Pigs are social but only dogs are attached, companion animal study finds

Researchers at the ELTE Department of Ethology in Budapest investigated whether an infant-mother analog attachment bond arises in intensively human-socialized companion pigs towards their owners similar to companion dogs.

The work is published in the journal Scientific Reports.

Using the so-called "Strange Situation Test," they compared the behaviors of young companion pigs and dogs towards their owner and a stranger. They found that dogs, but not pigs, exhibited the specific behavior pattern typical of the attachment bond. These findings suggest that the domestication process and intense early exposure to humans alone are not sufficient to trigger human-analog attachment in companion animals.

In the case of dogs, the unique selection for cooperation with and dependency on humans might be the key feature for the emergence of attachment to the caregiver.

Companion dogs' bond with their owners has long been described as special and unique. Indeed, from an ethological point of view, dogs' bond with their owners can be defined as "attachment," which is similar in function to that of a human mother with her infants.

In an attachment bond, the attached individual depends on the security-providing attachment figure.

Attachment has some well defined criteria: the attached individual should use the caregiver as a secure base when exploring a new environment, and as a "safe haven" in case of danger, and display specific behaviors upon reunion with the caregiver after separation.

"Besides dogs, there are only a few studies assessing if the behaviors of other companion animals fulfill the criteria of attachment," explains Anna Gábor from the Neuroethology of Communication Lab at the ELTE Department of Ethology, first author of the study.

"Therefore, whether human-analog attachment can arise in other domestic animals experiencing an intense socialization with humans, or, on the contrary, is facilitated by the artificial and unique selection for dependence to and cooperation with humans that dogs underwent during their domestication was still a question to answer."

To address this question, the researchers compared the behaviors of young companion dogs with those of young companion pigs.

"Like dogs, pigs are also group-living and extremely social animals, and when kept as companions, they have a similar role in human families to that of dogs" says Paula Pérez Fraga from the Neuroethology of Communication Lab at the ELTE Department of Ethology and co-first author of the study. "This is why they are a good model species for direct comparisons with companion dogs."

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The researchers tested the animals using the so-called "Strange Situation Test," a validated behavioral test to assess attachment behavior. This test's basic feature is to expose the animal to separations from their owner and encounters with a stranger.

The premise is that if attachment behaviors emerge, there will be a clear distinct behavioral pattern toward the owner compared to the stranger in the relevant situations. The research team found that the "attachment behavioral pattern" towards the owner was present only in dogs, but not in pigs.

"This finding suggests that the domestication process and intense human socialization alone are not enough to trigger human-analog attachment behavior to the human caregiver in animals," explains Gábor.

"We argue that dogs were selected for dependency on and for working in intense cooperation with humans, which is a unique characteristic of dog domestication."