https://phys.org/news/2018-11-transmission-antibiotic-resistant-coli-wild.html

Surprisingly, giraffes three months of age or younger were more likely to harbor antibiotic resistant E. coli than other age groups. "This is particularly surprising as giraffe neonates nurse from their mothers exclusively and interact minimally with other group members, leading us to predict they would have low exposure to resistant bacteria and residual antibiotics in their environment," said Dr. Miller. "These results suggest there may be competition between antibiotic resistant and susceptible E. coli strains in the giraffe neonatal gut, with resistant E. coli having a selective advantage.

Just why antibiotic resistant E. coli outcompete the susceptible strains is a matter of pure speculation at this point, said Dr. Miller. "One intriguing theory refers to iron acquisition genes that have been observed to occur on the same plasmid as resistance genes," she said. "Bacteria with these extra iron acquisition genes might have a selective advantage in this iron-depleted environment, and any antibiotic resistance genes associated with the iron resistant genes would be carried along for the ride," she said, noting that the theory had previously been suggested in a 2004 article in this journal.