Rats and other animals need to be highly at tuned to social signals from others so that can identify friends to cooperate with and enemies to avoid. To find out if this extends to non-living beings, Loleh Quinn at the University of California, San Diego, and her colleagues tested whether rats can detect social signals form robotic rats.

They housed eight adult rats with two types of robotic rat -- one social and one asocial -- for 5 our days. The robots rats were quite minimalist, resembling a chunkier version of a computer mouse with wheels -- to move around and colorful markings.

Next, the researchers trapped the robots in cages and gave the rats the opportunity to release them by pressing a lever.

During the experiment, the social robot rat followed the living rats around, played with the same toys, and opened caged doors to let trapped rats escape. Meanwhile, the asocial robot simply moved forwards and backwards and side to side.

Across 18 trials each, the living rats were 52 percent more likely on average to set the social robot free than the asocial one. This suggests that the rats perceived the social robot as a genuine social being. They may have bonded more with the social robot because it displayed behaviours like communal exploring and playing. This could lead to the rats better remembering having freed it earlier, and wanting the robot to return the favour when they get trapped, says Quinn.

The finding shows how sensitive rats are to social cues, even when they come from basic robots. Similarly, children tend to treat robots as if they are fellow beings, even when they display only simple social signals. "We humans seem to be fascinated by robots, and it turns out other animals are too," says Wiles.

The readiness of the rats to befriend the social robot was surprising given its minimal design. The robot was the same size as a regular rat but resembled a simple plastic box on wheels. "We'd assumed we'd have to give it a moving head and tail, facial features, and put a scene on it to make it smell like a real rat, but that wasn't necessary," says Janet Wiles at the University of Queensland in Australia, who helped with the research.