In the second stage, standard trials and probe trials were carried out on 6 giant pandas that passed the first stage (Table S3). Additionally, there was no significant difference in the rate of learning between females and males (Fig. For the upper and lower face groups, three giant pandas that were shown the upper face and three that were shown the lower face met the requirement, and there was no significant difference in the learning rate between the two groups (Fig. This indicated that the emotion associated with the human expression did not affect the learning speed of the giant pandas. Analysis of the Cox proportional hazards model showed that there was no significant difference in the speed of achieving the criterion between the happy and angry groups (Fig. Three giant pandas from the happy group and three from the angry group achieved the learning criterion of the first stage. To rule out the possibility that giant pandas can discriminate among facial expressions simply based on the geometric relationships among facial features, we presented giant pandas with pictures of halves of faces with happy or angry expression and tested whether they can choose correct stimuli. A previous study showed that giant pandas can recognize face-like geometric patterns 22. 30, we aimed to examine the ability of pandas to discriminate different emotion of human faces. In this paper, following the methods of Müller et al. For captive-bred giant pandas, constant contact with humans is inevitable during pre-release training, which is essential for their survival in the wild 29. Subsequently, a program was launched to release captive giant pandas into the wild to reinforce wild populations. The captive breeding of giant pandas has been so successful that the captive population of giant pandas reached 471 in 2016 28. Since giant pandas have visual and cognitive abilities, their response to human contact may lead to changes in behaviour and recognition, as has been found in domestic dogs ( Canis familiaris) 25 and cats ( Felis catus) 26 as well as birds 27. Thus, these giant pandas are exposed to the various expressions of a large number of visitors each day. In addition to their close interaction with keepers due to their daily care 24, nearly three million tourists visit the Chengdu Research Base of Giant Panda Breeding to see the approximately one hundred and fifty giant pandas and ten new cubs at the facility each year. Understanding the ability of the giant panda as a wild animal to discriminate human facial expressions can provide valuable information for their daily care. Giant pandas have been bred in captivity for only 64 years and have never been intentionally selected and bred for panda–human interactions 24. Therefore, many questions remain about the cognitive abilities of giant pandas. However, previous research has shown that giant pandas cannot recognize themselves in a mirror but instead consider the image to be a separate conspecific individual, indicating they do not have the capacity of self-recognition 23. In addition, pandas can discriminate between black-and-white objects with only subtle differences in shape, implying they can perception face-like stimuli 22. Giant pandas exhibit marking behaviour by peeling bark or disturbing the soil with their claws and directly transfer visual information through body language 20, 21. Giant pandas have been shown to maintain their social relationships through visual as well as chemical (e.g., glandular secretions or urine) communication 16, 17, 18, 19. Some research has also demonstrated that captive wildlife can recognize human faces 15, while it is still unknown whether non-primate wildlife can discriminate the emotional facial expressions of humans. Recent research has shown that domestic animals can discriminate human facial expressions 13, 14. Emotional facial expressions evolved for physiological functions 7, 8 and conspecific interactions 9, 10, 11, and it is also involved in interspecific interactions 12. Emotional facial expressions serve critical adaptive purposes 6. Some types of recognition are innate, but others are acquired through learning (acquisition) 4, 5. Animals adjust their behaviour based on their information-recognition ability 1, 2, 3.
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