Social interaction with a cagemate in pain increases allogrooming and induces pain hypersensitivity in the observer rats.

Empathy, which is a highly cognitive and emotional process, is the ability to share the emotional states of others. Empathy has also been observed in rodents. The empathic sharing of the distressful experience of a conspecific can even motivate altruistic behaviors, which are critical for survival. However, previous studies investigating empathy or prosocial behaviors in rodents mainly employed fearful or other stressful stimuli to elicit emotional changes; whether pain empathy can also motivate prosocial behaviors has yet to be investigated. By using the writhing test, the present study found that cagemate observer (CO) rats, compared with non-cagemate observer (NCO) rats, increased partner-directed grooming (allogrooming) toward conspecifics that had received an intraperitoneal injection of acetic acid during a dyadic social interaction. Following a dyadic social interaction with a demonstrator that received an intraperitoneal injection of acetic acid, the CO rats, compared with NCO rats, exhibited bilateral mechanical pain hypersensitivity and an enhanced acetic acid-induced writhing response. Our results here provided further evidence of pain empathy in rats, suggesting that empathy for pain may motivate prosocial behaviors in rats.