Non-contact respiratory measurements of outdoor-housed rhesus macaques ( Macaca mulatta ) using millimeter-wave radar systems

ABSTRACT Respiration is an invaluable signal that facilitates the real-time observation of physiological dynamics. In recent years, the advancement of non-contact measurement technology has gained momentum in capturing physiological dynamics in natural settings. This technology is anticipated to find utility in healthcare, not only in humans but also in captive animals, to enhance animal welfare. Currently, the predominant non-contact approach for captive animals involves measuring vital signs through subtle variations in skin color. However, this approach is limited when dealing with body regions covered with hair or outdoor environments under fluctuating sunlight. In contrast, millimeter-wave radar systems, which employ millimeter waves that can penetrate animal fur, exhibit minimal susceptibility to sunlight interference. Thus, this method holds promise for non-contact vital measurements in natural and outdoor settings. In this study, we validated a millimeter-wave radar methodology for capturing respiration in outdoor-housed rhesus macaques ( Macaca mulatta ). The radar was positioned beyond the captive enclosure and maintained at a distance of > 5 m from the target. Millimeter waves were transmitted to the target, and the reflected waves were used to estimate skin surface displacement associated with respiration. The results revealed periodic skin surface displacement, and the estimated respiratory rate was within the reported range of respiratory rates for rhesus macaques. This result suggests the potential applicability of millimeter-wave radar for non-contact respiration monitoring in outdoor-housed macaques. The continued advancement of non-contact vital measurement technology will contribute to the mental and physical monitoring of captive animals to establish comfortable captive environments. Research Highlights Millimeter-wave radar systems succeeded in the non-contact measurement of respiration in outdoor-housed rhesus macaques from > 5 m. Our results demonstrate the feasibility of radar-based remote monitoring to assess the welfare of zoo-housed animals.