What is a companion animal? An ethological approach based on Tinbergen's four questions. Critical review

Having one or more non-human animals in our company became a steadily growing custom around the world. The terminology that describes these non-consumable animals encompasses a wide range as well (e.g. ‘hobby animals’, ‘pets’, ‘exotic animals’). However, their customary dubbing as ‘companion animals’ raises several questions that warrant for a systematic approach. In this review, we based our approach on the fundamental four questions developed for studying behaviour by Tinbergen, through which behaviour-based descriptors can be found for the various criteria that consider an animal as being a companion to humans. There is little consensus about whether a companion animal would be (1) any animal that people keep for other than consumption or working purposes; or in a more narrow sense, (2) companion animals should provide hedonistic value through rewarding interactions with their owners; or in the strictest sense, (3) only those animals can be regarded as companion animals that stay with their owners of their own free will (without confinement). According to Tinbergen’s four recommended levels of behavioural research, we assessed the literature with regard to the (i) Mechanism, where we investigated what kind of behaviour is expected from a companion animal in the above defined clusters. Next, we surveyed (ii) the role of Ontogeny in causing potential changes in the behaviour of particular animals, thus making them more suitable for companionship with humans. The question about (iii) Evolutionary background of various companion animal types came next, whether and which kind of forces of selection were needed for their existence. Finally, (iv) regarding the Function of behaviour, our focus was directed to the advantages (adaptive value) that companionship with animals can provide to both parties if we consider the various definitions of companion animals. With this critical review our goal was to provide an ethology-based framework for the description of various approaches to this elusive human-animal relationship. Rather than relying on hard-to-quantify terms borrowed from human sciences (such as ‘attachment’, ‘bonding’ or ‘emotional closeness’), we believe that the essence of being a companion animal should be determined with the terminology of biology, more closely, with the help of behaviour.