By Victoria Gill
Science and nature reporter, BBC News
A prairie dog kiss may be a form of social reassurance, much like a human kiss
It seems humans are not the only animals that change their behaviour when they are being watched.
Captive prairie dogs - large and very sociable rodents - "kiss and cuddle" more when they are being watched by zoo visitors, scientists have found.
A research team studied 25 black-tailed prairie dogs at Saint Louis Zoo, US.
They say the findings will help others who study animal behaviour unpick the effects of being observed from their observations of "normal" behaviour.
They could also help to improve the design of zoo enclosures to benefit animals.
Adam Eltorai from Washington University in St. Louis led the study. He and his colleagues observed and recorded the behaviour of the prairie dogs throughout the course of a summer.
At the same time as recording the animals' behaviour, the scientists counted the number of people visiting their enclosure.
"In many situations, prairie dogs react to being watched in exactly the same way we would expect humans to react," Dr Eltorai told BBC News.
The animals appeared to enjoy the attention. As the size of the crowd increased, the adults relaxed more and spent less time looking out for potential threats.
"When more people were watching, the adult prairie dogs became much more affectionate, kissing and touching more and fighting less," said Dr Eltorai.
PRAIRIE DOG FACTS
Black-tailed prairie dog groups have 'look-outs' posted on mounds, which warn of approaching predators with loud, repetitious antipredator calls
The animals build underground tunnel systems that are interconnected by highways
Their language is surprisingly complex. Their squeaky barks contain information that can describe colour, size, direction of travel, speed and even different types of predator
"But the young prairie dogs actually seem to become more tense with larger audiences - fighting more and kissing less."
"It is possible that young prairie dogs are just behaving like any other rambunctious youngsters, so the added 'stress' of the visitors may simply amplify the animals' normal behaviours," said Dr Eltorai.
The prairie dogs relaxed behaviour surprised the team. The species has many natural predators - including bears, foxes, snakes, birds of prey and even humans - so the scientists expected them to become more tense and vigilant when watched.
But the reverse appeared to be true. "They actually became more relaxed," Dr Eltorai said. "And the larger the observing crowd, the more affectionate the animals became."
This prairie dog "greeting kiss" is remarkably similar to a human kiss - two rodents touch mouth to mouth and sometimes briefly press their tongues together.
And the kissing is often accompanied by other affectionate behaviours, such as playing or grooming one another.
Deborah Wells, a researcher from Queens University Belfast, who also studies the effects of visitors on captive animals, explained why it was important to understand how different species responded to human observers, so that zoo enclosures could be designed with the animal's welfare in mind.
"It is possible that the prairie dogs, a naturally social species, are enriched by visitors and seek out their 'company'," she told BBC News.
"But our work has shown that zoo-housed western lowland gorillas... gain welfare benefits from being buffered from the public, perhaps using netting designed to reduce people's visibility to the animals.
"So visitors to zoos have the potential to be enriching to some species, but stressful to others."