BackgroundMicrobiome composition is linked to host functional traits including metabolism and immune function. Drivers of microbiome composition are increasingly well-characterised; however, evidence of group-level microbiome convergence is limited and may represent a multi-level trait (i.e. across individuals and groups), whereby heritable phenotypes are influenced by social interactions. Here, we investigate the influence of spatial structuring and social interactions on the gut microbiome composition of Welsh mountain ponies.ResultsWe show that semi-feral ponies exhibit variation in microbiome composition according to band (group) membership, in addition to considerable within-individual variation. Spatial structuring was also identified within bands, suggesting that despite communal living, social behaviours still influence microbiome composition. Indeed, we show that specific interactions (i.e. mother-offspring and stallion-mare) lead to more similar microbiomes, further supporting the notion that individuals influence the microbiome composition of one another and ultimately the group. Foals exhibited different microbiome composition to sub-adults and adults, most likely related to differences in diet.ConclusionsWe provide novel evidence that microbiome composition is structured at multiple levels within populations of social mammals and thus may form a unit on which selection can act. High levels of within-individual variation in microbiome composition, combined with the potential for social interactions to influence microbiome composition, suggest the direction of microbiome selection may be influenced by the individual members present in the group. Although the functional implications of this require further research, these results lend support to the idea that multi-level selection can act on microbiomes.Electronic supplementary materialThe online version of this article (10.1186/s40168-018-0593-2) contains supplementary material, which is available to authorized users.

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Abstract1. Effective conservation and species management require an understanding of the causes of poor population growth. Conservation physiology uses biomarkers to identify factors that contribute to low individual fitness and population declines.Building on this, macrophysiology can use the same markers to assess how individual physiology varies with different ecological or demographic factors over large temporal and spatial scales.2. Here, we use a macrophysiological approach to identify the ecological and demographic correlates of poor population growth rates in the Cape mountain zebra metapopulation. We use two non-invasive biomarkers: faecal glucocorticoids as a measure of chronic stress, and faecal androgens as an indicator of male physiological status.3. We found that faecal glucocorticoid concentrations were highest in the spring prior to summer rainfall, and were elevated in individuals from populations associated with lowquality habitat (lower grass abundance). In addition, faecal androgen concentrations were higher in populations with a high proportion of non-breeding stallions (where male:female adult sex ratios exceed 2:1) suggesting sex ratio imbalances may intensify male competition. Finally, population growth rate was negatively associated with faecal glucocorticoid concentrations and female fecundity was negatively associated with faecal androgens, indicating a relationship between hormone profiles and fitness.4. Together, our results provide cross-population evidence for how poor population growth rates in Cape mountain zebra can be linked to individual physiological biomarkers. More broadly, we advocate physiological biomarkers as indicators of population viability, and as a way to evaluate the impact of variable ecological and demographic factors. In addition, conservation physiology can be used to assess the efficacy of management interventions for this subspecies, and this approach could inform models of species' responses to future environmental change. K E Y W O R D Sandrogens, conservation management, faecal hormone metabolites, glucocorticoids, habitat quality, macrophysiology, sex ratio skewThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.