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Here are eight findings about laughter from Becky and Emily's investigation into the natural world.

Animals make specific noises when playing

Sasha Winkler, a biological anthropologist at UCLA, examined which animals make noises to signal that they're playing. At least 65 species were found to make "play vocalisations", mostly including mammals and birds. Some are more obvious, such as primates, dogs and cats. But, notably, marine mammals such as dolphins and whales also make squeaking sounds when playing, and birds have highly specific types of play vocalisations. While reptiles and fish haven't been observed making play vocalisations, it's possible that they have an entirely different behaviour that performs the same function.

Play noises vary from animal to animal

Although the classic play vocalisation is a kind of chuckling or panting, there's a huge variety of sounds and signals across the animal kingdom. Elephants, for example, give a short trumpeting sound, while foxes squeal at one another. These noises are tied to breath: as animals play they exert energy, which leads to increased respiration, which combines with unique vocal tones to become a signal for play. Dogs are a prime example of this, having an entirely different panting pattern for when they’re overheated when compared with their excited panting during play.

There are two common theories for why animals play. The first is that animals might play to practice physical behaviours they'll need for when they're grown up, such as fighting or hunting. Play may also promote social bonding - when we as humans play, we build relationships with the other players and establish social boundaries.

Play vocalisations are therefore a great way to disambiguate whether two animals are actually fighting or merely playing. A study of rats found that mute rats who were unable to make play vocalisations had shorter play times and more aggressive encounters as a result.

Animal play noises become laughter

Sticking with rats for a moment, researchers have discovered that they actually can laugh - we just can't hear it because it's ultrasonic! You can hear what this sounds like in the clip below. Primates also like to have a laugh, whether they're being tickled or playing around. They can even have contagious laughter, where one chimp sets off the whole group.

Human laughter seems to be an extension of primate laughter, originally stemming from play vocalisations but later evolving alongside language. You can distinguish human laughs from non-human primate laughs with a neat trick: apes laugh on both the exhale and the inhale, whereas humans laugh on the exhale alone. Well, most humans…

Human laughter doesn’t just signal play: Professor Sophie Scott has found that laughter runs the full gamut of associations. It can be positive of course, showing affection, agreement or understanding. But laughter can also be negative, being used as a cover for anger or pain. Laughter can even change the mood, like nervous laughter which cuts through the tension.

These extra implications of laughter in humans derive from an entirely different brain system. The original brain system found in other mammals is involuntary, responsible for those uncontrollable laughing fits where your sides start to hurt! But the human-exclusive brain system governing laughter is actually voluntary. Does this mean laughter can be manipulative? Listen to the full episode here to hear Becky and Emily's thoughts.

Laughter builds trust

Robert Provine, a psychologist from the University of Maryland, noted that laughter is one of our best tools for building trust. In conversation, the person speaking is the one who laughs the most - it's a subconscious way to endear people to you! It's a product of our human drive to create and maintain social bonds.

That trust also serves a survival purpose: It helps us gain information. British anthropologist Robin Dunbar showed that if people are laughing they'll share more intimate information about themselves. First you laugh, then they laugh, then they spill the tea.

Laughter is a known stress-reliever, but it can be so powerful as to indicate whether a relationship will succeed or not. A long-running study by psychologist Robert Levenson of UC Berkeley looked at married couples' reactions when confronted with touchy issues from their own relationships. The couples were hooked up to polygraph machines to measure their stress levels as they discussed the topic.

Levenson found that couples who laughed and smiled about their problems stayed together longer, presumably thanks to laughter helping them get on top of their stress. Of course, that’s only if they're both laughing! The same is suspected to be true of platonic relationships.

The neuroscience of laughter

Naturally, there's a neurochemical explanation for all these social effects of laughter. Laughing, even when forced, has a hormonal effect on the body. Adrenaline levels drop, making you feel less confrontational. Levels of the stress hormone cortisol permanently drop over time. Endorphin levels also spike, giving you a warm feeling that helps you better tolerate pain. So laughing isn't just good for your relationships; it's good for your mental health!

So take a cue from playful animals and allow yourself time in your day to just have a good giggle.

Find out what else got Becky and Emily giggling, by listening to the full episode here.

To hear more about the links between us humans and beast, check out the entire series of Naturebang from Radio 4 here.