TL;DR it affects gut biome and the parasites on the monkeys
The findings showed that time spent in hot springs subtly changes how the monkeys interact with parasites and gut microbes. Monkeys that soaked in the warm water had different patterns of lice on their bodies and differences in certain gut bacteria. This suggests that immersion in hot water may interfere with lice activity or where they lay their eggs.
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The researchers also detected modest differences in the gut microbiome. Overall diversity of gut bacteria was similar between monkeys that bathed and those that did not. However, several bacterial genera were more common in individuals that skipped the hot springs. Importantly, sharing the pools did not appear to raise the risk of intestinal parasites. Bathing macaques showed no increase in parasite infection rates or severity.