Thought it was just humans that are ticklish? Think again - scientists are studying how animals respond to being tickled in a bid to shed light on how laughter evolved.
Tickling a gorilla is not for the faint-hearted. But keeper Phil Ridges is not worried at getting into the enclosure with Emmie at Port Lympne Wild Animal Park in Kent.
The gorilla, now 19, was hand-reared, and Phil has been her keeper for most of her life.
He says she has a tendency to be "a little bit frosty", but if she is in the mood, she cannot resist a chortle when she is tickled.
"I've worked with gorillas for a long time, and I've often seen gorillas tickling each other, so it is a nice feeling when they have accepted you enough and they don't mind you tickling them," he says.
But it is Emmie's response that has intrigued scientist Marina Davila-Ross from the University of Portsmouth, because the gorilla's reaction sounds a lot like human laughter.
Dr Davila-Ross says: "I was amazed about the way apes responded to being tickled - the apes seem to behave in the same way humans and children behave when they are being tickled."
In a study published in Current Biology in 2009, Dr Davila-Ross and colleagues compared the sounds that great apes made when they were tickled with the laughter that tickled humans produced.
They found many acoustic similarities, which has led them to believe that laughter in great apes shared the same evolutionary origin as laughter in humans, suggesting a common ancestor that giggled when tickled.
Dr Davila-Ross, who led the study, explains: "Based on the study, we can now say laughter is at least 30 million to 60 million years old."
But it is not only humans and apes that seem to be ticklish.
Several years ago, Jaak Panksepp, now based at the College of Veterinary Medicine at Washington State University, decided to see how rats reacted when they were tickled.
He says: "I had my tongue in my cheek when I first suggested that possibility to my student, Jeff Burgdorf.
"Waking up with that idea struck me as a delusional inspiration, but we tested it, and were blown away by the vigour and consistency of the result."
The researchers found that the rodents emitted high-pitched chirps when tickled by the scientists - outside the range of human hearing. They also produced the same noises when they played around with other rats.
In a series of published papers, the researchers said they believed these experiments revealed a primal form of laughter.
"The fact that it replicated in animal after animal coaxed us to drop other planned experiments, and follow where that lead us took us for months, then years," says Dr Panksepp.
"This is simply the best measure of social joy in animals we have."
But if animals as different as rats and primates can laugh when tickled - what about all of the animals in between?
This is a question that Dr Davila-Ross is now looking to answer.
She explains: "Over the course of several years, we started to hear about other research finding that other animals produced tickle-induced vocalisations.
"This was intriguing."
To investigate further, her team has turned to a rich source of animal footage: YouTube.
According to Dr Davila-Ross, more and more animal behaviour researchers are turning to the internet because it allows them to look through large volumes of footage, which can then be systematically assessed.
A quick trawl reveals an eclectic array of animals - owls, dogs, meerkats, penguins, and even a camel and a dolphin - that appear to noisily react when being tickled.
The team is especially interested in tickling, because it allows them to directly compare responses across these very different species. The researchers are looking to gather and compare as much tickling material of as many animals as possible.
Dr Davila-Ross explains: "It is such a systematic method, and tickling has many interesting elements to it - it is part of play behaviour, and it's very important that we have a surprise factor in there.
"There is certain coordination going on, and it is not just a passive behaviour."
The team is especially interested in the sounds animals make as they are being tickled.
Dr Davila-Ross does not describe these as laughter, rather she is looking for the animals that appear to make "positive vocalisations" - or expressions of joy.
She explains: "I'm interested in positive communications of animals.
"I believe that positive communication is closely linked to the evolution of laughter: by communicating with one another positively, we are interacting more with individuals - and it is likely that this played an important role for communication to develop."
Michael Owren, associate professor of psychology and neuroscience from Georgia State University in the US, believes there may be a simple explanation to why such a wide range of animals respond to tickling.
He says: "I suspect that it might be an artefact of what the mammalian nervous system is like.
"In the same way that touching or stroking or grooming animals can be pleasurable to them - without being possible for them because within their species they don't have the same hands and fingers that we have - it seems there is a commonality across mammals of pleasant feelings being evoked by touching."
Dr Marina Davila-Ross says that although it may take time to gather data and assess the results, she hopes this study across the animal kingdom could begin to shed light on how laughter evolved.
"A direct comparison across a range of species will give us some interesting insights into the evolution and co-evolution of play vocalisations and positive animal emotion," she says.
"I think it is important when one reconstructs evolutionary processes - particularly with positive expressions - that it is important to assess different types of animals.
"In this way we can assess in much detail how did these vocalisations emerge and why is it important for that animal to produce them."