Human relationships can be complex, so arguably, inter-species friendships can be even more complex. Instead of people with different personalities, perspectives and experiences, we have all of that plus different species with different wiring and capabilities.

One of the most recognizable pillars of human-animal relationships (though certainly not the only one) is domestication. But how does domestication happen? We’re all familiar with the quasi-scientific story of wolves scavenging human waste and somehow gradually transforming into modern dogs, but how did that actually happen? One long-term scientific experiment in Siberia has shown (and continues to show) us what that may have looked like.

In 1959, Dmitri Belyaev initiated a long-term experiment that would use silver foxes to explore how domestication may have occurred in other mammals from generation to generation. Generation after generation of captive wild foxes were selected for breeding based on signs of agreeability to human contact. Within the earlier generations, the selection processes were as simple as whether or not the fox chose to bite an assistant’s gloved hand or whether or not they could be trained to eat from an assistant’s hand.

In a few generations, a broad range of surprising separate genetic changes began to take shape. In addition to behavioral tendencies, the foxes began to develop floppy ears, piebald fur patterns, adolescent features, and other characteristics that we associate today with cute and domesticated animals. And, most importantly, these once-wild and -fearful foxes were now friendly and eager to please, some even learning to play fetch.

As we’ve mentioned, domestication is not the only way that (once-wild) animals can form friendships with people. Some non-domesticated animals can also become friends with people under certain circumstances. One correlation is intelligence. Corvids, dolphins, and other intelligent creatures are regularly reported to understand human cues and form friendly, if not dependent, relationships. In other cases, wild animals with a natural tendency to form parental fixations or social relationships, like ducks, have formed social connections with people who’ve fit into their existing wild social behaviors.

Another common form of loose social relationship is when wild animals learn not to fear people based on simple reward psychology. In many national parks, for example, rangers caution visitors against feeding wild animals for precisely this reason. If visitors feed animals they see, the animals will learn not to fear people and to expect food from them, which can lead to dangerous situations.