Do animals keep pets?

After reading an article from Psychology Today by Hal Herzog, it got me thinking about the idea of pet-keeping. The article "Are Humans The Only Animals That Keep Pets?", claims that humans are the only animal that keeps members of other species for an extended point of time purely for enjoyment. Herzog points out that while some animals are documented having pets, this behavior almost always happen in captive or semi-captive environment where food and shelter are provided. The author believes that humans are "true" pet owners because the owner-pet relationship occurs in a natural setting and argues that animal pet owners are not "true" pet owners because they do so in captive or semi-captive settings. Thus, Herzog believes that humans are the only animals that keep pets.

Cited in the article is a paper by Izar et al. (2006), on cross-genus adoption of a marmoset by wild capuchin monkeys (link to the paper is below on References). While the paper specifically refers to the behavior as an adoption, Herzog and paper co-author Dorothy Fragaszy think that there is a parallel between the capuchin-marmoset adoption and pet-keeping in humans. However, these capuchins live in a site where food are provided daily. So, Despite the similarities, these capuchins are not "true" pet owners according to Herzog's definition of pet-keeping,

A young marmoset taking food (cracked palm nut) from its adoptive mother’s (capuchin) hand. Photo by Jeanne Shirley (Izar et al., 2006).

What is a pet and how would you define one? Herzog (2010) defines a pet as a member of other species that are being kept for an extended period of time for enjoyment. According to the Merriam-Webster dictionary, the definition of a pet is "a domesticated animal kept for pleasure rather than utility". The definition of a pet in Oxford English Dictionary is "a domestic or tamed animal or bird kept for companionship or pleasure and treated with care and affection". Dr. James Serpell defines pet-keeping as a leisure activity but not necessarily without function, much like there are function in play or other recreational pursuits (Serpell, 1990). He thinks that pet-keeping is functional in a broad sense but not easily evaluable in economic terms.

Some primate pet owners include (left) Koko and (right) Tonda, who both had cats as pets. 

While it is impossible to define what a pet is from an animal standpoint, at least in humans, we can agree that a pet can be defined as a companion animal that we treat with affection whose function is to provide us with enjoyment. 

Did the behavior of animal domestication evolved into pet-keeping? Is pet-keeping a reflection of human's nurturing instinct? Or is pet-keeping a reflection of human's constant need of social interaction, even outside of our own species? What do you think? Well ... that's for another blog post.

References:

Herzog, H. 2010. Are Humans The Only Animals That Keep Pets? Retrieved July 1, 2010 http://www.psychologytoday.com/blog/animals-and-us/201006/are-humans-the-only-animals-keep-pets

Izar, P. Verderane, MP. Visalberghi E. Ottoni, E. De Oliveira, MG. Shirley, J. Fragaszy, D. 2006. Cross-Genus Adoption of a Marmoset (Callithrix jacchus) by Wild Capuchin Monkeys (Cebus libidinosus): Case Report. American Journal of Primatology 68:692-700. Retrieved July 1, 2010 http://psychology.uga.edu/primate/pub/Cross-genus%20adoption%20AJP%2068,%20692-700%202006.pdf

Serpell, JA. 1990. Pet-keeping and Animal Domestication: A reappraisal. In The Walking Larder. Clutton-Brock, J, ed. Pp. 10-21. Massachusetts: Unwin Hyman Inc. Retrieved July 1, 2010 http://research.vet.upenn.edu/Portals/36/media/Serpell_pet_keeping_domestication.pdf