This [nature.com] wins the prize for most awesome academic paper, ever. Why? Because it includes a figure that contains six—SIX!—photos of capybaras, with labels like "calm at baseline," "moderate comfortable," and "obvious comfortable."
The paper, written by Kengo Inaka and Tohru Kimura, is titled "Comfortable and dermatological effects of hot spring bathing provide demonstrative insight into improvement in the rough skin of Capybaras," and was published in Scientific Reports. The researchers studied 9 capybaras between the ages of 2 and 12 years old who live at Akiyoshidai Zoological Park Safari Land, examining the effects of hot baths on skin moisture, pH, sebum, melanin, and erythema. Before the experiment started, the capybaras had "rough skin." For 21 days, the researchers observed the capybaras soaking in a hot spring bath for 30 minutes each day, and they measured the animals' skin condition on days 1, 7, and 21. The researchers also studied how comfortable the animals were during the baths.
They found that daily bathing significantly improved the moisture content of the capybaras' skin—the "skin properties returned to the normal skin conditions (moisture, melanin and erythema values) observed in the summer," according to the paper's abstract.

And, to absolutely nobody's surprise, the animals were also very comfortable during their baths. The scientists studied both facial expression (specifically focusing on "eye shape"—i.e. do the capybaras display "peaceful eyelids"?) and "ear position." They found that, yes indeed, the capybara enjoyed the baths: "The evaluation of facial expression revealed that bathing in hot spring was comfortable for Capybaras." Coincidentally, an evaluation of my own facial expression when I see photos and videos of capybaras enjoying hot baths revealed high levels of comfort (and joy).
Here's the entire abstract:
The purpose of this study was to clarify dermatologically the favorable effects of hot spring bathing on the rough skin in Capybaras. Non-volcanic hot springs used in this study showed alkaline quality of water (pH 9.3), containing sodium and chloride ions. The normal skin in Capybaras was characterized by the presence of relatively thick epidermis with mild alkaline state (pH 8.26). The dorsal skin had melanin granules in the basal layer. Their rough skin affected in the Japanese cold winter was improved by daily bathing in an alkaline hot spring. The skin properties returned to the normal skin conditions (moisture, melanin and erythema values) observed in the summer. The facial expression mainly changes in the eyes was scored to evaluate comfortable status. The comfortable status during hot spring bathing significantly increased as compared with that observed before bathing (p < 0.01). The thermography revealed a heat retention effect of body temperature after hot spring bathing for 30 min. In conclusion, this study demonstrates that hot spring had significantly comfortable and dermatological effects on the basis of evaluation for the skin and body conditions in Capybaras.