Of hot springs and holobionts: linking hot spring bathing behavior, parasitism, and gut microbiome in Japanese macaques.

Japanese macaques at Jigokudani Snow Monkey Park, Nagano, exhibit hot spring bathing behavior (HSBB) during the cold season. HSBB is known to aid thermoregulation and reduce stress, but its impact on host-associated biota in nonhuman primates remains unexplored. In this study, conducted between December 2019 and March 2021, we investigated the relationship between HSBB and lice load, gastrointestinal (GI) parasitism, and the gut microbiome in Japanese macaques at Jigokudani. Fecal samples were collected from sixteen adult females (9 bathers and 7 non-bathers) varying by age, reproductive status, and social rank. Nit-picking rates, used to estimate lice load, differed between bathers and non-bathers in submerged and non-submerged areas. We detected four GI helminths and at least one protozoan parasite, but did not observe noticeable differences in the probability of infection or abundance of these endoparasites between bathers and non-bathers. Finally, the alpha and beta diversity of the gut microbiome did not differ between bathers and non-bathers, but we identified four microbial genera that were significantly more abundant in non-bathers. These findings suggest that HSBB may influence host-(micro)organism relationships. Further research is needed to explore potential health outcomes associated with HSBB.