While all of us know that an increase in our body temperature above 38°C means that we are experiencing a feverish immune response, most people don't know that body temperature can also show variations linked to how much we eat or how much stress we are experiencing. Fasting individuals, for example, produce less body heat than individuals that have unlimited access to food, and during stress, blood is redirected from the body surface, causing cold hands or feet. Although information about an animal's physiology, such as whether they are stressed or coping well, is interesting for researchers, obtaining physiological data from free-ranging animals is difficult and often puts additional stress on the animals. Animals often have to be captured multiple times to measure their body temperature and draw blood or collect faeces, which can be used to look at stress hormones in the animal's system.
In the search for other ways to obtain those data, a team of international researchers led by Paul Jerem, from the University of Glasgow, UK, made use of thermal imaging to measure body temperature. Thermal imaging cameras take a picture of the heat lost by an animal through its surface, which is higher when an animal is warmer and a lot of warm blood is allowed to flow close to the skin. In particular, the team wanted to know whether the non-invasive measurement of variations in the eye temperature of blue tits can give an indication of the bird's body condition; that is, how well fed and how stressed the birds are.
For their study, Jerem and colleagues measured the eye temperature of blue tits over a winter and the following breeding season. During the winter, they lured the birds with food into a trap and filmed the animals with thermal cameras while feeding. They then closed the trap and took blood samples and weighed the birds. In the breeding season – when it is important not to disturb the birds too much – they mounted thermal cameras inside nest boxes to continue monitoring their eye temperature and captured the birds once to weigh them.
As hoped, the team found that eye temperature reflected the bird's physiological state. Breeding birds, which were under stress, had higher eye temperatures than birds in the winter and the birds that had better body condition (weighed more) had higher eye temperatures during both seasons. And, when they looked at the amount of stress hormones in the blood of the birds during winter, they found that the birds with higher stress levels also had lower eye temperatures.
The study suggests that measuring body surface temperature via thermal imaging cameras can be used to draw conclusions about an animal's condition, whether it is stressed or how it is coping with seasonal food availability, providing new opportunities to study free-ranging animals noninvasively in the wild.