Fatal accidents in captive elephants occasionally occur because humans are unable to gauge elephants' emotions solely by their behavior. The intellectual capacity of elephants makes them capable of understanding circumstantial changes and associated emotions, allowing them to react accordingly. Physiological markers, such as heart rate variability, may be effective in determining an elephant's emotional state. In this study, a wearable heart rate monitor was used to determine the emotional state of a female Indian captive elephant (Elephas maximus indicus). The average heart rate was higher when the elephant underwent painful treatment than when it underwent non-painful treatment. In addition, the heart rate increased both before and after the treatment, which included radiography and blood collection.

Introduction Reduced activity and sleep–wake rhythm disturbances are essential features of depressive episodes. In addition, alterations in heart rate variability (HRV) have been implicated in depression. By using a wearable sensor that monitors 3‐dimensional acceleration and HRV simultaneously, we examined the activity and HRV indices in depressive episode of mood disorders. Methods Participants were 19 patients (13 major depressive disorder [MDD] and 6 bipolar depression; 11 females) and 18 controls (9 females) matched for age and ethnicity (all Japanese) who completed 3 consecutive days of all‐day monitoring by a small and light device attached to the chest. Results Activity magnitude was significantly reduced while lying/resting time was increased in depressed patients, compared with controls. When males and females were examined separately, male, but not female, patients showed significant reduction in activity. HRV indices such as R‐R interval and high‐frequency power (a parameter for the parasympathetic system) were significantly decreased in patients than in controls. Significant differences in activity and HRV indices were seen only in males. Sympathetic load during sleep significantly correlated with damped rest–activity rhythm in depressed patients. Limitations The number of participants was small, and the majority of the participants were taking psychotropic medications. Conclusions We obtained evidence for reduced activity, increased lying/resting time, and reduced HRV indices in male depressed patients. The simultaneous monitoring for activity and HRV suggested greater sympathetic load during sleep is associated with damped rest–activity rhythm (increased activity during sleep and decreased daytime activity), which might be a characteristic pathology of depression.