Review of depressive-like behaviours in some group-living mammals

Abstract

Thanks to animal models of depression, we are getting closer to understand the nature of this disorder in humans – but depressive disorders may not be specific only to humans. Although due to the inability to collect a verbal report from animals and the lack of direct insight into their mental states, the diagnosis of depression in animals is based on behavioural observations, which is considered insufficient to diagnose the disorder. Nevertheless, animal models of depression indicate the possibility of depression-like states in laboratory animals. This review aims to bring together previous reports of depressive disorder symptoms in group-living mammalian species living in unnatural environments and/or experiencing relatively frequent contact with humans: primates, marine mammals, domestic horses and elephants. The paper summarizes previous reports of depressive-like symptoms in those species, pointing out the similarity of depression-like symptoms in aforementioned animals to those observable in human depression. The efficacy of pharmacological treatments indicates that the brain biochemistry of depressive-like states is similar in both laboratory animals and humans. Those symptoms in laboratory animals are similar to certain species of group-living mammalian species – furthermore, the methods for inducing similar states are not uncommon in the training of the aforementioned mammals, indicating similar causes of the phenomenon. All previous reports of Major Depressive Disorder symptoms listed in the DSM-V and noticed in animals were considered and redescribed, such as: loss of interest (anhedonia), sleep disorders, psychomotor retardation, fatigue, feeling worthless, decreased concentration, self-harm and impairment in social. In addition, behavioural changes similar to mourning states resulting from changes in herd structure and stereotypies occurring in captive animals were taken into account. The data collected shows that, based on observed behaviours and similarities to symptoms in laboratory animals, at least some of group-living mammalian species experiencing relatively frequent contact with humans may suffer from depression-like symptoms, which may not only extend the knowledge of depression in humans, but also have a significant impact on improving the welfare of captive animals. Perhaps the only difference found between humans and some animals when it comes to depressive disorder is the ability to verbally report one's internal states.