Why are we interested in temperature measurements in the field of comparative psychology?
Research shows that the temperature of the human skin – even under constant weather conditions – fluctuates continuously. These fluctuations are related, at least in part, to changes in the activity of the vegetative nervous system. The vegetative nervous system influences – without our intervention and usually unconsciously – a great many aspects of our body, including skin temperature, depending on the emotions we are experiencing. For example, when we feel stress or anxiety, the vegetative nervous system gets our body alert and prepares this for activity. When we feel relaxed and at ease, the vegetative nervous system returns our body to a state of rest. Thermal imaging is a way of measuring such changes in the activity of the autonomic nervous system, and thus changes in the emotional state. In studies conducted to date, scientists have focused primarily on the area of the body that is most easily accessible and rarely covered: the face. In response to emotional agitation, there are above all changes to the temperature of the nasal region. When we feel stress, fear or guilt, the temperature of the nose falls. The same effect has been observed when we observe someone close to us in an unpleasant situation, or when we perform a challenging mental task. However, positive emotions such as pleasure (while we laugh heartily) also lower the temperature of the nose.
The first thermal imaging studies with non-human primates have been conducted in recent years. It has been shown that emotions also affect the nose temperature of apes. Chimpanzees at Leipzig Zoo, for example, were played recordings of fellow chimps fighting. In response, the nose temperature of the apes dropped by several degrees Celsius (Kano et al., 2016). Similar findings have been obtained from chimpanzees in the wild (Dezecache et al., 2017). This shows that chimpanzees generally find it very exciting to watch fights between fellow chimps. In another study, chimpanzees witnessed how a human they knew (seemingly) accidentally injured themselves (Sato et al., 2019). Here too, some of the apes responded by lowering their nose temperature, which suggests that the animals may have felt some kind of empathy with the human, or that observing the injury at least generated some agitation. There is also initial evidence that strong positive emotions, such as when playing exuberantly, lead to a reduction in nasal temperature in apes (Chotard et al., 2018; Heintz et al., 2019).
Thermal imaging thus enables us to gain insights into the emotional world of our closest living relatives in a contact-free and non-invasive way. Because the technology is flexible and mobile, we can conduct nose temperature measurements in a wide variety of situations: In response to video stimuli in a controlled study setup as well as during natural interactions between apes in their social group.
Our goal is to further establish thermal imaging in research with great apes, thereby gaining more knowledge about the emotions of chimpanzees, bonobos, gorillas and orang-utans. It is hoped that the newly gained knowledge will enable us to learn more about the evolutionary origins of our own emotional world.