IR covers a considerable range of wavelengths. The typical silicon sensor, such as in a camera can see to about 0.9-1um wavelength. (0.7um is about the limit of the human eye). IR sensors designed for sensitive night vision typically operate in the long wavelenght 5um to 10um region (objects at room temperature radiate most strongly in the 10um region). To operate at these long wavelength regions with best sensitivity requires cooled detectors, but some of the new thermal type detectors have remarkeably good sensitivity operating uncooled at room temperature.
A natural IR detector example is a rattlesnake, which has two IR thermal sensors, one in each of the pits between its eyes and the nostril. Since a rattlesnake is cold-blooded the sensors are naturally somewhat cool as compared to any warm-blooded prey which improves sensitivity. Probably don't work so well on a hot day.