattered people signaled their social affinities with culturally distinct syntax and built themselves into communi- ties and places, even perhaps in the absence of actual people. Triliths (2300–1700 cal BP) could be built as accumula- tions of distinct elements by successive visitors, a few at a time. The high variance in trilith volumes hints at this; even as the iconic trilith form was relatively consistent over a large region [112,113], some triliths had one element while others had 40. Triliths appear in extremely arid zones after the introduction of camels, which extended human range and herder intervals between water sources. Accretive triliths and boat graves appear in the wake of important climate changes. Boat graves appear after 1150 cal BP when climate proxies in Southern Oman indicate increased monsoon moisture after a particularly arid period. Unlike previous episodes of monument construction (HCT, triliths), boat graves coincide with a return to slightly moister con- ditions (Fig 2). Some boat graves appeared in the context of abandoned settlements, where mobile builders minimized labor costs by quarrying nearby buildings [39,114]. People could settle and farm, and we suggest a resilient commitment to mobile pastoralism—for its social and ideological traditions as much as for its ecological inheritance. These changes in monuments coincided with climate thresholds in Holocene Arabia, yet change was not always adjust-

ment to aridification. Instead, we see changes in monument location, construction, and requisite labor parties as shifts in the criteria for participation in mobile, monument-linked communities adjusting to various environmental and social stressors. We suggest that such communities rallied at population density thresholds where people lacked habitual contact, whether through aridification (HCT, triliths) or a wetter era that relaxed water constraints on mobility and herds (boat graves). Southern Arabia is a significant case that informs our contemporary challenges by showcasing the adaptive social resil- ience of people committed to specific economic strategies, like pastoralism or small-scale farming, in the face of climate and environmental dynamics. Across the African Sahel, monument construction appears among pastoralists from the ear- liest Neolithic [115] and was manifest, with adjustments, through time [21,116]. In the American Southwest, pueblo devel- opment has been linked to drier and more variable conditions [117,118]. Much prior focus has been on the establishment of Arabian oasis sedentism with changes in economy and mobility [119]. We offer a view of the behaviors that sustained desert pastoralism. Our analysis suggests socioeconomic resiliency to a dynamic environment. All mobile pastoralists need communities to exchange information, animals and other goods, reproductive partners, and commitments. These circumstances forced repeated adjustment in the technologies and institutions of group identity even as pastoralism remained an economic

mainstay. We accept prior claims that Bronze Age tomb-building reflects ancestry as a foundation of social identity and resource access [8], and we accept the appearance of tombs and oases (beyond Dhofar) correlated with climate change [24,26]. By linking all monuments, our analysis newly demonstrates that this was a resilient social adaptation from earlier,

PLOS One | https://doi.org/10.1371/journal.pone.0323544 May 28, 2025 18 / 24 large, social gatherings of contemporaries living in a more abundant environment. Furthermore, the appearance of Ara- bian genealogical organization as adaptive social resilience belongs to a larger pattern: over time, ever-smaller groups fragmenting into culturally distinct regions began to build accretive monuments to maintain collective social identities among persistent pastoralists who seldom, if ever, coalesced. Conclusions Derived from the metrics of labor scheduling and location, and testable in new data sets, our empirical model theorizes eras of monument building as the exercises of social capacities to absorb risk in the dynamic climates and environments of the arid sub-tropics of Arabia. By decentering analysis from a single era or type of monument, we show how monument classes are proxies for social behavior of mobile, persistent pastoralists. Our model highlights a reliance on monuments as touchstones of social belonging and as flexible technologies of social resilience in a changing world. While built on archaeological observations of monuments in Arabia, our model may be applicable and adaptable to assess social resil- ience in other regions, such as Saharan, Mongolian, or the high Andes. Supporting information S1 Table. Codes and Metrics of 371 monuments in Dhofar used in this analysis. (XLSX) S2 Table. Description of monument types with age ranges of archaeological periods. (XLSX)

S3 Table. Samples, contexts, and radiocarbon determinations shaping the chronology of monuments in Dhofar. For comparison with published Bayesian analysis of radiocarbon determinations [38], see horizontal bars in Fig 2. (XLSX) S1 Fig. Partial dependence plots showing the relationship between each environmental variable and the pre- dicted probability of monument occurrence. (PDF) S2 Fig. Multinomial deviance across values of log λ from 11 models with different α parameter settings. We used the model with α = 0.4 (elastic net regression), which had the lowest multinomial deviance. (PDF) S3 Fig. Multinomial deviance across values of log λ with α = 0.4 (elastic net regression). We used the ‘one-standard error’ rule, to select the model with λ = 0.049 (log λ = -3.023). (PDF) S4 Fig. Mean-squared error across values of log λ from 11 models with different α parameter settings. We used the model with α = 0.1 (elastic net regression), which had the lowest mean-squared error. (PDF) S5 Fig. Mean-squared error across values of log λ with α = 0.1 (elastic net regression). We selected the model with the minimum λ = 0.129 (log λ = -2.047). (PDF) S6 Fig. The elastic net regression coefficient paths for each monument type. As λ decreases, the maximum permis- sible value of the L1 norm increases and more coefficients enter the model. Variables enter the model based on their true linear regression coefficient, and therefore variables that enter the model early have a higher predictive power, compared

PLOS One | https://doi.org/10.1371/journal.pone.0323544 May 28, 2025 19 / 24 to those that enter the model later. When a new variable enters the model, it affects the slope of the coefficient paths of the other predictors, depending on the magnitude of collinearity. (PDF) S7 Fig. The elastic net regression coefficient paths for largest stone volume. Penalized coefficients are col- ored grey. The regularized multiple linear regression assessed whether labor input responded to environmental factors; however, using the ‘one-standard error’ rule, no variables were preserved in the model. Eleven of the twelve environ- mental variables were preserved in the minimum lambda model. The variables with the highest coefficients entered the model early and have the greatest predictive power. Notably, labor input (total stone volume) increased with fog density and topographic position at large scales but decreased with maximum NDVI. Like the results of bootstrap aggregat- ing, the minimum lambda result suggests a threshold in population dispersal at which humans engage monuments for messaging-while-absent. At the higher population densities supported in the vegetated mountains, people had sufficient direct contact that monuments were not needed as social touchstones. Fog density is high in the near Nejd backslope of the plateau, where mobile families would readily find seasonal grazing and yet be infrequently in contact, using monu- ments to communicate. (PDF) S8 Fig. The regularized multiple linear regression model coefficients show the relationship between total stone

Formal analysis: Lawrence Ball, Ian M. Hamilton. Funding acquisition: Joy McCorriston, Ian M. Hamilton. Investigation: Joy McCorriston, Michael J. Harrower, Matthew J. Senn, Tara Steimer-Herbet, Abigail F. Buffington, 'Ali Ahmad Al-Kathiri, 'Ali Musalam Al-Mahri. Methodology: Joy McCorriston, Lawrence Ball, Michael J. Harrower, Ian M. Hamilton, Matthew J. Senn, Tara Steimer- Herbet, Abigail F. Buffington.

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