Both Los Tuxtlas and Uxpanapa are areas of intense bird migratory activity, which could be a contributing factor to the ATBR detected in arboreal mammal species. It is not clear why howler monkeys generally had lower levels of ATBR than spider monkeys. However, both home range area and group size are greater in spider monkeys. This could expose spider monkeys to larger amounts of antibiotics and/or bacteria from human origin, or to more individuals carrying resistant bacteria. On the other hand, spider monkeys may be more prone to descend to the ground, increasing their exposure to ATBR determinants. However, there is little data available regarding the frequencies of these behaviors. Finally, it is important to emphasize that factors that affect the composition of the microbiota could also modify the prevalence of resistance Acrivastine reported here. For instance, should one kind of animals be prone to carry more Enterobacter spp., the prevalence of AMand AMC-resistance should also rise, as these species commonly carry a chromosomal beta-lactamase. That could be the case for spider monkeys, where nearly half of the isolates were Enterobacter spp., and, accordingly, the prevalence of AM and AMC resistance rose significantly. On the other hand, as all four mammals sampled here have different diets, it would be expected that their microbiota is different. Whether the selection of antibioticresistance traits affect the composition, or other factors that affect the composition influence the prevalence of resistance, cannot be inferred from these data. Overall, this study shows that resistance to old, naturallyoccurring antibiotics is common in the fecal microbiota of wild mammals. The counterintuitive nature of the data on E. coli resistance, that also goes against other resistance FIPI indicators used in this study, suggests that E. coli might not be a reliable indicator of the human impact on resistance in wildlife bacteria and demonstrates that examining non-E.coli species when conducting phenotypical screenings, is essential to get a better picture of ATBR in wildlife.