Events with Different Emotional Valence Affect the Eye’s Lacrimal Caruncle Temperature Changes in Sheep

Infrared thermography (IRT) has been recently applied to measure lacrimal caruncle
temperature non-invasively since this region is related to the sympathetic response, and it seems
a promising technique that is able to infer negative emotions in sheep (e.g., fear). However, the
scientific literature so far is limited in understanding whether a caruncle’s temperature changes also
in response to positive emotional states in sheep. Through classical conditioning, we aimed to assess
how a positive or a negative event affects the physiological (lacrimal caruncle temperature measured
with IRT and cortisol levels) and behavioral responses of sheep (ear position). Fourteen ewes from
the same flock were randomly assigned to two treatment groups: positive (n = 7) and negative (n = 7).
Each group was then trained through classical conditioning to associate a neutral auditory (ring bell)
stimulus to an oncoming event: for the positive group, the presence of a food reward (maize grains),
while for the negative one, the opening of an umbrella. After three weeks of training, before (at rest)
and after (post-treatment), lacrimal caruncle temperature was non-invasively measured via IRT, and
saliva samples were gently collected to measure cortisol levels. During treatment, sheep behavior
was videorecorded and then analyzed using a focal animal sampling technique. At rest, the eye’s
lacrimal caruncle temperature was similar in both groups, while post-treatment, a significant increase
was shown only in the negative group (t-test; p = 0.017). In the anticipation phase, sheep in the
positive group kept their ears forward longer compared to those in the negative one (Mann–Whitney;
p < 0.014), 8.3 ± 2.1 s and 5.2 ± 4.2 s, respectively. The behavioral response observed reflects a
learnt association between a neutral stimulus and events with different emotional valence. Cortisol
concentration slightly increased in both groups post-treatment. Our results confirm that IRT is a
non-invasive technique that can be useful when applied to assess how positive and negative events
may affect the physiological response in sheep.