The animal is connected to the external environment (e.g., social environment, habitat environment) via their brain, and the hormone plays an important role as its transmitter substance. In other words, when the external environment changes, hormones are secreted to adjust the internal environment in response. Therefore, hormone analysis is a useful tool that enables researchers to know animal's physiological state under various environments.
Since the measurement of hormone concentration can be carried out at a relatively low cost and does not require advanced technique, it is applied to a wide research field. For example, many relations between behaviors and hormones were clarified, such as behaviors in estrus and rut, infant rearing, attacking behaviors, intra/inter-species communications, and response to the change of habitat environment. Recently noninvasive samples are often used for the hormone analysis in both captive and wild animals (e.g., urine, feces, and hair). However, when excrement is used as a sample, it should be noted that there is a species specificity in the excretion route of hormones and the time taken for excretion. Also, depending on hormones, it is necessary to thoroughly examine the sampling frequency according to the change of hormonal concentration. As Beach (1948) summarized, no behavior depends only on one type of hormone, and conversely, no hormone has only one kind of physiological function. Not one but multiple mechanisms are involved in the hormonal control of behavior. Therefore, focusing on multiple hormones and evaluating results from various aspects are also important keys to capturing their invisible physiological state accurately.